linux-kernel - Re: [RFC][PATCH] sched: Have do_idle() call _

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Message-ID: <20170412162157.ukmhksytgrc2qx2e@hirez.programming.kicks-ass.net>
Date:   Wed, 12 Apr 2017 18:21:57 +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>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH] sched: Have do_idle() call __schedule() without
 enabling preemption

On Wed, Apr 12, 2017 at 10:41:24AM -0400, Steven Rostedt wrote:
> [ tl;dr; version ]
> 
> Peter, In order to have synchronize_rcu_tasks() work, the idle task can
> never be preempted. There's a very small window in
> schedule_preempt_disable() that enables preemption, and when this
> happens, it breaks synchronize_rcu_tasks() (see above email for
> details).
> 
> Is there any reason to enable preemption, or can we simply have idle
> call into schedule without ever allowing it to be preempted, as in my
> patch?

Dunno,.. this changelog should convince me, not make me do the work :-)

> Note, it is almost good enough to just change
> schedule_preempt_disable() to do the exact same thing, but there's one
> instance in kernel/locking/mutex.c that calls it in a non running state.

The point being that that must not happen because of
sched_submit_work(), which, for idle, should not matter.