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

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Message-ID: <20151028050314.GC11242@worktop.amr.corp.intel.com>
Date:	Wed, 28 Oct 2015 06:03:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Turner <commonly@...il.com>
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 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.

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