linux-kernel - Re: Use case for TASKS

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Message-ID: <20170523153939.7122e892@vmware.local.home>
Date:   Tue, 23 May 2017 15:39:39 -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 Mon, 22 May 2017 17:00:36 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> > 
> > 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.  
> 
> And please see below for such a patch, which does add (just barely)
> fewer than 100 lines net.
> 
> Unfortunately, it does not work, as I should have known ahead of time
> from the dyntick-idle experience.  Not all context switches go through
> context_switch().  :-/

Wait. What context switch doesn't go through a context switch? Or do
you mean a user/kernel context switch?

-- Steve

> 
> I believe this is fixable, more or less like dyntick-idle's
> half-interrupts were fixable, but it will likely be a few days.  Not
> clear whether the result will be simpler than current TASKS_RCU, but
> there is only one way to find out.  ;-)
>