`On Sat, Dec 23, 2017 at 05:21:20PM -0800, Paul E. McKenney wrote:> 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:Sigh.  They were five-hour tests, not ten-hour tests.  1.	Baseline: 3.0+2.5+5=10.52.	Four patches from Anna-Marie and Thomas: 5+2.7+1.7=9.43.	Ditto plus the patch below: 5+4.3+5=14.3Oh, and the reason for my suspecting that #2 is actually an improvementover #1 is that my patch by itself produced a very small improvement inreliability.  This leads to the hypothesis that #2 really is helping outin some way or another.							Thanx, Paul> 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@linux.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@linux.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);> `