linux-kernel - Re: [PATCH] sched/fair: refine maximum periods for decay

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Date:	Fri, 5 Aug 2016 10:31:32 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Leo Yan <leo.yan@...aro.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: refine maximum periods for decay_load

On 5 August 2016 at 10:17, Leo Yan <leo.yan@...aro.org> wrote:
> Hi Vincent,
>
> Thanks for review.
>
> On Fri, Aug 05, 2016 at 09:56:46AM +0200, Vincent Guittot wrote:
>> Hi Leo,
>>
>> On 5 August 2016 at 08:34, Leo Yan <leo.yan@...aro.org> wrote:
>> >
>> > In current code, decay_load() will consider to set load value to zero
>> > after passing 32*64 ms. So this means max_load * (0.5^64) ~= 0.
>> >
>> > Kernel can support maximum number of processes and threads to 2^29 and
>> > set task with highest priority with nice=-20 (weight = 88761). So in
>> > worst case, one CPU may have maximum load value is:
>> >
>> >     max_load = 2^29 * 88761 < 2^46
>> >
>> > In theory after pass 46 periods we can ensure load value will be decayed
>> > to zero. So this patch is to change maximum periods from 64 to 48.
>> >
>> > Signed-off-by: Leo Yan <leo.yan@...aro.org>
>> > ---
>> >  kernel/sched/fair.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> > index e342159..55cb134 100644
>> > --- a/kernel/sched/fair.c
>> > +++ b/kernel/sched/fair.c
>> > @@ -2622,7 +2622,7 @@ static __always_inline u64 decay_load(u64 val, u64 n)
>> >
>> >         if (!n)
>> >                 return val;
>> > -       else if (unlikely(n > LOAD_AVG_PERIOD * 63))
>> > +       else if (unlikely(n > LOAD_AVG_PERIOD * 47))
>>
>> In the equation above, you use 46 then you mentioned : "change maximum
>> periods from 64 to 48." and finally you use 47.
>
> Sorry introduce confusion. I want to align to 16 so choose maximum
> periods to 48, and due to condition is (n > LOAD_AVG_PERIOD * 47) so
> it equeals to (n >= LOAD_AVG_PERIOD * 48).
>
> Precisely I think the code should be:
>     else if (unlikely(n >= LOAD_AVG_PERIOD * 46))
>
> Do you think this is okay?

yes, this looks okay


>
>> >                 return 0;
>> >
>> >         /* after bounds checking we can collapse to 32-bit */
>> > --
>> > 1.9.1
>> >