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338 lines
14 KiB
338 lines
14 KiB
2016-01-22 Torvald Riegel <triegel@redhat.com> |
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* beginend.cc (GTM::gtm_thread::serial_lock): Put on cacheline |
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boundary. |
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(htm_fastpath): Remove. |
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(gtm_thread::begin_transaction): Fix HTM fastpath. |
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(_ITM_commitTransaction): Adapt. |
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(_ITM_commitTransactionEH): Adapt. |
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* libitm/config/linux/rwlock.h (gtm_rwlock): Add htm_fastpath member |
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and accessors. |
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* libitm/config/posix/rwlock.h (gtm_rwlock): Likewise. |
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* libitm/config/posix/rwlock.cc (gtm_rwlock::gtm_rwlock): Adapt. |
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* libitm/libitm_i.h (htm_fastpath): Remove declaration. |
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* libitm/method-serial.cc (htm_mg): Adapt. |
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(gtm_thread::serialirr_mode): Adapt. |
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* libitm/query.cc (_ITM_inTransaction, _ITM_getTransactionId): Adapt. |
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--- libitm/beginend.cc |
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+++ libitm/beginend.cc |
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@@ -32,7 +32,11 @@ using namespace GTM; |
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extern __thread gtm_thread_tls _gtm_thr_tls; |
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#endif |
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-gtm_rwlock GTM::gtm_thread::serial_lock; |
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+// Put this at the start of a cacheline so that serial_lock's writers and |
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+// htm_fastpath fields are on the same cacheline, so that HW transactions |
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+// only have to pay one cacheline capacity to monitor both. |
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+gtm_rwlock GTM::gtm_thread::serial_lock |
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+ __attribute__((aligned(HW_CACHELINE_SIZE))); |
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gtm_thread *GTM::gtm_thread::list_of_threads = 0; |
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unsigned GTM::gtm_thread::number_of_threads = 0; |
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@@ -54,9 +58,6 @@ static pthread_mutex_t global_tid_lock = PTHREAD_MUTEX_INITIALIZER; |
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static pthread_key_t thr_release_key; |
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static pthread_once_t thr_release_once = PTHREAD_ONCE_INIT; |
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-// See gtm_thread::begin_transaction. |
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-uint32_t GTM::htm_fastpath = 0; |
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- |
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/* Allocate a transaction structure. */ |
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void * |
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GTM::gtm_thread::operator new (size_t s) |
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@@ -174,9 +175,11 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb) |
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// lock's writer flag and thus abort if another thread is or becomes a |
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// serial transaction. Therefore, if the fastpath is enabled, then a |
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// transaction is not executing as a HW transaction iff the serial lock is |
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- // write-locked. This allows us to use htm_fastpath and the serial lock's |
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- // writer flag to reliable determine whether the current thread runs a HW |
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- // transaction, and thus we do not need to maintain this information in |
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+ // write-locked. Also, HW transactions monitor the fastpath control |
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+ // variable, so that they will only execute if dispatch_htm is still the |
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+ // current method group. This allows us to use htm_fastpath and the serial |
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+ // lock's writers flag to reliable determine whether the current thread runs |
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+ // a HW transaction, and thus we do not need to maintain this information in |
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// per-thread state. |
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// If an uninstrumented code path is not available, we can still run |
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// instrumented code from a HW transaction because the HTM fastpath kicks |
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@@ -187,9 +190,14 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb) |
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// indeed in serial mode, and HW transactions should never need serial mode |
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// for any internal changes (e.g., they never abort visibly to the STM code |
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// and thus do not trigger the standard retry handling). |
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- if (likely(htm_fastpath && (prop & pr_hasNoAbort))) |
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+ if (likely(serial_lock.get_htm_fastpath() && (prop & pr_hasNoAbort))) |
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{ |
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- for (uint32_t t = htm_fastpath; t; t--) |
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+ // Note that the snapshot of htm_fastpath that we take here could be |
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+ // outdated, and a different method group than dispatch_htm may have |
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+ // been chosen in the meantime. Therefore, take care not not touch |
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+ // anything besides the serial lock, which is independent of method |
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+ // groups. |
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+ for (uint32_t t = serial_lock.get_htm_fastpath(); t; t--) |
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{ |
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uint32_t ret = htm_begin(); |
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if (htm_begin_success(ret)) |
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@@ -197,9 +205,11 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb) |
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// We are executing a transaction now. |
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// Monitor the writer flag in the serial-mode lock, and abort |
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// if there is an active or waiting serial-mode transaction. |
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+ // Also checks that htm_fastpath is still nonzero and thus |
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+ // HW transactions are allowed to run. |
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// Note that this can also happen due to an enclosing |
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// serial-mode transaction; we handle this case below. |
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- if (unlikely(serial_lock.is_write_locked())) |
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+ if (unlikely(serial_lock.htm_fastpath_disabled())) |
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htm_abort(); |
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else |
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// We do not need to set a_saveLiveVariables because of HTM. |
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@@ -210,9 +220,12 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb) |
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// retrying the transaction will be successful. |
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if (!htm_abort_should_retry(ret)) |
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break; |
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+ // Check whether the HTM fastpath has been disabled. |
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+ if (!serial_lock.get_htm_fastpath()) |
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+ break; |
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// Wait until any concurrent serial-mode transactions have finished. |
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// This is an empty critical section, but won't be elided. |
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- if (serial_lock.is_write_locked()) |
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+ if (serial_lock.htm_fastpath_disabled()) |
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{ |
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tx = gtm_thr(); |
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if (unlikely(tx == NULL)) |
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@@ -618,7 +631,7 @@ _ITM_commitTransaction(void) |
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// a serial-mode transaction. If we are, then there will be no other |
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// concurrent serial-mode transaction. |
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// See gtm_thread::begin_transaction. |
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- if (likely(htm_fastpath && !gtm_thread::serial_lock.is_write_locked())) |
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+ if (likely(!gtm_thread::serial_lock.htm_fastpath_disabled())) |
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{ |
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htm_commit(); |
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return; |
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@@ -634,7 +647,7 @@ _ITM_commitTransactionEH(void *exc_ptr) |
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{ |
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#if defined(USE_HTM_FASTPATH) |
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// See _ITM_commitTransaction. |
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- if (likely(htm_fastpath && !gtm_thread::serial_lock.is_write_locked())) |
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+ if (likely(!gtm_thread::serial_lock.htm_fastpath_disabled())) |
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{ |
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htm_commit(); |
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return; |
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--- libitm/config/linux/rwlock.h |
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+++ libitm/config/linux/rwlock.h |
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@@ -39,16 +39,29 @@ struct gtm_thread; |
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// |
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// In this implementation, writers are given highest priority access but |
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// read-to-write upgrades do not have a higher priority than writers. |
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+// |
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+// Do not change the layout of this class; it must remain a POD type with |
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+// standard layout, and the writers field must be first (i.e., so the |
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+// assembler code can assume that its address is equal to the address of the |
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+// respective instance of the class), and htm_fastpath must be second. |
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class gtm_rwlock |
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{ |
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- // TODO Put futexes on different cachelines? |
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std::atomic<int> writers; // Writers' futex. |
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+ // We put the HTM fastpath control variable here so that HTM fastpath |
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+ // transactions can check efficiently whether they are allowed to run. |
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+ // This must be accessed atomically because threads can load this value |
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+ // when they are neither a registered reader nor writer (i.e., when they |
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+ // attempt to execute the HTM fastpath). |
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+ std::atomic<uint32_t> htm_fastpath; |
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+ // TODO Put these futexes on different cachelines? (writers and htm_fastpath |
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+ // should remain on the same cacheline. |
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std::atomic<int> writer_readers;// A confirmed writer waits here for readers. |
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std::atomic<int> readers; // Readers wait here for writers (iff true). |
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public: |
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- gtm_rwlock() : writers(0), writer_readers(0), readers(0) {}; |
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+ gtm_rwlock() : writers(0), htm_fastpath(0), writer_readers(0), readers(0) |
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+ { } |
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void read_lock (gtm_thread *tx); |
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void read_unlock (gtm_thread *tx); |
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@@ -59,12 +72,28 @@ class gtm_rwlock |
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bool write_upgrade (gtm_thread *tx); |
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void write_upgrade_finish (gtm_thread *tx); |
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- // Returns true iff there is a concurrent active or waiting writer. |
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- // This is primarily useful for simple HyTM approaches, and the value being |
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- // checked is loaded with memory_order_relaxed. |
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- bool is_write_locked() |
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+ // Returns true iff there is a concurrent active or waiting writer, or |
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+ // htm_fastpath is zero. This is primarily useful for simple HyTM |
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+ // approaches, and the values being checked are loaded with |
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+ // memory_order_relaxed. |
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+ bool htm_fastpath_disabled () |
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+ { |
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+ return writers.load (memory_order_relaxed) != 0 |
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+ || htm_fastpath.load (memory_order_relaxed) == 0; |
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+ } |
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+ |
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+ // This does not need to return an exact value, hence relaxed MO is |
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+ // sufficient. |
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+ uint32_t get_htm_fastpath () |
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+ { |
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+ return htm_fastpath.load (memory_order_relaxed); |
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+ } |
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+ // This must only be called while having acquired the write lock, and other |
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+ // threads do not need to load an exact value; hence relaxed MO is |
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+ // sufficient. |
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+ void set_htm_fastpath (uint32_t val) |
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{ |
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- return writers.load (memory_order_relaxed) != 0; |
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+ htm_fastpath.store (val, memory_order_relaxed); |
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} |
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protected: |
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--- libitm/config/posix/rwlock.h |
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+++ libitm/config/posix/rwlock.h |
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@@ -44,19 +44,32 @@ struct gtm_thread; |
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// |
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// In this implementation, writers are given highest priority access but |
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// read-to-write upgrades do not have a higher priority than writers. |
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+// |
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+// Do not change the layout of this class; it must remain a POD type with |
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+// standard layout, and the summary field must be first (i.e., so the |
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+// assembler code can assume that its address is equal to the address of the |
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+// respective instance of the class), and htm_fastpath must be second. |
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class gtm_rwlock |
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{ |
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- pthread_mutex_t mutex; // Held if manipulating any field. |
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- pthread_cond_t c_readers; // Readers wait here |
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- pthread_cond_t c_writers; // Writers wait here for writers |
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- pthread_cond_t c_confirmed_writers; // Writers wait here for readers |
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- |
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static const unsigned a_writer = 1; // An active writer. |
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static const unsigned w_writer = 2; // The w_writers field != 0 |
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static const unsigned w_reader = 4; // The w_readers field != 0 |
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std::atomic<unsigned int> summary; // Bitmask of the above. |
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+ |
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+ // We put the HTM fastpath control variable here so that HTM fastpath |
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+ // transactions can check efficiently whether they are allowed to run. |
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+ // This must be accessed atomically because threads can load this value |
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+ // when they are neither a registered reader nor writer (i.e., when they |
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+ // attempt to execute the HTM fastpath). |
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+ std::atomic<uint32_t> htm_fastpath; |
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+ |
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+ pthread_mutex_t mutex; // Held if manipulating any field. |
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+ pthread_cond_t c_readers; // Readers wait here |
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+ pthread_cond_t c_writers; // Writers wait here for writers |
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+ pthread_cond_t c_confirmed_writers; // Writers wait here for readers |
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+ |
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unsigned int a_readers; // Nr active readers as observed by a writer |
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unsigned int w_readers; // Nr waiting readers |
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unsigned int w_writers; // Nr waiting writers |
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@@ -74,12 +87,28 @@ class gtm_rwlock |
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bool write_upgrade (gtm_thread *tx); |
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void write_upgrade_finish (gtm_thread *tx); |
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- // Returns true iff there is a concurrent active or waiting writer. |
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- // This is primarily useful for simple HyTM approaches, and the value being |
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- // checked is loaded with memory_order_relaxed. |
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- bool is_write_locked() |
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+ // Returns true iff there is a concurrent active or waiting writer, or |
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+ // htm_fastpath is zero. This is primarily useful for simple HyTM |
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+ // approaches, and the values being checked are loaded with |
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+ // memory_order_relaxed. |
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+ bool htm_fastpath_disabled () |
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+ { |
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+ return (summary.load (memory_order_relaxed) & (a_writer | w_writer)) |
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+ || htm_fastpath.load (memory_order_relaxed) == 0; |
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+ } |
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+ |
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+ // This does not need to return an exact value, hence relaxed MO is |
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+ // sufficient. |
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+ uint32_t get_htm_fastpath () |
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+ { |
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+ return htm_fastpath.load (memory_order_relaxed); |
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+ } |
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+ // This must only be called while having acquired the write lock, and other |
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+ // threads do not need to load an exact value; hence relaxed MO is |
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+ // sufficient. |
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+ void set_htm_fastpath (uint32_t val) |
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{ |
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- return summary.load (memory_order_relaxed) & (a_writer | w_writer); |
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+ htm_fastpath.store (val, memory_order_relaxed); |
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} |
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protected: |
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--- libitm/config/posix/rwlock.cc |
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+++ libitm/config/posix/rwlock.cc |
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@@ -30,11 +30,12 @@ namespace GTM HIDDEN { |
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// ??? Move this back to the header file when constexpr is implemented. |
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gtm_rwlock::gtm_rwlock() |
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- : mutex (PTHREAD_MUTEX_INITIALIZER), |
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+ : summary (0), |
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+ htm_fastpath (0), |
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+ mutex (PTHREAD_MUTEX_INITIALIZER), |
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c_readers (PTHREAD_COND_INITIALIZER), |
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c_writers (PTHREAD_COND_INITIALIZER), |
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c_confirmed_writers (PTHREAD_COND_INITIALIZER), |
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- summary (0), |
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a_readers (0), |
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w_readers (0), |
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w_writers (0) |
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--- libitm/libitm_i.h |
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+++ libitm/libitm_i.h |
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@@ -336,10 +336,6 @@ extern abi_dispatch *dispatch_htm(); |
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extern gtm_cacheline_mask gtm_mask_stack(gtm_cacheline *, gtm_cacheline_mask); |
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-// Control variable for the HTM fastpath that uses serial mode as fallback. |
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-// Non-zero if the HTM fastpath is enabled. See gtm_thread::begin_transaction. |
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-extern uint32_t htm_fastpath; |
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- |
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} // namespace GTM |
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#endif // LIBITM_I_H |
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--- libitm/method-serial.cc |
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+++ libitm/method-serial.cc |
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@@ -222,13 +222,13 @@ struct htm_mg : public method_group |
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// Enable the HTM fastpath if the HW is available. The fastpath is |
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// initially disabled. |
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#ifdef USE_HTM_FASTPATH |
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- htm_fastpath = htm_init(); |
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+ gtm_thread::serial_lock.set_htm_fastpath(htm_init()); |
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#endif |
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} |
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virtual void fini() |
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{ |
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// Disable the HTM fastpath. |
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- htm_fastpath = 0; |
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+ gtm_thread::serial_lock.set_htm_fastpath(0); |
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} |
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}; |
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@@ -288,7 +288,7 @@ GTM::gtm_thread::serialirr_mode () |
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#if defined(USE_HTM_FASTPATH) |
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// HTM fastpath. If we are executing a HW transaction, don't go serial but |
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// continue. See gtm_thread::begin_transaction. |
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- if (likely(htm_fastpath && !gtm_thread::serial_lock.is_write_locked())) |
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+ if (likely(!gtm_thread::serial_lock.htm_fastpath_disabled())) |
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return; |
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#endif |
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--- libitm/query.cc |
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+++ libitm/query.cc |
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@@ -49,7 +49,7 @@ _ITM_inTransaction (void) |
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// a transaction and thus we can't deduce this by looking at just the serial |
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// lock. This function isn't used in practice currently, so the easiest |
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// way to handle it is to just abort. |
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- if (htm_fastpath && htm_transaction_active()) |
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+ if (gtm_thread::serial_lock.get_htm_fastpath() && htm_transaction_active()) |
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htm_abort(); |
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#endif |
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struct gtm_thread *tx = gtm_thr(); |
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@@ -69,7 +69,7 @@ _ITM_getTransactionId (void) |
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{ |
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#if defined(USE_HTM_FASTPATH) |
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// See ITM_inTransaction. |
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- if (htm_fastpath && htm_transaction_active()) |
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+ if (gtm_thread::serial_lock.get_htm_fastpath() && htm_transaction_active()) |
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htm_abort(); |
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#endif |
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struct gtm_thread *tx = gtm_thr();
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