Re: [tip:locking/core] sched/cputime: Fix invalid gtime in proc

[Ingo Molnar <mingo@kernel.org>]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Fri, Dec 04, 2015 at 03:53:13AM -0800, tip-bot for Hiroshi Shimamoto wrote:
> > Commit-ID:  2541117b0cf79977fa11a0d6e17d61010677bd7b
> > Gitweb:     http://git.kernel.org/tip/2541117b0cf79977fa11a0d6e17d61010677bd7b
> > Author:     Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> > AuthorDate: Thu, 19 Nov 2015 16:47:28 +0100
> > Committer:  Ingo Molnar <mingo@kernel.org>
> > CommitDate: Fri, 4 Dec 2015 10:18:49 +0100
> > 
> > sched/cputime: Fix invalid gtime in proc
> > 
> > /proc/stats shows invalid gtime when the thread is running in guest.
> > When vtime accounting is not enabled, we cannot get a valid delta.
> > The delta is calculated with now - tsk->vtime_snap, but tsk->vtime_snap
> > is only updated when vtime accounting is runtime enabled.
> > 
> > This patch makes task_gtime() just return gtime without computing the
> > buggy non-existing tickless delta when vtime accounting is not enabled.
> > 
> > Use context_tracking_is_enabled() to check if vtime is accounting on
> > some cpu, in which case only we need to check the tickless delta. This
> > way we fix the gtime value regression on machines not running nohz full.
> > 
> > The kernel config contains CONFIG_VIRT_CPU_ACCOUNTING_GEN=y and
> > CONFIG_NO_HZ_FULL_ALL=n and boot without nohz_full.
> > 
> > I ran and stop a busy loop in VM and see the gtime in host.
> > Dump the 43rd field which shows the gtime in every second:
> > 
> > 	 # while :; do awk '{print $3" "$43}' /proc/3955/task/4014/stat; sleep 1; done
> > 	S 4348
> > 	R 7064566
> > 	R 7064766
> > 	R 7064967
> > 	R 7065168
> > 	S 4759
> > 	S 4759
> > 
> > During running busy loop, it returns large value.
> > 
> > After applying this patch, we can see right gtime.
> > 
> > 	 # while :; do awk '{print $3" "$43}' /proc/10913/task/10956/stat; sleep 1; done
> > 	S 5338
> > 	R 5365
> > 	R 5465
> > 	R 5566
> > 	R 5666
> > 	S 5726
> > 	S 5726
> > 
> > Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Chris Metcalf <cmetcalf@ezchip.com>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Luiz Capitulino <lcapitulino@redhat.com>
> > Cc: Mike Galbraith <efault@gmx.de>
> > Cc: Paul E . McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Link: http://lkml.kernel.org/r/1447948054-28668-2-git-send-email-fweisbec@gmail.com
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > ---
> 
> Thanks for applying the patchset!
> 
> However we may want to backport this one, it's a regression fix affecting
> CONFIG_NO_HZ_FULL=y with nohz_full off (99% of many distros defaults).
> 
> Thanks.

Absolutely, not sure why I forgot that tag - I added it for other recent fixes.

Greg, please consider applying this upstream fix to -stable as well:

  2541117b0cf7 ("sched/cputime: Fix invalid gtime in proc")

Note, the commit will cherry-pick cleanly all the way back to v3.9 (released 2.5+ 
years ago), but it will only build on v3.14 and later kernels.

Thanks,

	Ingo