linux-kernel - Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS

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Message-ID: <CAKfTPtCXuHzoFMsL7CwKzkqi0sDbhaOTKjdqPrZfdkC_7g=jkg@mail.gmail.com>
Date:   Fri, 22 Dec 2017 09:04:35 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK

On 22 December 2017 at 08:56, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Dec 21, 2017 at 05:23:27PM +0100, Vincent Guittot wrote:
>> Hi Peter,
>>
>> I think that part of the proposal is missing.
>>
>> One goal of the patchset was to kick an update of the stats of idle
>> cpu when a task wake up on a cpu but the statistic has not been
>> updated for a while.
>>
>> That's why there where a call to nohz_kick_needed in the proposal to
>> kick ilb but only for updating blocked load and not a full idle load
>> balance
>>
>> I can't find this call any more in your patchset
>
> Yeah, I took it out because it didn't make sense to me. If you're waking
> to a stale CPU, you're too late.

I agree that's it's too late for the current wake up but that's the
trade off with delaying the wake up until all blocked load has been
updated.

>