[commit: ghc] wip/gc/nonmoving-nonconcurrent, wip/gc/preparation, wip/gc/sync-without-capability: rts/Schedule: Allow synchronization without holding a capability (3ce969b)
git at git.haskell.org
git at git.haskell.org
Thu Feb 21 15:12:12 UTC 2019
Repository : ssh://git@git.haskell.org/ghc
On branches: wip/gc/nonmoving-nonconcurrent,wip/gc/preparation,wip/gc/sync-without-capability
Link : http://ghc.haskell.org/trac/ghc/changeset/3ce969bb518ad5f056be18756d93d1472f372b61/ghc
>---------------------------------------------------------------
commit 3ce969bb518ad5f056be18756d93d1472f372b61
Author: Ben Gamari <ben at well-typed.com>
Date: Thu Jul 19 21:57:14 2018 -0400
rts/Schedule: Allow synchronization without holding a capability
The concurrent mark-and-sweep will be performed by a GHC task which will
not hold a capability. This is necessary to avoid a concurrent mark from
interfering with minor generation collections.
However, the major collector must synchronize with the mutators at the
end of marking to flush their update remembered sets. This patch extends
the `requestSync` mechanism used to synchronize garbage collectors to
allow synchronization without holding a capability.
This change is fairly straightforward as the capability was previously
only required for two reasons:
1. to ensure that we don't try to re-acquire a capability that we
the sync requestor already holds.
2. to provide a way to suspend and later resume the sync request if
there is already a sync pending.
When synchronizing without holding a capability we needn't worry about
consideration (1) at all.
(2) is slightly trickier and may happen, for instance, when a capability
requests a minor collection and shortly thereafter the non-moving mark
thread requests a post-mark synchronization. In this case we need to
ensure that the non-moving mark thread suspends his request until after
the minor GC has concluded to avoid dead-locking. For this we introduce
a condition variable, `sync_finished_cond`, which a
non-capability-bearing requestor will wait on and which is signalled
after a synchronization or GC has finished.
>---------------------------------------------------------------
3ce969bb518ad5f056be18756d93d1472f372b61
rts/Schedule.c | 90 +++++++++++++++++++++++++++++++++++++++++++---------------
rts/Schedule.h | 6 ++++
2 files changed, 73 insertions(+), 23 deletions(-)
diff --git a/rts/Schedule.c b/rts/Schedule.c
index 02055d2..683871c 100644
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -110,6 +110,19 @@ Mutex sched_mutex;
#define FORKPROCESS_PRIMOP_SUPPORTED
#endif
+/*
+ * sync_finished_cond allows threads which do not own any capability (e.g. the
+ * concurrent mark thread) to participate in the sync protocol. In particular,
+ * if such a thread requests a sync while sync is already in progress it will
+ * block on sync_finished_cond, which will be signalled when the sync is
+ * finished (by releaseAllCapabilities).
+ */
+#if defined(THREADED_RTS)
+static Condition sync_finished_cond;
+static Mutex sync_finished_mutex;
+#endif
+
+
/* -----------------------------------------------------------------------------
* static function prototypes
* -------------------------------------------------------------------------- */
@@ -130,7 +143,6 @@ static void scheduleYield (Capability **pcap, Task *task);
static bool requestSync (Capability **pcap, Task *task,
PendingSync *sync_type, SyncType *prev_sync_type);
static void acquireAllCapabilities(Capability *cap, Task *task);
-static void releaseAllCapabilities(uint32_t n, Capability *cap, Task *task);
static void startWorkerTasks (uint32_t from USED_IF_THREADS,
uint32_t to USED_IF_THREADS);
#endif
@@ -1368,17 +1380,24 @@ scheduleNeedHeapProfile( bool ready_to_gc )
* change to the system, such as altering the number of capabilities, or
* forking.
*
+ * pCap may be NULL in the event that the caller doesn't yet own a capability.
+ *
* To resume after stopAllCapabilities(), use releaseAllCapabilities().
* -------------------------------------------------------------------------- */
#if defined(THREADED_RTS)
-static void stopAllCapabilities (Capability **pCap, Task *task)
+void stopAllCapabilities (Capability **pCap, Task *task)
+{
+ stopAllCapabilitiesWith(pCap, task, SYNC_OTHER);
+}
+
+void stopAllCapabilitiesWith (Capability **pCap, Task *task, SyncType sync_type)
{
bool was_syncing;
SyncType prev_sync_type;
PendingSync sync = {
- .type = SYNC_OTHER,
+ .type = sync_type,
.idle = NULL,
.task = task
};
@@ -1387,9 +1406,10 @@ static void stopAllCapabilities (Capability **pCap, Task *task)
was_syncing = requestSync(pCap, task, &sync, &prev_sync_type);
} while (was_syncing);
- acquireAllCapabilities(*pCap,task);
+ acquireAllCapabilities(pCap ? *pCap : NULL, task);
pending_sync = 0;
+ signalCondition(&sync_finished_cond);
}
#endif
@@ -1400,6 +1420,16 @@ static void stopAllCapabilities (Capability **pCap, Task *task)
* directly, instead use stopAllCapabilities(). This is used by the GC, which
* has some special synchronisation requirements.
*
+ * Note that this can be called in two ways:
+ *
+ * - where *pcap points to a capability owned by the caller: in this case
+ * *prev_sync_type will reflect the in-progress sync type on return, if one
+ * *was found
+ *
+ * - where pcap == NULL: in this case the caller doesn't hold a capability.
+ * we only return whether or not a pending sync was found and prev_sync_type
+ * is unchanged.
+ *
* Returns:
* false if we successfully got a sync
* true if there was another sync request in progress,
@@ -1424,13 +1454,25 @@ static bool requestSync (
// After the sync is completed, we cannot read that struct any
// more because it has been freed.
*prev_sync_type = sync->type;
- do {
- debugTrace(DEBUG_sched, "someone else is trying to sync (%d)...",
- sync->type);
- ASSERT(*pcap);
- yieldCapability(pcap,task,true);
- sync = pending_sync;
- } while (sync != NULL);
+ if (pcap == NULL) {
+ // The caller does not hold a capability (e.g. may be a concurrent
+ // mark thread). Consequently we must wait until the pending sync is
+ // finished before proceeding to ensure we don't loop.
+ // TODO: Don't busy-wait
+ ACQUIRE_LOCK(&sync_finished_mutex);
+ while (pending_sync) {
+ waitCondition(&sync_finished_cond, &sync_finished_mutex);
+ }
+ RELEASE_LOCK(&sync_finished_mutex);
+ } else {
+ do {
+ debugTrace(DEBUG_sched, "someone else is trying to sync (%d)...",
+ sync->type);
+ ASSERT(*pcap);
+ yieldCapability(pcap,task,true);
+ sync = pending_sync;
+ } while (sync != NULL);
+ }
// NOTE: task->cap might have changed now
return true;
@@ -1445,9 +1487,9 @@ static bool requestSync (
/* -----------------------------------------------------------------------------
* acquireAllCapabilities()
*
- * Grab all the capabilities except the one we already hold. Used
- * when synchronising before a single-threaded GC (SYNC_SEQ_GC), and
- * before a fork (SYNC_OTHER).
+ * Grab all the capabilities except the one we already hold (cap may be NULL is
+ * the caller does not currently hold a capability). Used when synchronising
+ * before a single-threaded GC (SYNC_SEQ_GC), and before a fork (SYNC_OTHER).
*
* Only call this after requestSync(), otherwise a deadlock might
* ensue if another thread is trying to synchronise.
@@ -1477,29 +1519,30 @@ static void acquireAllCapabilities(Capability *cap, Task *task)
}
}
}
- task->cap = cap;
+ task->cap = cap == NULL ? tmpcap : cap;
}
#endif
/* -----------------------------------------------------------------------------
- * releaseAllcapabilities()
+ * releaseAllCapabilities()
*
- * Assuming this thread holds all the capabilities, release them all except for
- * the one passed in as cap.
+ * Assuming this thread holds all the capabilities, release them all (except for
+ * the one passed in as keep_cap, if non-NULL).
* -------------------------------------------------------------------------- */
#if defined(THREADED_RTS)
-static void releaseAllCapabilities(uint32_t n, Capability *cap, Task *task)
+void releaseAllCapabilities(uint32_t n, Capability *keep_cap, Task *task)
{
uint32_t i;
for (i = 0; i < n; i++) {
- if (cap->no != i) {
- task->cap = capabilities[i];
- releaseCapability(capabilities[i]);
+ Capability *tmpcap = capabilities[i];
+ if (keep_cap != tmpcap) {
+ task->cap = tmpcap;
+ releaseCapability(tmpcap);
}
}
- task->cap = cap;
+ task->cap = keep_cap;
}
#endif
@@ -1801,6 +1844,7 @@ delete_threads_and_gc:
// reset pending_sync *before* GC, so that when the GC threads
// emerge they don't immediately re-enter the GC.
pending_sync = 0;
+ signalCondition(&sync_finished_cond);
GarbageCollect(collect_gen, heap_census, gc_type, cap, idle_cap);
#else
GarbageCollect(collect_gen, heap_census, 0, cap, NULL);
diff --git a/rts/Schedule.h b/rts/Schedule.h
index 66cf839..a477229 100644
--- a/rts/Schedule.h
+++ b/rts/Schedule.h
@@ -49,6 +49,12 @@ StgWord findRetryFrameHelper (Capability *cap, StgTSO *tso);
/* Entry point for a new worker */
void scheduleWorker (Capability *cap, Task *task);
+#if defined(THREADED_RTS)
+void stopAllCapabilitiesWith (Capability **pCap, Task *task, SyncType sync_type);
+void stopAllCapabilities (Capability **pCap, Task *task);
+void releaseAllCapabilities(uint32_t n, Capability *keep_cap, Task *task);
+#endif
+
/* The state of the scheduler. This is used to control the sequence
* of events during shutdown. See Note [shutdown] in Schedule.c.
*/
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