diff options
| -rw-r--r-- | build2/scheduler | 11 | ||||
| -rw-r--r-- | build2/scheduler.cxx | 36 |
2 files changed, 34 insertions, 13 deletions
diff --git a/build2/scheduler b/build2/scheduler index 563ec31..42f82e0 100644 --- a/build2/scheduler +++ b/build2/scheduler @@ -73,8 +73,11 @@ namespace build2 void async (atomic_count& task_count, F&&, A&&...); - // Wait until the task count reaches 0. If the scheduler is shutdown - // while waiting, throw system_error(ECANCELED). + // Wait until the task count reaches 0. If the scheduler is shutdown while + // waiting, throw system_error(ECANCELED). + // + // Note that it is valid to wait on another thread's task count (that is, + // without making any async() calls in this thread). // void wait (atomic_count& task_count); @@ -288,11 +291,15 @@ namespace build2 // depends on the number of waiters that we can have which cannot be // greater than the total number of threads. // + // The pointer to the task count is used to identify the already waiting + // group of threads for collision statistics. + // struct wait_slot { std::mutex mutex; std::condition_variable condv; size_t waiters = 0; + const atomic_count* tcount; bool shutdown = true; }; diff --git a/build2/scheduler.cxx b/build2/scheduler.cxx index 47d38e4..c151847 100644 --- a/build2/scheduler.cxx +++ b/build2/scheduler.cxx @@ -18,17 +18,20 @@ namespace build2 // See if we can run some of our own tasks. // - task_queue& tq (*task_queue_); // Must have been set by async() or task - // would have been 0. - - for (lock ql (tq.mutex); !tq.shutdown && !empty_back (tq); ) - pop_back (tq, ql); - - // Note that empty task queue doesn't automatically mean the task count - // is zero (some might still be executing asynchronously). + // If we are waiting on someone else's task count then there migh still + // be no queue which is set by async(). // - if (task_count == 0) - return; + if (task_queue* tq = task_queue_) + { + for (lock ql (tq->mutex); !tq->shutdown && !empty_back (*tq); ) + pop_back (*tq, ql); + + // Note that empty task queue doesn't automatically mean the task count + // is zero (some might still be executing asynchronously). + // + if (task_count == 0) + return; + } suspend (task_count); } @@ -66,7 +69,18 @@ namespace build2 bool collision; { lock l (s.mutex); - collision = (s.waiters++ != 0); + + // We have a collision if there is already a waiter for a different + // task count. + // + collision = (s.waiters++ != 0 && s.tcount != &tc); + + // This is nuanced: we want to always have the task count of the last + // thread to join the queue. Otherwise, if threads are leaving and + // joining the queue simultaneously, we may end up with a task count of + // a thread group that is no longer waiting. + // + s.tcount = &tc; // Since we use a mutex for synchronization, we can relax the atomic // access. |