linux-kernel - Re: [PATCH 4/6 v5] sched: propagate load during synchronous attach/detach

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Message-ID: <CAKfTPtC-cLRLe5TNh3CtMcMzSa_6iG4XCSJJGm9RZny_2M_raw@mail.gmail.com>
Date:   Wed, 26 Oct 2016 14:31:01 +0200
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>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Yuyang Du <yuyang.du@...el.com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        "linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Wanpeng Li <kernellwp@...il.com>
Subject: Re: [PATCH 4/6 v5] sched: propagate load during synchronous attach/detach

On 26 October 2016 at 12:54, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Oct 17, 2016 at 11:14:11AM +0200, Vincent Guittot wrote:
>>  /*
>> + * Signed add and clamp on underflow.
>> + *
>> + * Explicitly do a load-store to ensure the intermediate value never hits
>> + * memory. This allows lockless observations without ever seeing the negative
>> + * values.
>> + */
>> +#define add_positive(_ptr, _val) do {                                \
>> +     typeof(_ptr) ptr = (_ptr);                              \
>> +     typeof(_val) res, val = (_val);                         \
>> +     typeof(*ptr) var = READ_ONCE(*ptr);                     \
>> +     res = var + val;                                        \
>> +     if (res < 0)                                            \
>> +             res = 0;                                        \
>
> I think this is broken, and inconsistent with sub_positive().

I agree that the behavior is different from sub_positive which deals
with unsigned value, but i was not able to come with a short name that
highlight this signed/unsigned difference

>
> The thing is, util_avg, on which you use this, is an unsigned type.

The delta that is added to util_avg, is a signed value

> Checking for unsigned underflow can be done by comparing against either
> one of the terms.
>
>> +     WRITE_ONCE(*ptr, res);                                  \
>> +} while (0)
>
>> +     add_positive(&cfs_rq->avg.util_avg, delta);