Re: [PATCH 1/5] mm, compaction: pass classzone_idx and alloc_flags to watermark checking

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On 10/07/2014 11:33 AM, Vlastimil Babka wrote:
Compaction relies on zone watermark checks for decisions such as if it's worth
to start compacting in compaction_suitable() or whether compaction should stop
in compact_finished(). The watermark checks take classzone_idx and alloc_flags
parameters, which are related to the memory allocation request. But from the
context of compaction they are currently passed as 0, including the direct
compaction which is invoked to satisfy the allocation request, and could
therefore know the proper values.

The lack of proper values can lead to mismatch between decisions taken during
compaction and decisions related to the allocation request. Lack of proper
classzone_idx value means that lowmem_reserve is not taken into account.
This has manifested (during recent changes to deferred compaction) when DMA
zone was used as fallback for preferred Normal zone. compaction_suitable()
without proper classzone_idx would think that the watermarks are already
satisfied, but watermark check in get_page_from_freelist() would fail. Because
of this problem, deferring compaction has extra complexity that can be removed
in the following patch.

The issue (not confirmed in practice) with missing alloc_flags is opposite in
nature. For allocations that include ALLOC_HIGH, ALLOC_HIGHER or ALLOC_CMA in
alloc_flags (the last includes all MOVABLE allocations on CMA-enabled systems)
the watermark checking in compaction with 0 passed will be stricter than in
get_page_from_freelist(). In these cases compaction might be running for a
longer time than is really needed.

This patch fixes these problems by adding alloc_flags and classzone_idx to
struct compact_control and related functions involved in direct compaction and
watermark checking. Where possible, all other callers of compaction_suitable()
pass proper values where those are known. This is currently limited to
classzone_idx, which is sometimes known in kswapd context. However, the direct
reclaim callers should_continue_reclaim() and compaction_ready() do not
currently know the proper values, so the coordination between reclaim and
compaction may still not be as accurate as it could. This can be fixed later,
if it's shown to be an issue.

The effect of this patch should be slightly better high-order allocation
success rates and/or less compaction overhead, depending on the type of
allocations and presence of CMA. It allows simplifying deferred compaction
code in a followup patch.

When testing with stress-highalloc, there was some slight improvement (which
might be just due to variance) in success rates of non-THP-like allocations.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>

Acked-by: Rik van Riel <riel@xxxxxxxxxx>

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