Re: page fault scalability patch V11 [0/7]: overview

From: Nick Piggin <nickpiggin_at_yahoo.com.au>
Date: 2004-11-20 15:29:29
William Lee Irwin III wrote:
> William Lee Irwin III wrote:
> 
>>>/proc/ triggering NMI oopses was a persistent problem even before that
>>>code was merged. I've not bothered testing it as it at best aggravates it.
> 
> 
> On Sat, Nov 20, 2004 at 03:03:17PM +1100, Nick Piggin wrote:
> 
>>It isn't a problem. If it ever became a problem then we can just
>>touch the nmi oopser in the loop.
> 
> 
> Very, very wrong. The tasklist scans hold the read side of the lock
> and aren't even what's running with interrupts off. The contenders
> on the write side are what the NMI oopser oopses.
> 

*blinks*

So explain how this is "very very wrong", then?

> And supposing the arch reenables interrupts in the write side's
> spinloop, you just get a box that silently goes out of service for
> extended periods of time, breaking cluster membership and more. The
> NMI oopser is just the report of the problem, not the problem itself.
> It's not a false report. The box is dead for > 5s at a time.
> 

The point is, adding a for-each-thread loop or two in /proc isn't
going to cause a problem that isn't already there.

If you had zero for-each-thread loops then you might have a valid
complaint. Seeing as you have more than zero, with slim chances of
reducing that number, then there is no valid complaint.

> 
> William Lee Irwin III wrote:
> 
>>>And thread groups can share mm's. do_for_each_thread() won't suffice.
> 
> 
> On Sat, Nov 20, 2004 at 03:03:17PM +1100, Nick Piggin wrote:
> 
>>I think it will be just fine.
> 
> 
> And that makes it wrong on both counts. The above fails any time
> LD_ASSUME_KERNEL=2.4 is used, we well as when actual Linux features
> are used directly.
> 

See my followup.
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Received on Fri Nov 19 23:57:43 2004

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