Re: [BUG nohz]: wrong user and system time accounting

From	Frederic Weisbecker <fweisbec@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [BUG nohz]: wrong user and system time accounting
Date	2017-04-03 17:30 +0200
Message-ID	<tsc5B-cE-45@gated-at.bofh.it> (permalink)
References	(5 earlier) <tqJfk-7FK-17@gated-at.bofh.it> <tqPNL-3S3-5@gated-at.bofh.it> <trb1T-QL-3@gated-at.bofh.it> <tre9r-2KJ-3@gated-at.bofh.it> <trhK1-5pA-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Fri, Mar 31, 2017 at 11:11:19PM -0400, Luiz Capitulino wrote:
> On Sat, 1 Apr 2017 01:24:54 +0200
> Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > On Fri, Mar 31, 2017 at 04:09:10PM -0400, Luiz Capitulino wrote:
> > > On Thu, 30 Mar 2017 17:25:46 -0400
> > > Luiz Capitulino <lcapitulino@redhat.com> wrote:
> > >   
> > > > On Thu, 30 Mar 2017 16:18:17 +0200
> > > > Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > > >   
> > > > > On Thu, Mar 30, 2017 at 09:59:54PM +0800, Wanpeng Li wrote:    
> > > > > > 2017-03-30 21:38 GMT+08:00 Frederic Weisbecker <fweisbec@gmail.com>:      
> > > > > > > If it works, we may want to take that solution, likely less performance sensitive
> > > > > > > than using sched_clock(). In fact sched_clock() is fast, especially as we require it to
> > > > > > > be stable for nohz_full, but using it involves costly conversion back and forth to jiffies.      
> > > > > > 
> > > > > > So both Rik and you agree with the skew tick solution, I will try it
> > > > > > tomorrow. Btw, if we should just add random offset to the cpu in the
> > > > > > nohz_full mode or add random offset to all cpus like the codes above?      
> > > > > 
> > > > > Lets just keep it to all CPUs for simplicty.
> > > > > Also please add a comment that explains why we need that skew_tick on nohz_full.    
> > > > 
> > > > I've tried all the test-cases we discussed in this thread with skew_tick=1
> > > > and it worked as expected in bare-metal and KVM guests.
> > > > 
> > > > However, I found a test-case that works in bare-metal but show problems
> > > > in KVM guests. It could something that's KVM specific, or it could be
> > > > something that's harder to reproduce in bare-metal.  
> > > 
> > > After discussing some findings on this issue with Rik, I realized that
> > > we don't add the skew when restarting the tick in tick_nohz_restart().
> > > Adding the offset there seems to solve this problem.  
> > 
> > Are you sure? tick_nohz_restart() doesn't seem to override the initial skew. It
> > always forwards the expiration time on top of the last tick.
> 
> OK, I'll double check. Without my change the bug triggers almost
> instantly with the described reproducer. With my change it didn't
> trig for several minutes (but it does look wrong looking at it now).

Do you observe aligned ticks with trace events (hrtimer_expire_entry)?

You might want to enforce the global clock to trace that:

    echo "global" > /sys/kernel/debug/tracing/trace_clock

Thread

Re: [BUG nohz]: wrong user and system time accounting Rik van Riel <riel@redhat.com> - 2017-03-27 19:40 +0200
  Re: [BUG nohz]: wrong user and system time accounting Wanpeng Li <kernellwp@gmail.com> - 2017-03-28 09:30 +0200
  Re: [BUG nohz]: wrong user and system time accounting Rik van Riel <riel@redhat.com> - 2017-03-28 23:10 +0200
    Re: [BUG nohz]: wrong user and system time accounting Luiz Capitulino <lcapitulino@redhat.com> - 2017-03-28 23:30 +0200
      Re: [BUG nohz]: wrong user and system time accounting Wanpeng Li <kernellwp@gmail.com> - 2017-03-29 12:00 +0200
        Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-03-29 15:00 +0200
  Re: [BUG nohz]: wrong user and system time accounting Rik van Riel <riel@redhat.com> - 2017-03-28 23:30 +0200
    Re: [BUG nohz]: wrong user and system time accounting Luiz Capitulino <lcapitulino@redhat.com> - 2017-03-28 23:40 +0200
  Re: [BUG nohz]: wrong user and system time accounting Rik van Riel <riel@redhat.com> - 2017-03-29 22:20 +0200
    Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-03-30 01:00 +0200
      Re: [BUG nohz]: wrong user and system time accounting Rik van Riel <riel@redhat.com> - 2017-03-30 15:00 +0200
    Re: [BUG nohz]: wrong user and system time accounting Wanpeng Li <kernellwp@gmail.com> - 2017-03-30 04:00 +0200
      Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-03-30 14:50 +0200
        Re: [BUG nohz]: wrong user and system time accounting Mike Galbraith <efault@gmx.de> - 2017-03-30 15:20 +0200
    Re: [BUG nohz]: wrong user and system time accounting Mike Galbraith <efault@gmx.de> - 2017-03-30 06:30 +0200
      Re: [BUG nohz]: wrong user and system time accounting Wanpeng Li <kernellwp@gmail.com> - 2017-03-30 08:50 +0200
        Re: [BUG nohz]: wrong user and system time accounting Wanpeng Li <kernellwp@gmail.com> - 2017-03-30 14:00 +0200
          Re: [BUG nohz]: wrong user and system time accounting Mike Galbraith <efault@gmx.de> - 2017-03-30 14:40 +0200
        Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-03-30 15:40 +0200
          Re: [BUG nohz]: wrong user and system time accounting Wanpeng Li <kernellwp@gmail.com> - 2017-03-30 16:10 +0200
            Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-03-30 16:30 +0200
              Re: [BUG nohz]: wrong user and system time accounting Luiz Capitulino <lcapitulino@redhat.com> - 2017-03-30 23:30 +0200
                Re: [BUG nohz]: wrong user and system time accounting Luiz Capitulino <lcapitulino@redhat.com> - 2017-03-31 22:10 +0200
                Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-04-01 01:30 +0200
                Re: [BUG nohz]: wrong user and system time accounting Luiz Capitulino <lcapitulino@redhat.com> - 2017-04-01 05:20 +0200
                Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-04-03 17:30 +0200
                Re: [BUG nohz]: wrong user and system time accounting Luiz Capitulino <lcapitulino@redhat.com> - 2017-04-03 21:10 +0200
                Re: [BUG nohz]: wrong user and system time accounting Luiz Capitulino <lcapitulino@redhat.com> - 2017-04-04 20:10 +0200
                Re: [BUG nohz]: wrong user and system time accounting Rik van Riel <riel@redhat.com> - 2017-04-05 16:30 +0200
      Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-03-30 15:00 +0200
        Re: [BUG nohz]: wrong user and system time accounting Rik van Riel <riel@redhat.com> - 2017-03-30 15:10 +0200
          Re: [BUG nohz]: wrong user and system time accounting Mike Galbraith <efault@gmx.de> - 2017-03-30 15:40 +0200
            Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-04-03 16:50 +0200
              Re: [BUG nohz]: wrong user and system time accounting Mike Galbraith <efault@gmx.de> - 2017-04-04 09:40 +0200
          Re: [BUG nohz]: wrong user and system time accounting Frederic Weisbecker <fweisbec@gmail.com> - 2017-03-30 15:50 +0200
  Re: [BUG nohz]: wrong user and system time accounting Wanpeng Li <kernellwp@gmail.com> - 2017-03-30 00:50 +0200
    Re: [BUG nohz]: wrong user and system time accounting Luiz Capitulino <lcapitulino@redhat.com> - 2017-03-30 04:20 +0200
      Re: [BUG nohz]: wrong user and system time accounting Wanpeng Li <kernellwp@gmail.com> - 2017-03-30 14:30 +0200

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