linux-kernel - Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu

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Message-ID: <20190805144755.GH28441@linux.ibm.com>
Date:   Mon, 5 Aug 2019 07:47:55 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rcu@...r.kernel.org, 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, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop()
 enable tick on all online CPUs

On Mon, Aug 05, 2019 at 10:07:36AM +0200, Peter Zijlstra wrote:
> On Sun, Aug 04, 2019 at 09:19:01PM -0700, Paul E. McKenney wrote:
> > On Sun, Aug 04, 2019 at 01:24:46PM -0700, Paul E. McKenney wrote:
> 
> > > For whatever it is worth, the things on my list include using 25 rounds
> > > of resched_cpu() on each CPU with ten-jiffy wait between each (instead of
> > > merely 10 rounds), using waitqueues or some such to actually force a
> > > meaningful context switch on the other CPUs, etc.
> 
> That really should not be needed. What are those other CPUs doing?

Excellent question.  It would be really nice to have a CPU-stopper stall
warning, wouldn't it?  But who knows?  Maybe I am the only one to have
run into this.  However, the comment in multi_cpu_stop() just before
the call to touch_nmi_watchdog() leads me to believe otherwise.  ;-)

> > Which appears to have reduced the bug rate by about a factor of two.
> > (But statistics and all that.)
> 
> Which is just weird..

Indeed.  Your point being?

> > I am now trying the same test, but with CONFIG_PREEMPT=y and without
> > quite so much hammering on the scheduler.  This is keying off Peter's
> > earlier mention of preemption.  If this turns out to be solid, perhaps
> > we outlaw CONFIG_PREEMPT=n && CONFIG_NO_HZ_FULL=y?
> 
> CONFIG_PREEMPT=n should work just fine, _something_ is off.

Thank you, that is what I needed to know.

							Thanx, Paul