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

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Date:   Fri, 25 Aug 2017 13:25:42 -0700
From:   Vikram Mulukutla <markivx@...eaurora.org>
To:     Will Deacon <will.deacon@....com>
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

On 2017-08-25 12:48, Vikram Mulukutla wrote:

> 
> If I understand the code correctly, the upper 32 bits of an ARM64 
> virtual
> address will overflow when 1 is added to it, and so we'll keep WFE'ing 
> on
> every subsequent cpu_relax invoked from the same PC, until we cross the
> hard-coded threshold, right?
> 

Oops, misread that. Second time we enter cpu_relax from the same PC, we
do a WFE. Then we stop doing the WFE until we hit the threshold using 
the
per-cpu counter. So with a higher threshold, we wait for more 
cpu_relax()
calls before starting the WFE again.

So a lower threshold implies we should hit WFE branch sooner. It seems
that since my test keeps the while loop going for a full 5 seconds, a 
lower
threshold will obviously result in more WFEs and lower the 
lock-acquired-count.

I guess we want a high threshold but not so high that the little CPU has
to wait a while before the big CPU counts up to the threshold, is that 
correct?

Thanks,
Vikram

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