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Subject: Re: [RFC][PATCH v2 1/7] taskstats: Add new taskstats command
 TASKSTATS_CMD_ATTR_PIDS
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
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Date: Tue, 16 Nov 2010 13:36:12 +0100
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Cc: Shailabh Nagar <nagar1234@in.ibm.com>, Andrew Morton <akpm@linux-foundation.org>, Venkatesh Pallipadi <venki@google.com>, Suresh Siddha <suresh.b.siddha@intel.com>, Ingo Molnar <mingo@elte.hu>, Oleg Nesterov <oleg@redhat.com>, John stultz <johnstul@us.ibm.com>, Thomas Gleixner <tglx@linutronix.de>, Balbir Singh <balbir@linux.vnet.ibm.com>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Heiko Carstens <heiko.carstens@de.ibm.com>, Roland McGrath <roland@redhat.com>, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org
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On Tue, 2010-11-16 at 13:16 +0100, Michael Holzheu wrote:

> Ok, thanks. So sched_clock() seems to be a bad idea for our purposes.
> 
> An alternative approach could be to have a global counter for the task
> snapshots, which is increased each time a snapshot is created for
> userspace. In addition to that we had to add a snapshot counter field to
> the task_struct that is set to the current value of the global counter
> each time a task leaves a CPU. Then userspace could ask for all tasks
> that have been active after snapshot number x. In the response userspace
> gets all tasks that have a snapshot number bigger than x together with
> the new snapshot number y that can be used for the next query.
> 
> Still it would be useful to add a timestamp of the creation of the
> taskstats data in the response to userspace for calculating the interval
> time between two snapshots. Would the usage of ktime_get() be valid for
> that purpose?

ktime_get() can be insanely slow, but yes that's an option. Another
option is using local_clock() and living with the fact that it may be
out of sync (up to a jiffy or so) between CPUs.

The advantage of using ktime_get() as opposed to any other clock is that
userspace has access to it as well through:
clock_gettime(CLOCK_MONOTONIC), although that's arguably not too
relevant when all you're interested in is deltas.