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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <p73k5ohav4n.fsf@bingen.suse.de>
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

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/