linux-kernel - Re: [PATCH tip/core/rcu 02/15] rcu: Use timer as backstop for NOCB deferred wakeups

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Date:   Tue, 25 Jul 2017 12:18:14 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 02/15] rcu: Use timer as backstop for NOCB
 deferred wakeups

On Tue, Jul 25, 2017 at 02:12:20PM -0400, Steven Rostedt wrote:
> On Mon, 24 Jul 2017 14:44:31 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > The handling of RCU's no-CBs CPUs has a maintenance headache, namely
> > that if call_rcu() is invoked with interrupts disabled, the rcuo kthread
> > wakeup must be defered to a point where we can be sure that scheduler
> > locks are not held.  Of course, there are a lot of code paths leading
> > from an interrupts-disabled invocation of call_rcu(), and missing any
> > one of these can result in excessive callback-invocation latency, and
> > potentially even system hangs.
> 
> What about using irq_work? That's what perf and ftrace use for such a
> case.

I hadn't looked at irq_work before, thank you for the pointer!

I nevertheless believe that timers work better in this particular case
because they can be cancelled (which appears to be the common case), they
normally are not at all time-critical, and because running in softirq
is just fine -- no need to run out of the scheduling-clock interrupt.

Seem reasonable?

							Thanx, Paul

> -- Steve
> 
> > 
> > This commit therefore uses a timer to guarantee that the wakeup will
> > eventually occur.  If one of the deferred-wakeup points kicks in, then
> > the timer is simply cancelled.
> > 
> > This commit also fixes up an incomplete removal of commits that were
> > intended to plug remaining exit paths, which should have the added
> > benefit of reducing the overhead of RCU's context-switch hooks.  In
> > addition, it simplifies leader-to-follower callback-list handoff by
> > introducing locking.  The call_rcu()-to-leader handoff continues to
> > use atomic operations in order to maintain good real-time latency for
> > common-case use of call_rcu().
> > 
>