Re: [PATCH-tip] locking/rwsem: Make handoff writer optimistically spin on owner

[Peter Zijlstra]

On Tue, Jun 25, 2019 at 10:39:13AM -0400, Waiman Long wrote:
> 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@xxxxxxxxxx>

Thanks.. sorry for taking so long.