Date: Tue, 24 May 2005 17:09:50 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Christoph Hellwig <hch@infradead.org>, Andrew Morton <akpm@osdl.org>,
       Arjan van de Ven <arjanv@infradead.org>,
       Nick Piggin <nickpiggin@yahoo.com.au>, linux-kernel@vger.kernel.org
Subject: Re: [patch] Voluntary Kernel Preemption, 2.6.12-rc4-mm2
Message-ID: <20050524150950.GA10736@elte.hu>
References: <20050524121541.GA17049@elte.hu> <20050524132105.GA29477@elte.hu> <20050524145636.GA15943@infradead.org>
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* Christoph Hellwig <hch@infradead.org> wrote:

> I still disagree with this one violently. [...]

(then you must be disagreeing with CONFIG_PREEMPT too to a certain 
degree i guess?)

> [...] If you want a cond_resched() add it where nessecary, but don't 
> hide it behind might_sleep - there could be quite a lot might_sleeps 
> in common codepathes and they should stay purely a debug aid.

The recent prolifation of might_sleep() points was a direct result of 
the -VP patch. I _did_ measure and lay out the might_sleep()s so that 
key latency paths get cut. If we did what you propose we'd end up
duplicating 95% of the current might_sleep() invocations. So instead of
sprinking the source with cond_resched()s, we implicitly get them via
might_sleep().

there's another argument as well: if a function truly might sleep, it's 
in most cases complex enough to not worry about one extra need_resched() 
check. So might_sleep() and cond_resched() pair better than one would 
think.

(it is also a debugging helper: by actually sleeping at might_sleep() 
points we truly explore whether preemption at that point is safe.)

or if you think we can get away with using just a couple of 
cond_resched()s then you are my guest to prove me wrong: take the -RT 
kernel it has both -VP and the latency measurement tools integrated, and 
remove the cond_resched() from might_sleep() and try to find the points 
that are necessary to cut down latencies so that they fall into the 
1msec range on typical hw.

	Ingo
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