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

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Message-ID: <1289840968.1916.85.camel@holzheu-laptop>
Date:	Mon, 15 Nov 2010 18:09:28 +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 Mon, 2010-11-15 at 17:06 +0100, Peter Zijlstra wrote:
> On Mon, 2010-11-15 at 16:53 +0100, Michael Holzheu wrote:
> > 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?
> 
> That you should not use sched_clock(),

What should we use instead?

> and if you do (you really
> shouldn't) you should have disabled IRQs around it.
> 
> > > 
> > > 
> > > > +       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).
> 
> What does last departed mean? That is what timeline are you counting in?
> Do you want time as tasks see it, or time as your wallclock sees it?

"last_depart" should be the time stamp, where the task has left a CPU
the last time.

We assume that we can compare "last_depart" with "time_ns" in the
taskstats structure, if we use task_rq(t)->clock for last_depart and
sched_clock() for stats->time_ns. We also assume that we get wallclock
intervals in nanoseconds, if we look at two sched_clock() timestamps.

"stats->time_ns" is used as timestamp for the next snapshot query and
for calculation of the snapshot interval time. So there are three
important timestamps:
* struct task_struct:
  sched_info.last_depart: Last time task has left CPU
* struct taskstats:
  time_ns: Timestamp where taskstats data is generated
* sturuct cmd_pids:
  time_ns: Timestamp for TASKSTATS_CMD_ATTR_PIDS command. 

Example:
1. Get initial snapshot with cmd_pids->time_ns=0:
   - All tasks are returned.
    snapshot_time = MIN(stats->time_ns) for all received taskstats
2. Get second snapshot with cmd_pids->time_ns = snapshot_time
   - Only tasks that were active after "snapshot_time" are returned.

Michael

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