Re: [PATCH v3 4/7] mm/compaction: update defer counter when allocation is expected to succeed

[Vlastimil Babka]

On 12/03/2015 08:11 AM, Joonsoo Kim wrote:
> It's rather strange that compact_considered and compact_defer_shift aren't
> updated but compact_order_failed is updated when allocation is expected
> to succeed. Regardless actual allocation success, deferring for current
> order will be disabled so it doesn't result in much difference to
> compaction behaviour.

The difference is that if the defer reset was wrong, the next compaction 
attempt that fails would resume the deferred counters?

> Moreover, in the past, there is a gap between expectation for allocation
> succeess in compaction and actual success in page allocator. But, now,
> this gap would be diminished due to providing classzone_idx and alloc_flags
> to watermark check in compaction and changed watermark check criteria
> for high-order allocation. Therfore, it's not a big problem to update
> defer counter when allocation is expected to succeed. This change
> will help to simplify defer logic.

I guess that's true. But at least some experiment would be better.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>   include/linux/compaction.h |  2 --
>   mm/compaction.c            | 27 ++++++++-------------------
>   mm/page_alloc.c            |  1 -
>   3 files changed, 8 insertions(+), 22 deletions(-)

...

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7002c66..f3605fd 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2815,7 +2815,6 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>   		struct zone *zone = page_zone(page);
>
>   		zone->compact_blockskip_flush = false;

While we are here, I wonder if this is useful at all? 
compact_blockskip_flush is set true when scanners meet. That typically 
means the compaction wasn't successful. Rarely it can be, but I doubt 
this will make much difference, so we could remove this line as well.

> -		compaction_defer_reset(zone, order, true);
>   		count_vm_event(COMPACTSUCCESS);
>   		return page;
>   	}

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