linux-kernel - Re: [PATCH 1/7] x86/fpu: Simplify the fpu->last_cpu logic and rename it to fpu->fpregs

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Message-ID: <20170126150955.GA12274@gmail.com>
Date:   Thu, 26 Jan 2017 16:09:55 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Rik van Riel <riel@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 1/7] x86/fpu: Simplify the fpu->last_cpu logic and rename
 it to fpu->fpregs_cached


* Rik van Riel <riel@...hat.com> wrote:

> On Thu, 2017-01-26 at 12:26 +0100, Ingo Molnar wrote:
> 
> > index c56fb57f2991..7eb2f3041fde 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1253,6 +1253,8 @@ void set_task_cpu(struct task_struct *p,
> > unsigned int new_cpu)
> >  			p->sched_class->migrate_task_rq(p);
> >  		p->se.nr_migrations++;
> >  		perf_event_task_migrate(p);
> > +
> > +		arch_task_migrate(p);
> >  	}
> > 
> 
> Does it really count as a "simplification" if you add a
> scheduler callback?
> 
> This code does not seem any easier to understand than
> the old code...

See the extra commit I added on top:

  7deff4369276 x86/fpu: Unify the naming of the FPU register cache validity flags

which makes it clearer, we now have:
	
	->fpregs_owner             [bool]
          fpregs_owner_ctx         [ptr]

That are set to 1 and the context pointer when a task with no FPU state is 
scheduled in and where the state of the previous task is preserved (cached) in the 
FPU registers - and which FPU register state cache can be invalidated after this 
by clearing any of the two flags.

That should make its overall meaning clearer, in that they represent a single 
'cache' where the cache validity flag is split into two copies, where any of which 
can be used to invalidate the cache.

Thanks,

	Ingo