linux-kernel - Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce global impact

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Date:   Thu, 25 Aug 2016 19:14:07 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Om Dhyade <odhyade@...eaurora.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>,
        Colin Cross <ccross@...gle.com>, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce
 global impact

On Wed, Aug 24, 2016 at 2:30 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello, John.
>
> On Wed, Aug 24, 2016 at 02:16:52PM -0700, John Stultz wrote:
>> Hey Peter, Tejun, Oleg,
>>   So while you're tweaks for the percpu-rwsem have greatly helped the
>> regression folks were seeing (many thanks, by the way), as noted
>> above, the performance regression with the global lock compared to
>> earlier kernels is still ~3x slower (though again, much better then
>> the 80x slower that was seen earlier).
>>
>> So I was wondering if patches to go back to the per signal_struct
>> locking would still be considered? Or is the global lock approach the
>> only way forward?
>
> We can't simply revert but we can make the lock per signal_struct
> again.  It's just that it'd be quite a bit more complex (but, again,
> if we need it...) and for cases where migrations aren't as frequent
> percpu-rwsem would be at least a bit lower overhead.  Can you please
> test with the following patch applied just in case?
>
>  https://git.kernel.org/cgit/linux/kernel/git/tj/cgroup.git/commit/?h=for-4.8-fixes&id=568ac888215c7fb2fabe8ea739b00ec3c1f5d440


Hey! Good news. This patch along with Peter's locking changes pushes
the latencies down to an apparently acceptable level!

Many thanks for the pointer!

thanks
-john