netdev - Re: [patch] timers: add mod_timer

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Message-ID: <20090218185850.GA2494@redhat.com>
Date:	Wed, 18 Feb 2009 19:58:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Patrick McHardy <kaber@...sh.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	Rick Jones <rick.jones2@...com>,
	Eric Dumazet <dada1@...mosbay.com>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, tglx@...utronix.de,
	Martin Josefsson <gandalf@...g.westbo.se>
Subject: Re: [patch] timers: add mod_timer_pending()

On 02/18, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@...hat.com> wrote:
>
> > Unlike __mod_timer(..., bool pending_only), it preserves the CPU on
> > which the timer is pending.
> >
> > Or, perhaps, we can modify __mod_timer() further,
>
> if then i'd put it into a separate commit.
>
> I think the auto-migration of all the mod_timer() variants is a
> scalability feature: if for example a networking socket's main
> user migrates to another CPU, then the timer 'follows' it - even
> if the timer never actually expires (which is quite common for
> high-speed high-reliability networking transports).

OK.

But sometimes it is better (or necessary) to prevent the migration.
Since you already are changed __mod_timer() it would be ugly to
add yet another helper. Perhaps we should turn "bool pending_only"
into "int flags" right now?

This is minor, and perhaps we will never need the TIMER_DONT_MIGRATE
flag. But if ever need, then we have to audit all callers.

Oleg.

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