linux-kernel - Re: [RFC] schedule_timeout

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Date:	Tue, 22 Jul 2008 00:12:02 -0400
From:	David Woodhouse <dwmw2@...radead.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, arjan@...radead.org
Subject: Re: [RFC] schedule_timeout_range()

On Tue, 2008-07-22 at 13:56 +1000, Nick Piggin wrote:
> Rather than specific "deadline" values (which we can't guarantee anyway),
> or vague "can defer" values,

We already _have_ those vague 'can defer' timers. They'll get run the
next time the CPU happens to be awake after they expire.

>  I would prefer just a small selection of maybe orders of magnitude
> flags, maybe SECONDS, MILLISECONDS, MICROSECONDS which gives an amount
> of delay the kernel might add to the timer.

As far as I can tell, any implementation of that ends up being converted
into what we have at the moment -- a deferrable timer which gets run
some time after it expires, and a timer which would actually _wake_ a
sleeping CPU. You have to create a value for that final timer anyway, so
why not just let the in-kernel caller provide it?

There's no point in trying to coalesce the 'final' timeouts; if just one
of them wakes the CPU and we're in the range for any other 'range
timers', those others will happy immediately anyway.

We did ponder the idea of a per-process setting which affects userspace
delays like nanosleep/poll/select, and introduces a variable extra delay
if the CPU is actually sleeping. So we can reduce the number of CPU
wakeup events for thosee userspace apps which aren't timing-sensitive.

We were also thinking of extending nanosleep/ppoll/pselect also to take
a 'range', for those cases where the process-wide setting needs to be
overridden. The prctl is a simple solution which doesn't involve
modifying large chunks of userspace to use new system calls, but it's
not a panacea -- in some places, an app might _want_ a prompt wakeup.

For kernel timers, though, I think it's better to let the caller set a
non-deferrable timer at a specific time. Although you're right that
'deadline' is probably a bad name for it. 

How about 'start' and 'end'? Or 'early' and 'late'? I really don't care
too much what it's called.

-- 
dwmw2

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