linux-kernel - Re: getting processor numbers

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Message-ID: <4612D175.30604@redhat.com>
Date:	Tue, 03 Apr 2007 15:13:09 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: getting processor numbers

Andrew Morton wrote:
> Did we mean to go off-list?

Oops, no, pressed the wrong button.

>> Andrew Morton wrote:
>>> So I'd have thought that in general an application should be querying its
>>> present affinity mask - something like sched_getaffinity()?  That fixes the
>>> CPU hotplug issues too, of course.
>> Does it really?
>>
>> My recollection is that the affinity masks of running processes is not
>> updated on hotplugging.  Is this addressed?
> 
> ah, yes, you're correct.
> 
> Inside a cpuset:
> 
>   sched_setaffinity() is constrained to those CPUs which are in the
>   cpuset.
> 
>   If a cpu if on/offlined we update each cpuset's cpu mask appropriately
>   but we do not update all the tasks presently running in the cpuset.
> 
> Outside a cpuset:
> 
>   sched_setaffinity() is constrained to all possible cpus
> 
>   We don't update each task's cpus_allowed when a CPU is removed.
> 
> 
> I think we trivially _could_ update each tasks's cpus_allowed mask when a
> CPU is removed, actually.

I think it has to be done.  But that's not so trivial.  What happens if
all the CPUs a process was supposed to be runnable on vanish.
Shouldn't, if no affinity mask is defined, new processors be added?  I
agree that if the process has a defined affinity mask no new processors
should be added _automatically_.


>> If yes, sched_getaffinity is a solution until the NUMA topology
>> framework can provide something better.  Even without a popcnt
>> instruction in the CPU (64-bit albeit) it's twice as fast as the the
>> stat() method proposed.
> 
> I'm surprised - I'd have expected sched_getaffinity() to be vastly quicker
> that doing fileystem operations.

You mean because it's only a factor of two?  Well, it's not once you
count the whole overhead.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖


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