linux-kernel - Re: [RFC][PATCH v2 1/7] taskstats: Add new taskstats command TASKSTATS_CMD_ATTR

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Date:	Mon, 15 Nov 2010 16:53:00 +0100
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Shailabh Nagar <nagar1234@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...e.hu>, Oleg Nesterov <oleg@...hat.com>,
	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>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [RFC][PATCH v2 1/7] taskstats: Add new taskstats command
 TASKSTATS_CMD_ATTR_PIDS

Hello Peter,

On Sat, 2010-11-13 at 20:20 +0100, Peter Zijlstra wrote:
> On Thu, 2010-11-11 at 18:03 +0100, Michael Holzheu wrote:
> > As clock for 'now' and 'time' the sched_clock() function is used and the patch
> 
> > +       preempt_disable();
> > +       stats->time_ns = sched_clock();
> > +       preempt_enable();
> 
> > +       task_snap_time = sched_clock();
> 
> That's just plain broken...

What exactly do you mean? Do you mean that we should not use
sched_clock() in general or that it is called twice?

> 
> 
> > +       t->sched_info.last_depart = task_rq(t)->clock;
> 
> Are you sure you don't mean task_rq(t)->clock_task ?

Maybe... I want to save in "last_depart" a sched_clock() timestamp that
is as precise as possible.

We use "last_depart" for the TASKSTATS_CMD_ATTR_PIDS command to find out
which tasks have been running on a CPU since the last taskstats
snapshot. We return all tasks where last_depart > MIN(stats->time_ns for
all tasks of last snapshot).

Michael



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