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

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Message-ID: <20160805081702.GA21369@leoy-linaro>
Date:	Fri, 5 Aug 2016 16:17:02 +0800
From:	Leo Yan <leo.yan@...aro.org>
To:	Vincent Guittot <vincent.guittot@...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

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?

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