linux-kernel - Re: [RFC] Thread Migration Preemption

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Message-ID: <4690A8E9.9060605@yahoo.com.au>
Date:	Sun, 08 Jul 2007 19:05:45 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Thread Migration Preemption

Steven Rostedt wrote:
> On Fri, 2007-07-06 at 16:12 +1000, Nick Piggin wrote:
> 
>>Mathieu Desnoyers wrote:
> 
> 
>>>migration_disable();
>>>local_inc(&__get_cpu_var(&my_local_t_var));
>>>migration_enable();
>>>
> 
> 
> [...]
> 
> 
>>This seems like way too much stuff to add just for this type of thing. Why
>>not just disable and reenable preempt? Surely local_inc is not going to take
>>so long that disabling preemption matters.
> 
> 
> For this given example, it may be too much fine tuning. But there are
> other things (at least in RT) where this would be very helpful. One
> thing is that in RT an IRQ thread might service a softirq if that
> softirq thread is of the same priority as the IRQ thread. The difference
> between an IRQ thread and a softirq thread is that the IRQ thread may
> migrate but the softirq thread may not. So to do this performance
> enhancement, we need to temporarily pin the IRQ thread to the CPU, which
> is expensive (set_cpus_allowed).  This would make it much simpler and
> light weight to implement.

Well if this was just intended for -rt, then OK.


>>The task struct is not something we should just be carefree putting crap
>>into because it is seemingly free :(
>>
> 
> 
> Agreed, but as the subject says "RFC".  Perhaps we can make it a bit
> more complex and put this as one of the most significant bits in the
> preempt_count. We would just need to mask off that bit in all the archs
> when determining if we should preempt or not.  That's more complex, but
> keeps the task struct free from more luggage.

Just so long as it stays out of mainline without a good reason
that's fine.

-- 
SUSE Labs, Novell Inc.
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