linux-kernel - Re: [Question]: try to fix contention between expire_timers and try_to_del_timer

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Date:   Mon, 31 Jul 2017 14:13:21 +0100
From:   Will Deacon <will.deacon@....com>
To:     Vikram Mulukutla <markivx@...eaurora.org>
Cc:     qiaozhou <qiaozhou@...micro.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>, sboyd@...eaurora.org,
        LKML <linux-kernel@...r.kernel.org>,
        Wang Wilbur <wilburwang@...micro.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel-owner@...r.kernel.org, sudeep.holla@....com
Subject: Re: [Question]: try to fix contention between expire_timers and
 try_to_del_timer_sync

Hi Vikram,

On Fri, Jul 28, 2017 at 12:09:38PM -0700, Vikram Mulukutla wrote:
> On 2017-07-28 02:28, Will Deacon wrote:
> >On Thu, Jul 27, 2017 at 06:10:34PM -0700, Vikram Mulukutla wrote:
> 
> <snip>
> 
> >>
> >>I think we should have this discussion now - I brought this up earlier
> >>[1]
> >>and I promised a test case that I completely forgot about - but here it
> >>is (attached). Essentially a Big CPU in an acquire-check-release loop
> >>will have an unfair advantage over a little CPU concurrently attempting
> >>to acquire the same lock, in spite of the ticket implementation. If the
> >>Big
> >>CPU needs the little CPU to make forward progress : livelock.
> >>
> 
> <snip>
> 
> >>
> >>One solution was to use udelay(1) in such loops instead of cpu_relax(),
> >>but
> >>that's not very 'relaxing'. I'm not sure if there's something we could
> >>do
> >>within the ticket spin-lock implementation to deal with this.
> >
> >Does bodging cpu_relax to back-off to wfe after a while help? The event
> >stream will wake it up if nothing else does. Nasty patch below, but I'd be
> >interested to know whether or not it helps.
> >
> >Will
> >
> This does seem to help. Here's some data after 5 runs with and without the
> patch.

Blimey, that does seem to make a difference. Shame it's so ugly! Would you
be able to experiment with other values for CPU_RELAX_WFE_THRESHOLD? I had
it set to 10000 in the diff I posted, but that might be higher than optimal.
It would be interested to see if it correlates with num_possible_cpus()
for the highly contended case.

Will