linux-kernel - Re: [PATCH v2] sched: Have do_idle() call __schedule() without enabling preemption

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Message-ID: <20170413143807.gukvroojodipwuvg@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Apr 2017 16:38:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] sched: Have do_idle() call __schedule() without
 enabling preemption

On Thu, Apr 13, 2017 at 09:48:40AM -0400, Steven Rostedt wrote:
> On Thu, 13 Apr 2017 10:44:53 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, Apr 12, 2017 at 02:27:44PM -0400, Steven Rostedt wrote:
> > > + * schedule_idle() is similar to schedule_preempt_disable() except
> > > + * that it never enables preemption.  
> > 
> > That's not right. The primary distinction is that it doesn't call
> > sched_submit_work().
> 
> That has nothing to do with fixing synchronize_rcu_tasks(), which is
> the entire point of my patch, thus it is *not* the primary distinction.

It is. The only reason you _can_ avoid that preemption window is by not
doing sched_submit_work().

The moment you need to call that, the preemption window comes back.