linux-kernel - Re: [RFC PATCH v2 2/3] restartable sequences: x86 ABI

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Date:	Tue, 27 Oct 2015 22:19:24 -0700
From:	Paul Turner <commonly@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andrew Hunter <ahh@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Dave Watson <davejwatson@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
	Josh Triplett <josh@...htriplett.org>,
	Ingo Molnar <mingo@...hat.com>, Chris Lameter <cl@...ux.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH v2 2/3] restartable sequences: x86 ABI

On Tue, Oct 27, 2015 at 10:03 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Oct 27, 2015 at 04:57:05PM -0700, Paul Turner wrote:
> > +static void rseq_sched_out(struct preempt_notifier *pn,
> > +                        struct task_struct *next)
> > +{
> > +     set_thread_flag(TIF_NOTIFY_RESUME);
> > +}
> >
> >  static __read_mostly struct preempt_ops rseq_preempt_ops = {
> >       .sched_in = rseq_sched_in_nop,
> > -     .sched_out = rseq_sched_out_nop,
> > +     .sched_out = rseq_sched_out,
> >  };
>
> Since we're unconditionally setting this TIF flag for these tasks, can't
> we introduce something similar to the (contested) TIF_NOHZ_FULL thing
> which is kept on the task indefinitely.
>
So Andy and I talked about this also, I'm in favor, in particular this
has two nice effects:
 a) In exit_to_usermode_loop() we can ensure that this is evaluated
prior to _TIF_SIGPENDING.  This removes the current requirement that
we also validate this state in setup_rt_frame() [which can perturb
this state prior to our existing notifier].
 b) We avoid spurious interactions with other things that use notify resume.

> That avoids having the preempt notifiers and this atomic op in the
> schedule path.


So we still want something there (although it can be definitely be
inlined as opposed to a preempt_notifier) since this allows us to only
evaluate this check on returns to user-space that might matter as
opposed to every syscall.
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