linux-kernel - Re: [patch 0/4] timer/nohz: Fix timer/nohz woes

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Date:   Sat, 23 Dec 2017 17:21:20 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [patch 0/4] timer/nohz: Fix timer/nohz woes

On Fri, Dec 22, 2017 at 09:09:07AM -0800, Paul E. McKenney wrote:
> On Fri, Dec 22, 2017 at 03:51:11PM +0100, Thomas Gleixner wrote:
> > Paul was observing weird stalls which are hard to reproduce and decode. We
> > were finally able to reproduce and decode the wreckage on RT.
> > 
> > The following series addresses the issues and hopefully nails the root
> > cause completely.
> > 
> > Please review carefully and expose it to the dreaded rcu torture tests
> > which seem to be the only way to trigger it.
> 
> Best Christmas present ever, thank you!!!
> 
> Just started up three concurrent 10-hour runs of the infamous rcutorture
> TREE01 scenario, and will let you know how it goes!

Well, I messed up the first test and then reran it.  Which had the benefit
of giving me a baseline.  The rerun (with all four patches) produced
failures, so I ran it again with an additional patch of mine.  I score
these tests by recording the time at first failure, or, if there is no
failure, the duration of the test.  Summing the values gives the score.
And here are the scores, where 30 is a perfect score:

1.	Baseline: 3.0+2.5+10=15.5

2.	Four patches from Anna-Marie and Thomas: 10+2.7+1.7=14.4

3.	Ditto plus the patch below: 10+4.3+10=24.3

Please note that these are nowhere near anything even resembling
statistical significance.  However, they are encouraging.  I will do
more runs, but also do shorter five-hour runs to increase the amount
of data per unit time.  Please note also that my patch by itself never
did provide that great of an improvement, so there might be some sort
of combination effect going on here.  Or maybe it is just luck, who knows?

Please note that I have not yet ported my diagnostic patches on top of
these, however, the stacks have the usual schedule_timeout() entries.
This is not too surprising from a software-engineering viewpoint:
Locating several bugs at a given point of time usually indicates that
there are more to be found.  So in a sense we are lucky that the
same test triggers at least one of those additional bugs.

							Thanx, Paul

------------------------------------------------------------------------

commit accb0edb85526a05b934eac49658d05ea0216fc4
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Thu Dec 7 13:18:44 2017 -0800

    timers: Ensure that timer_base ->clk accounts for time offline

    The timer_base ->must_forward_clk is set to indicate that the next timer
    operation on that timer_base must check for passage of time.  One instance
    of time passage is when the timer wheel goes idle, and another is when
    the corresponding CPU is offline.  Note that it is not appropriate to set
    ->is_idle because that could result in IPIing an offline CPU.  Therefore,
    this commit instead sets ->must_forward_clk at CPU-offline time.

    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index ffebcf878fba..94cce780c574 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1875,6 +1875,7 @@ int timers_dead_cpu(unsigned int cpu)

 		BUG_ON(old_base->running_timer);

+		old_base->must_forward_clk = true;
 		for (i = 0; i < WHEEL_SIZE; i++)
 			migrate_timer_list(new_base, old_base->vectors + i);