linux-kernel - Re: [patch v2 4/4] taskstats: Export "cdata

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Message-ID: <20101201185128.GA7656@redhat.com>
Date:	Wed, 1 Dec 2010 19:51:28 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
Cc:	Shailabh Nagar <nagar1234@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	John stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Roland McGrath <roland@...hat.com>, Valdis.Kletnieks@...edu,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [patch v2 4/4] taskstats: Export "cdata_wait" CPU times with
	taskstats

On 11/29, Michael Holzheu wrote:
>
> * I left the struct signal locking in the patch, because I think
>   that in order to get consistent data this is necessary.

OK, I guess you mean that we want to read utime/stime "atomically",
and thus we need ->siglock to prevent the race with __exit_signal().

> @@ -30,6 +30,7 @@ void bacct_add_tsk(struct taskstats *sta
>  {
>  	const struct cred *tcred;
>  	struct timespec uptime, ts;
> +	unsigned long flags;
>  	u64 ac_etime;
>
>  	BUILD_BUG_ON(TS_COMM_LEN < TASK_COMM_LEN);
> @@ -71,6 +72,12 @@ void bacct_add_tsk(struct taskstats *sta
>  	stats->ac_majflt = tsk->maj_flt;
>
>  	strncpy(stats->ac_comm, tsk->comm, sizeof(stats->ac_comm));
> +	if (thread_group_leader(tsk) && lock_task_sighand(tsk, &flags)) {
> +		struct cdata *cd = &tsk->signal->cdata_wait;
> +		stats->ac_cutime = cputime_to_usecs(cd->utime);
> +		stats->ac_cstime = cputime_to_usecs(cd->stime);

perhaps we need a small comment to explain ->siglock...

But in fact I don't really understand this anyway. This is called
before we reparent our children. This means that ac_cutime/ac_cstime
can be changed after that (multithreading, or full_cdata_enabled).

Say, taskstats_exit()->fill_stats()->bacct_add_tsk(). Every thread
does this, including the group_leader. But, it is possible that
group_leader exits first, before other threads. IOW, what
stats->ac_cXtime actually mean?

Nevermind, the whole series looks correct to me. Although I still
can't find the time to read it ;) But at first glance this version
addresses all concerns we discussed before.

Oleg.

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