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

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Date:   Thu, 9 Feb 2017 21:28:39 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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


* Thomas Gleixner <tglx@...utronix.de> wrote:

> 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.

Indeed!

> 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.

Ok, I'm convinced!

Thanks,

	Ingo