Re: [Lhms-devel] Re: [PATCH 2.6.12-rc5 0/10] mm: manual page migration-rc3 -- overview

[Ray Bryant <raybry@engr.sgi.com>]

Christoph Lameter wrote:

> 
> 
> There is PF_FREEZE flag used by the suspend feature that could 
> be used here to send the process into the "freezer" first. Using regular 
> signals to stop a process may cause races with user space code also doing
> SIGSTOP SIGCONT on a process while migrating it.
> 
> 

In general, process flags are only updatable by the current process.
There is no locking applied.  Having the migrating task set the PF_FREEZE
bit in the migrated process runs the risk of losing the update to some other
flags bit that is simultaneously set by the (running) migrated process.

I suppose this could be fixed as well by introducing a second flags word
in the task_struct.  But this starts to sound like a reimplemtnation of
signals.

The other concern (probably not a problem on Altix  :-) ), is what happens
if a process migration is underway at the time of a suspend.  When the
resume occurs, all processes will be unfrozen, including the task that
is under migration.

At the moment, I'm not convinced that this is a better path than depending
on SIGSTOP/SIGCONT.  It is a resonable restriction that processes eligble for
migration are not allowed to use those signals themselves, in particular for
the batch environment this is targeted at.

-- 
Best Regards,
Ray
-----------------------------------------------
                   Ray Bryant
512-453-9679 (work)         512-507-7807 (cell)
raybry@sgi.com             raybry@austin.rr.com
The box said: "Requires Windows 98 or better",
            so I installed Linux.
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