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 18:21:45 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	holzheu@...ux.vnet.ibm.com
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

On Mon, 2010-11-15 at 18:09 +0100, Michael Holzheu wrote:

> > That you should not use sched_clock(),
> 
> What should we use instead?

Depends on what you want, look at kernel/sched_clock.c

> > 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,

I think you assume I actually know anything about taskstat :-), its the
thing I always say =n to in my config file and have so far happily
ignored all code of.

>  if we use task_rq(t)->clock for last_depart and
> sched_clock() for stats->time_ns.

Then you're up shit creek because rq->clock doesn't necessarily have any
correlation to sched_clock().

>  We also assume that we get wallclock
> intervals in nanoseconds, if we look at two sched_clock() timestamps.

Invalid assumption.

> "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

So you're essentially replicating the data in
sched_entity::statistics::wait_start ?

> * 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.

/me can only hope all this will only increase overhead for those of us
who actually use any of this..

I'm still upset over ->[us]timescaled existing, this patch set looks to
me like it will only upset me more.
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