Date: Sat, 28 Jan 2006 12:06:20 -0800
From: Paul Jackson <pj@sgi.com>
To: Nathan Lynch <ntl@pobox.com>
Cc: mingo@elte.hu, steiner@sgi.com, linux-kernel@vger.kernel.org,
       rml@novell.com, akpm@osdl.org
Subject: Re: 2.6.16 - sys_sched_getaffinity & hotplug
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Nathan wrote:
> Task finishes work and does sched_setaffinity(saved_mask).

Stupid task.  If task wants to run on -all- cpus on a
hotplug system, task should not pass a saved mask, but
rather construct a mask with all bits set and pass that:

  cpu_set_t mask;
  unsigned int i;

  /* set all bits in mask - code totally untested */
  for (i = 0; i < sizeof(cpu_set_t) / sizeof (__cpu_mask); i++)
    mask.__bits[i] = ~0;

  sched_setaffinity(&mask);

Similar problems exist for a task running in a cpuset under
migration.  Saved masks are useless in all but static systems,
having no migration, no hotplug.

That, or use a library on top of this that lets the task work
with relative (to whatever is available) CPU and (for the
mbind/mempolicy calls) Memory Node numbers and that handles
the above details.  If all goes well, I should be releasing
such a library in the not distant future.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401
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