Re: Processes stuck on D state on Dual Opteron

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:
> Nick Piggin wrote:
> 
>> Chen, Kenneth W wrote:
> 
> 
>>> I like the patch a lot and already did bench it on our db setup.  
>>> However,
>>> I'm seeing a negative regression compare to a very very crappy patch 
>>> (see
>>> attached, you can laugh at me for doing things like that :-).
>>>
>>
>> OK - if we go that way, perhaps the following patch may be the
>> way to do it.
>>
> 
> Here.
> 

Actually yes this is good I think.

What I was worried about is that you could lose some fairness due
to not being put on the queue before allocation.

This is probably a silly thing to worry about, because up until
that point things aren't really deterministic anyway (and before this
patchset it would try doing a GFP_ATOMIC allocation first anyway).

However after the subsequent locking rework, both these get_request()
calls will be performed under the same lock - giving you the same
fairness. So it is nothing to worry about anyway!

It is a bit subtle: get_request may only drop the lock and return NULL
(after retaking the lock), if we fail on a memory allocation. If we
just fail due to unavailable queue slots, then the lock is never
dropped. And the mem allocation can't fail because it is a mempool
alloc with GFP_NOIO.

Nick

-- 
SUSE Labs, Novell Inc.