linux-kernel - Re: Use case for TASKS

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Date:   Fri, 19 May 2017 12:06:09 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, May 19, 2017 at 10:23:31AM -0400, Steven Rostedt wrote:
> 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().

Hmmm...  The goal is to make sure that any task that was preempted or
running at a given point in time passes through a voluntary context switch
(or userspace execution, or, ...).

What is the simplest way to get this job done?  To Ingo's point, I bet
that there is a simpler way than the current TASKS_RCU implementation.

Ingo, if I make it fit into 100 lines of code, would you be OK with it?
I probably need a one-line hook at task-creation time and another
at task-exit time, if that makes a difference.

							Thanx, Paul