linux-kernel - Re: [PATCH 4/5] select: make select() use schedule

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Message-ID: <alpine.LFD.1.10.0808290915560.3300@nehalem.linux-foundation.org>
Date:	Fri, 29 Aug 2008 09:20:18 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...x.de
Subject: Re: [PATCH 4/5] select: make select() use schedule_hrtimeout()

On Fri, 29 Aug 2008, Arjan van de Ven wrote:
> 
> now that we have schedule_hrtimeout(), make select() use it.
> But only for short delays; really long delays are assumed to not
> need the highres level of accuracy but rather want the regular
> timer behavior for now.

This is _really_ ugly.

Can't you just do this relaxation in "schedule_hrtimeout()" instead, and 
just document the fact that the "high resolution" of hrtimeout is relative 
to the length of the timeout.

It's not that select() doesn't care, it's that *NOBODY* cares. If somebody 
asks for a timeout of one second and 2 microseconds, the two microseconds 
simply don't matter. Ever. But if somebody asks for a timeout of 12 
microseconds, individual microseconds probably _do_ matter.

So if you want high-resolution select/poll, then get rid of the "use_hr" 
logic entirely, and just do it unconditionally. Then, relax the scheduler 
timeouts in the scheduler.

(But, that's probably _generally_ true. Even now, when people do 
"schedule_timeout()", there's a big difference between asking for two 
ticks and asking for two seconds. The latter should probably try to round 
to a nice timer tick basis for power reasons).

		Linus
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