linux-kernel - Re: Use case for TASKS

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Message-ID: <20170519102331.0d5a8536@gandalf.local.home>
Date:   Fri, 19 May 2017 10:23:31 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Use case for TASKS_RCU

On Fri, 19 May 2017 10:04:21 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 19 May 2017 06:35:50 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > Simpler would be better!
> > 
> > However, is it really guaranteed that one SCHED_IDLE thread cannot
> > preempt another?  If not, then the trampoline-freeing SCHED_IDLE thread
> > might preempt some other SCHED_IDLE thread in the middle of a trampoline.
> > I am not seeing anything that prevents such preemption, but it is rather
> > early local time, so I could easily be missing something.
> > 
> > However, if SCHED_IDLE threads cannot preempt other threads, even other
> > SCHED_IDLE threads, then your approach sounds quite promising to me.
> > 
> > Steve, Peter, thoughts?  
> 
> SCHED_IDLE is the swapper task. There's one on each CPU, and they don't
> migrate. And they only get called when there's no other task running.

Peter just "schooled" me on IRC. I stand corrected (and he may respond
to this email too). I guess any task can become SCHED_IDLE.

But that just makes this an even less likely option for
synchronize_rcu_tasks().

-- Steve