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

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Message-ID: <20170413084453.ad4falb7kc53viri@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Apr 2017 10:44:53 +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 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().

And because that function is a no-op for the idle thread, the idle
thread can do without calling that and therefore avoid the preemption
window.

You also need a few words about fake idle threads, search play_idle()
callers.

You could also make schedule_idle() more robust by adding a WARN for the
blk_schedule_flush_plug() condition.

You Changelog is still entirely long and rambling but fails to mention
the fundamental important stuff :-(