Messages in this thread | | | Date | Mon, 13 Jan 2020 16:48:29 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v3] locking/osq: Use optimized spinning loop for arm64 |
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On Mon, Jan 13, 2020 at 10:07:35AM -0500, Waiman Long wrote: > Arm64 has a more optimized spinning loop (atomic_cond_read_acquire) > using wfe for spinlock that can boost performance of sibling threads > by putting the current cpu to a wait state that is broken only when > the monitored variable changes or an external event happens. > > OSQ has a more complicated spinning loop. Besides the lock value, it > also checks for need_resched() and vcpu_is_preempted(). The check for > need_resched() is not a problem as it is only set by the tick interrupt > handler. That will be detected by the spinning cpu right after iret. > > The vcpu_is_preempted() check, however, is a problem as changes to the > preempt state of of previous node will not affect the wait state. For > ARM64, vcpu_is_preempted is not currently defined and so is a no-op. > Will has indicated that he is planning to para-virtualize wfe instead > of defining vcpu_is_preempted for PV support. So just add a comment in > arch/arm64/include/asm/spinlock.h to indicate that vcpu_is_preempted() > should not be defined as suggested. > > On a 2-socket 56-core 224-thread ARM64 system, a kernel mutex locking > microbenchmark was run for 10s with and without the patch. The > performance numbers before patch were: > > Running locktest with mutex [runtime = 10s, load = 1] > Threads = 224, Min/Mean/Max = 316/123,143/2,121,269 > Threads = 224, Total Rate = 2,757 kop/s; Percpu Rate = 12 kop/s > > After patch, the numbers were: > > Running locktest with mutex [runtime = 10s, load = 1] > Threads = 224, Min/Mean/Max = 334/147,836/1,304,787 > Threads = 224, Total Rate = 3,311 kop/s; Percpu Rate = 15 kop/s > > So there was about 20% performance improvement. > > Signed-off-by: Waiman Long <longman@redhat.com> > --- > arch/arm64/include/asm/spinlock.h | 9 +++++++++ > kernel/locking/osq_lock.c | 17 ++++------------- > 2 files changed, 13 insertions(+), 13 deletions(-)
Acked-by: Will Deacon <will@kernel.org>
Thanks,
Will
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