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authorWaiman Long <longman@redhat.com>2019-06-25 10:39:13 -0400
committerPeter Zijlstra <peterz@infradead.org>2019-08-06 12:49:15 +0200
commit91d2a812dfb98b3b4dad661529c33bc38d303461 (patch)
tree05cbc48c0e107931c0d39a373a714396d001db62 /kernel/locking
parent8c779229d0f4fe83ead90bdcbbf08b02989aa200 (diff)
locking/rwsem: Make handoff writer optimistically spin on owner
When the handoff bit is set by a writer, no other tasks other than the setting writer itself is allowed to acquire the lock. If the to-be-handoff'ed writer goes to sleep, there will be a wakeup latency period where the lock is free, but no one can acquire it. That is less than ideal. To reduce that latency, the handoff writer will now optimistically spin on the owner if it happens to be a on-cpu writer. It will spin until it releases the lock and the to-be-handoff'ed writer can then acquire the lock immediately without any delay. Of course, if the owner is not a on-cpu writer, the to-be-handoff'ed writer will have to sleep anyway. The optimistic spinning code is also modified to not stop spinning when the handoff bit is set. This will prevent an occasional setting of handoff bit from causing a bunch of optimistic spinners from entering into the wait queue causing significant reduction in throughput. On a 1-socket 22-core 44-thread Skylake system, the AIM7 shared_memory workload was run with 7000 users. The throughput (jobs/min) of the following kernels were as follows: 1) 5.2-rc6 - 8,092,486 2) 5.2-rc6 + tip's rwsem patches - 7,567,568 3) 5.2-rc6 + tip's rwsem patches + this patch - 7,954,545 Using perf-record(1), the %cpu time used by rwsem_down_write_slowpath(), rwsem_down_write_failed() and their callees for the 3 kernels were 1.70%, 5.46% and 2.08% respectively. Signed-off-by: Waiman Long <longman@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: x86@kernel.org Cc: Ingo Molnar <mingo@redhat.com> Cc: Will Deacon <will.deacon@arm.com> Cc: huang ying <huang.ying.caritas@gmail.com> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: "H. Peter Anvin" <hpa@zytor.com> Link: https://lkml.kernel.org/r/20190625143913.24154-1-longman@redhat.com
Diffstat (limited to 'kernel/locking')
-rw-r--r--kernel/locking/rwsem.c32
1 files changed, 26 insertions, 6 deletions
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index bd0f0d05724c..354238a08b7a 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -724,11 +724,12 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
rcu_read_lock();
for (;;) {
- if (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF) {
- state = OWNER_NONSPINNABLE;
- break;
- }
-
+ /*
+ * When a waiting writer set the handoff flag, it may spin
+ * on the owner as well. Once that writer acquires the lock,
+ * we can spin on it. So we don't need to quit even when the
+ * handoff bit is set.
+ */
new = rwsem_owner_flags(sem, &new_flags);
if ((new != owner) || (new_flags != flags)) {
state = rwsem_owner_state(new, new_flags, nonspinnable);
@@ -974,6 +975,13 @@ static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
{
return false;
}
+
+static inline int
+rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
+{
+ return 0;
+}
+#define OWNER_NULL 1
#endif
/*
@@ -1206,6 +1214,18 @@ wait:
raw_spin_unlock_irq(&sem->wait_lock);
+ /*
+ * After setting the handoff bit and failing to acquire
+ * the lock, attempt to spin on owner to accelerate lock
+ * transfer. If the previous owner is a on-cpu writer and it
+ * has just released the lock, OWNER_NULL will be returned.
+ * In this case, we attempt to acquire the lock again
+ * without sleeping.
+ */
+ if ((wstate == WRITER_HANDOFF) &&
+ (rwsem_spin_on_owner(sem, 0) == OWNER_NULL))
+ goto trylock_again;
+
/* Block until there are no active lockers. */
for (;;) {
if (signal_pending_state(state, current))
@@ -1240,7 +1260,7 @@ wait:
break;
}
}
-
+trylock_again:
raw_spin_lock_irq(&sem->wait_lock);
}
__set_current_state(TASK_RUNNING);