linux-kernel - Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

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Date:	Thu, 14 Aug 2014 18:12:47 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sanjay Rao <srao@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with
	seqlock

On 08/14, Rik van Riel wrote:
>
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On 08/14/2014 10:24 AM, Oleg Nesterov wrote:
> > On 08/13, Rik van Riel wrote:
> >>
> >> @@ -862,11 +862,9 @@ void do_sys_times(struct tms *tms) {
> >> cputime_t tgutime, tgstime, cutime, cstime;
> >>
> >> -	spin_lock_irq(&current->sighand->siglock);
> >> thread_group_cputime_adjusted(current, &tgutime, &tgstime);
> >> cutime = current->signal->cutime; cstime =
> >> current->signal->cstime; -
> >> spin_unlock_irq(&current->sighand->siglock);
> >
> > Ah, wait, there is another problem afaics...
>
> Last night I worked on another problem with this code.
>
> After propagating the stats from a dying task to the signal struct,
> we need to make sure that that task's stats are not counted twice.

Heh indeed ;) Can't understand how I missed that.

> This requires zeroing the stats under the write_seqlock, which was
> easy enough to add.

Or you can expand the scope of write_seqlock/write_sequnlock, so that
__unhash_process in called from inside the critical section. This looks
simpler at first glance.

Hmm, wait, it seems there is yet another problem ;) Afaics, you also
need to modify __exit_signal() so that ->sum_sched_runtime/etc are
accounted unconditionally, even if the group leader exits.

Probably this is not a big problem, and sys_times() or clock_gettime()
do not care at all because they use current.

But without this change thread_group_cputime(reaped_zombie) won't look
at this task_struct at all, this can lead to non-monotonic result if
it was previously called when this task was alive (non-reaped).

Oleg.

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