Re: [PATCH v3 05/13] mm, compaction: report compaction as contended only due to lock contention

[Zhang Yanfei]

Hello Minchan

Thank you for your explain. Actually, I read the kernel with an old
version. The latest upstream kernel has the behaviour like you described
below. Oops, how long didn't I follow the buddy allocator change.

Thanks.

On 06/24/2014 07:35 AM, Minchan Kim wrote:
>>> Anyway, most big concern is that you are changing current behavior as
>>> > > I said earlier.
>>> > > 
>>> > > Old behavior in THP page fault when it consumes own timeslot was just
>>> > > abort and fallback 4K page but with your patch, new behavior is
>>> > > take a rest when it founds need_resched and goes to another round with
>>> > > async, not sync compaction. I'm not sure we need another round with
>>> > > async compaction at the cost of increasing latency rather than fallback
>>> > > 4 page.
>> > 
>> > I don't see the new behavior works like what you said. If need_resched
>> > is true, it calls cond_resched() and after a rest it just breaks the loop.
>> > Why there is another round with async compact?
> One example goes
> 
> Old:
> page fault
> huge page allocation
> __alloc_pages_slowpath
> __alloc_pages_direct_compact
> compact_zone_order
>         isolate_migratepages
>         compact_checklock_irqsave
>                 need_resched is true
>                 cc->contended = true;
>         return ISOLATE_ABORT
> return COMPACT_PARTIAL with *contented = cc.contended;
> COMPACTFAIL
> if (contended_compaction && gfp_mask & __GFP_NO_KSWAPD)
>         goto nopage;
> 
> New:
> 
> page fault
> huge page allocation
> __alloc_pages_slowpath
> __alloc_pages_direct_compact
> compact_zone_order
>         isolate_migratepages
>         compact_unlock_should_abort
>                 need_resched is true
>                 cc->contended = COMPACT_CONTENDED_SCHED;
>                 return true;
>         return ISOLATE_ABORT
> return COMPACT_PARTIAL with *contended = cc.contended == COMPACT_CONTENDED_LOCK (1)
> COMPACTFAIL
> if (contended_compaction && gfp_mask & __GFP_NO_KSWAPD)
>         no goto nopage because contended_compaction was false by (1)
> 
> __alloc_pages_direct_reclaim
> if (should_alloc_retry)
> else
>         __alloc_pages_direct_compact again with ASYNC_MODE
>                         
> 


-- 
Thanks.
Zhang Yanfei
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