runtime: split mprof locks

The profiles for memory allocations, sync.Mutex contention, and general
blocking store their data in a shared hash table. The bookkeeping work
at the end of a garbage collection cycle involves maintenance on each
memory allocation record. Previously, a single lock guarded access to
the hash table and the contents of all records. When a program has
allocated memory at a large number of unique call stacks, the
maintenance following every garbage collection can hold that lock for
several milliseconds. That can prevent progress on all other goroutines
by delaying acquirep's call to mcache.prepareForSweep, which needs the
lock in mProf_Free to report when a profiled allocation is no longer in
use. With no user goroutines making progress, it is in effect a
multi-millisecond GC-related stop-the-world pause.

Split the lock so the call to mProf_Flush no longer delays each P's call
to mProf_Free: mProf_Free uses a lock on the memory records' N+1 cycle,
and mProf_Flush uses locks on the memory records' accumulator and their
N cycle. mProf_Malloc also no longer competes with mProf_Flush, as it
uses a lock on the memory records' N+2 cycle. The profiles for
sync.Mutex contention and general blocking now share a separate lock,
and another lock guards insertions to the shared hash table (uncommon in
the steady-state). Consumers of each type of profile take the matching
accumulator lock, so will observe consistent count and magnitude values
for each record.

For #45894

Change-Id: I615ff80618d10e71025423daa64b0b7f9dc57daa
Reviewed-on: https://go-review.googlesource.com/c/go/+/399956
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Rhys Hiltner <rhys@justin.tv>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index f65be2bc74..14bf9a583f 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -413,7 +413,12 @@ func mallocinit() {
 	mheap_.init()
 	mcache0 = allocmcache()
 	lockInit(&gcBitsArenas.lock, lockRankGcBitsArenas)
-	lockInit(&proflock, lockRankProf)
+	lockInit(&profInsertLock, lockRankProfInsert)
+	lockInit(&profBlockLock, lockRankProfBlock)
+	lockInit(&profMemActiveLock, lockRankProfMemActive)
+	for i := range profMemFutureLock {
+		lockInit(&profMemFutureLock[i], lockRankProfMemFuture)
+	}
 	lockInit(&globalAlloc.mutex, lockRankGlobalAlloc)
 
 	// Create initial arena growth hints.