From: jak@rudolph.ccur.com (Joe Korty)
Message-Id: <200203152214.WAA27958@rudolph.ccur.com>
Subject: Re: [PATCH] 2.4.18 scheduler bugs
To: mingo@elte.hu
Date: Fri, 15 Mar 2002 17:14:20 -0500 (EST)
Cc: joe.korty@ccur.com (Joe Korty), alan@lxorguk.ukuu.org.uk (Alan Cox),
        marcelo@conectiva.com.br (Marcelo Tosatti),
        torvalds@transmeta.com (Linus Torvalds), linux-kernel@vger.kernel.org
Reply-To: joe.korty@ccur.com (Joe Korty)
In-Reply-To: <Pine.LNX.4.44.0203152156270.23324-100000@elte.hu> from "Ingo Molnar" at Mar 15, 2002 09:57:06 PM
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Sender: linux-kernel-owner@vger.kernel.org

>>> but even in the Athlon case an IPI is still an IRQ entry, which will add
>>> at least 200 cycles or more to the idle wakeup latency.
>> 
>> It is an idle cpu that is spending those 200 cycles.
> 
> wrong. When it's woken up it's *not* an idle CPU anymore, and it's the
> freshly woken up task that is going to execute 200 cycles later...

I have to disagree.  It is the woken up task *running on the
otherwise idle CPU* that burns up 200 cycles at the tail.

A cpu is wasting, say, 5,000,000 cycles (1GHz/100/2, or 1/2 tick) in
hlt when it could have been doing work.  Why worry about an
alternative wakeup path that burns up 200-400 cycles of that on the
otherwise idling cpu, even if it is at the tail.

Joe
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