linux-kernel - Re: [PATCH 05/10] sched/core: Remove the tsk_cpus

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Message-ID: <alpine.DEB.2.20.1702091243200.3604@nanos>
Date:   Thu, 9 Feb 2017 12:46:43 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 05/10] sched/core: Remove the tsk_cpus_allowed()
 wrapper

On Thu, 9 Feb 2017, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, Feb 08, 2017 at 07:34:18PM +0100, Ingo Molnar wrote:
> >
> > > So the original intention of tsk_cpus_allowed() was to 'future-proof' the 
> > > field - but it's pretty ineffectual at that, because half of the code uses 
> > > ->cpus_allowed directly ...
> > > 
> > > Also, the wrapper makes the code longer than the original expression!
> > 
> > I still object to taking this out without replacement.
> 
> Yeah, that would have been my next suggestion.
> 
> > This leaves RT stranded.
> 
> Well, no, it leaves -rt with slightly more patching work than it already has...
> 
> Because note how the wrappery is _already_ incomplete to a significant degree:
> 
>   triton:~/tip> git grep -Ee '->cpus_allowed' | grep -vE 'tsk_|cpuset|core.c' | wc -l
>   27
>   triton:~/tip> git grep tsk_cpus_allowed | wc -l
>   43
> 
> I.e. around 40% of the places that use ->cpus_allowed in the upstream kernel are 
> not properly wrapped. That fact already 'wrecks' -rt.

Nope it does not. The places which use cpumask directly are not interfering
with the decisions which are made by the scheduler whether migration can
happen or not. All decision code pathes use the wrapper and we make sure on
every update that this is the case.

I completely agree that your idea with the const *ptr is the better
solution, but without that replacement RT is stranded and left alone with
the mop up.

Thanks,

	tglx