Re: [BUG] long freezes on thinkpad t60

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 18 Jun 2007 10:12:04 +0200 Ingo Molnar <mingo@elte.hu> wrote:

> ---------------------------------------------------->
> Subject: [patch] x86: fix spin-loop starvation bug
> From: Ingo Molnar <mingo@elte.hu>
> 
> Miklos Szeredi reported very long pauses (several seconds, sometimes 
> more) on his T60 (with a Core2Duo) which he managed to track down to 
> wait_task_inactive()'s open-coded busy-loop. He observed that an 
> interrupt on one core tries to acquire the runqueue-lock but does not 
> succeed in doing so for a very long time - while wait_task_inactive() on 
> the other core loops waiting for the first core to deschedule a task 
> (which it wont do while spinning in an interrupt handler).
> 
> The problem is: both the spin_lock() code and the wait_task_inactive() 
> loop uses cpu_relax()/rep_nop(), so in theory the CPU should have 
> guaranteed MESI-fairness to the two cores - but that didnt happen: one 
> of the cores was able to monopolize the cacheline that holds the 
> runqueue lock, for extended periods of time.
> 
> This patch changes the spin-loop to assert an atomic op after every REP 
> NOP instance - this will cause the CPU to express its "MESI interest" in 
> that cacheline after every REP NOP.

Kiran, if you're still able to reproduce that zone->lru_lock starvation problem,
this would be a good one to try...
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