linux-kernel - Re: [PATCH] sched,fair: Remove > u32 weight handling for delta

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Date:	Mon, 6 Jul 2015 19:04:07 +0530
From:	Afzal Mohammed <afzal.mohd.ma@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched,fair: Remove > u32 weight handling for delta

Hi,

On Mon, Jul 06, 2015 at 01:44:30PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 06, 2015 at 08:14:37AM +0530, Afzal Mohammed wrote:

> > scaled down weight 'fact' would not be > u32 rather than unlikely as the
> > values being passed for delta is either NICE_O_LOAD or the weight of the
> > 'se' which would be a value that can be accomodated in a u32.
> 
> This needs a bit more on why se->load.weight must fit u32 (its true, but
> not evident from this text).

Okay, I will add an equivalent of the below to the log,

"se->load.weight can have either the values in prio_to_weight[] for
cases where 'se' is a task or capped to MAX_SHARES (1 << 18) when it
is a group. And these values can be accomodated in a u32.",

and send the patch, unless a negative opinion on the above.

> Now as long as we never call __calc_delta() on a rq weight -- which is a
> sum of weights and can indeed be larger than u32, we can indeed remove
> this.

My understanding is that we do not call __calc_delta() on rq weight.

> And I think we already assume such, see this story on why shift will
> remain positive.

ok

> > The hunk being removed here
> > would not make a difference to it as this is on scaled weight > u32.
> > And pre-"9dbdb15553239" doesn't seem to have logical equivalent of hunk
> > removed here either.
> 
> -ENOPARSE.

Reading 9dbdb15553239 ("sched/fair: Rework sched_fair time
accounting") again, realized that I am wrong on this, that was
referring to the below statement removed in that commit,

        if (likely(weight > (1UL << SCHED_LOAD_RESOLUTION)))
                tmp = (u64)delta_exec * scale_load_down(weight);

earlier came to a reasoning that as scale_load_down(weight) was not
separately typecasted, value above u32 would be discarded, that non
parsable statement meant that weight > u32 was not considered. Since
cast has precedence over multiply, that statement of mine was wrong.

Regards
Afzal
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