linux-kernel - RE: [PATCH] cputime: fix invalid gtime

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Message-ID: <7F861DC0615E0C47A872E6F3C5FCDDBD05F3598A@BPXM14GP.gisp.nec.co.jp>
Date:	Thu, 17 Sep 2015 23:47:05 +0000
From:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: RE: [PATCH] cputime: fix invalid gtime

> Subject: Re: [PATCH] cputime: fix invalid gtime
> 
> On Thu, Sep 03, 2015 at 12:45:50AM +0000, Hiroshi Shimamoto wrote:
> > From: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
> >
> > /proc/stats shows invalid gtime when the thread is running in guest.
> 
> Why is this a problem?

In host, when I monitored cpu usage in guest I noticed that the cpu time
is not stable.

> 
> > When vtime accounting is not enabled, we cannot get a valid delta.
> > Just return gtime when vtime accounting is not enabled in task_gtime().
> 
> But isn't other stuff then also broken, like fetch_task_cputime(). Tell
> me more about why you think your patch is the right one.

No, because I think vtime_snap_whence keeps VTIME_SLEEPING until
vtime_accounting_enabled() returns true. Then no delta is added.

> 
> > Before
> > 10987 (qemu-kvm) S 1 10923 10923 0 -1 138428608 7521 0 90 0 3776 460 0 0 20 0 24 0 11960 8090398720 151288 18446744073709551615
> 1 1 0 0 0 0 2147220671 4096 25155 18446744073709551615 0 0 -1 9 0 0 0 3554 0 0 0 0 0 0 0 0 0
> 
> It would have been helpful if you'd used a small script to take out the
> right column. As is I've no clue which field to look at.

Sorry for inconvenience with little explanation.
I watched /proc/<pid>/stats.
When guest running busy, gtime looks big number.

10987 (qemu-kvm) S ... 3554 ...
10987 (qemu-kvm) R ... 21415 ...

and keep it busy. after 1 second.

10987 (qemu-kvm) R ... 21616 ...

few second later, I stopped busy program on guest.
The gtime back to sane value.

10987 (qemu-kvm) S ... 4084 ...


thanks,
Hiroshi

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