Re: [PATCH 2/2] mm: compaction: Minimise the time IRQs are disabledwhile isolating pages for migration

[Minchan Kim]

On Wed, Mar 2, 2011 at 7:34 AM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 2 Mar 2011 07:22:33 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>
>> On Wed, Mar 2, 2011 at 1:19 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
>> > On Wed, Mar 02, 2011 at 12:35:58AM +0900, Minchan Kim wrote:
>> >> On Tue, Mar 01, 2011 at 01:49:25PM +0900, KAMEZAWA Hiroyuki wrote:
>> >> > On Tue, 1 Mar 2011 13:11:46 +0900
>> >> > Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>> >> >
>>
>> ...
>>
>> > pages freed from irq shouldn't be PageLRU.
>>
>> Hmm..
>> As looking code, it seems to be no problem and I didn't see the any
>> comment about such rule. It should have been written down in
>> __page_cache_release.
>> Just out of curiosity.
>> What kinds of problem happen if we release lru page in irq context?
>
> put_page() from irq context has been permissible for ten years. ÂI
> expect there are a number of sites which do this (via subtle code
> paths, often). ÂIt might get messy.
>
>> >
>> > deferring freeing to workqueue doesn't look ok. firewall loads runs
>> > only from irq and this will cause some more work and a delay in the
>> > freeing. I doubt it's worhwhile especially for the lru_lock.
>> >
>>
>> As you said, if it is for decreasing lock contention in SMP to deliver
>> overall better performance, maybe we need to check again how much it
>> helps.
>> If it doesn't help much, could we remove irq_save/restore of lru_lock?
>> Do you know any benchmark to prove it had a benefit at that time or
>> any thread discussing about that in lkml?
>
>
> : commit b10a82b195d63575958872de5721008b0e9bef2d
> : Author: akpm <akpm>
> : Date: Â Thu Aug 15 18:21:05 2002 +0000
> :
> : Â Â [PATCH] make pagemap_lru_lock irq-safe
> :
> : Â Â It is expensive for a CPU to take an interrupt while holding the page
> : Â Â LRU lock, because other CPUs will pile up on the lock while the
> : Â Â interrupt runs.
> :
> : Â Â Disabling interrupts while holding the lock reduces contention by an
> : Â Â additional 30% on 4-way. ÂThis is when the only source of interrupts is
> : Â Â disk completion. ÂThe improvement will be higher with more CPUs and it
> : Â Â will be higher if there is networking happening.
> :
> : Â Â The maximum hold time of this lock is 17 microseconds on 500 MHx PIII,
> : Â Â which is well inside the kernel's maximum interrupt latency (which was
> : Â Â 100 usecs when I last looked, a year ago).
> :
> : Â Â This optimisation is not needed on uniprocessor, but the patch disables
> : Â Â IRQs while holding pagemap_lru_lock anyway, so it becomes an irq-safe
> : Â Â spinlock, and pages can be moved from the LRU in interrupt context.
> :
> : Â Â pagemap_lru_lock has been renamed to _pagemap_lru_lock to pick up any
> : Â Â missed uses, and to reliably break any out-of-tree patches which may be
> : Â Â using the old semantics.
> :
> : Â Â BKrev: 3d5bf1110yfdAAur4xqJfiLBDJ2Cqw
>
>
> Ancient stuff, and not a lot of detail. ÂBut I did measure it. ÂI
> measured everything ;) And, as mentioned, I'd expect that the
> contention problems would worsen on higher CPU machines and higher
> interrupt frequencies.

Thanks for giving the important information.

>
> I expect we could eliminate the irqsave requirement from
> rotate_reclaimable_page() simply by switching to a trylock. ÂSome pages
> will end up at the wrong end of the LRU but the effects may be
> negligible. ÂOr perhaps they may not - disk seeks are costly.
>
>

Releasing 14 pages should not have much cost about interrupt latency
and It's a general concept we have been used. If it really has a
problem, I think it would be better to reduce PAGEVEC_SIZE rather than
fixing the rotate_reclaimable_page.




-- 
Kind regards,
Minchan Kim
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