linux-kernel - Re: [NET]: rt_check_expire() can take a long time, add a cond

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Date:	Sat, 17 Nov 2007 13:56:08 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	davem@...emloft.net, netdev@...t.kernel.org
Subject: Re: [NET]: rt_check_expire() can take a long time, add a cond_resched()

Arjan van de Ven <arjan@...radead.org> writes:
>> > 
>> > Its not that cheap. The ChangeLog included my own numbers, on a
>> > Pentium M machine. (i686, 1.6 GHz, 1.5 GB ram)
>> > 
>> > Without "if (need_resched())" (so calling need_resched() X.XXX.XXX 
>> > times), each run takes 88ms
>> > 
>> > With the extra check (and *much* less function calls), each run
>> > takes 25ms

ms?!? The numbers sound wrong. Wrong unit? 

>> > 
>> 
>> Looking at cond_resched(), I think the extra cost comes from
>> "mov %esp,%edx ; and $0xffffe000,%edx" (current_thread_info())
>> 
>> I dont have oprofile numbers yet, but I suspect CPU may have some
>> delays to compute this pointer value, since %esp is probably 'busy'
>> because of the preceding "call"
>
> yeah the explicit reference makes the stack pointer tracking engine do a
> commit I suspect which then also creates a data dependency in the code
> flow.
>
> however... this is likely a good argument for making cond_resched() as a
> whole a #define (or inline) that does this test and then calls the out
> of line code (which then doesn't need to retest, so it avoids the
> double test)...

Disadvantage would be that might_sleep would be commonly skipped then
(unless you actually need to reschedule) 
But perhaps that's not a big issue.

-Andi

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