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

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Date:	Mon, 6 Jul 2015 13:44:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Afzal Mohammed <afzal.mohd.ma@...il.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched,fair: Remove > u32 weight handling for delta

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).

> Remove the initial > u32 handling on 'fact'.
> 
> 9dbdb15553239 ("sched/fair: Rework sched_fair time accounting") in
> addition to fixing the original issue of time moving backwards elsewhere
> in the code, handled delta > u32 case (due to NO_HZ_FULL) which brought
> in as it's part the change changed here.

Because the unsigned long weight is a u64 on 64bit..

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.

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

> 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.
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