linux-kernel - Re: [RFC PATCH] sched: Fix sched

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Date:	Mon, 08 Jun 2015 08:55:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, rostedt <rostedt@...dmis.org>,
	Francis Giraldeau <francis.giraldeau@...il.com>
Subject: Re: [RFC PATCH] sched: Fix sched_wakeup tracepoint

On Fri, 2015-06-05 at 13:23 +0000, Mathieu Desnoyers wrote:
> sched_wakeup: when try_to_wake_up{,_local} is called in the waker.
> sched_activate_task: when the wakee is marked runnable.
> sched_switch: when scheduling actually happens.
> 
> We can then calculate wakeup latency as
> 
>   time@...ed_activate - time@...ed_wakeup

One more thing, I think I would disagree with this. I would suggest
never to use the 'wakeup' (or 'waking' in my proposal) for timing. I
would suggest to use your interrupt tracepoint (or whatever else causes
wakeup to be called for this).

The wakeup times should be measured in tasktime -- of course, if
interrupts/preemption are disabled then tasktime == walltime.

The scheduling bit OTOH always needs to be measured in walltime, and is
most affected by the presence of other tasks on the system.

This too is why I'm not sure it makes sense to combine the two into a
single measurement. They should be measured in different time domains.
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