Re: [PATCH 11/12] mm, page_alloc: Reserve pageblocks for high-order atomic allocations on demand

From: Vlastimil Babka
Date: Wed Aug 26 2015 - 08:45:04 EST

On 08/24/2015 02:29 PM, Mel Gorman wrote:
High-order watermark checking exists for two reasons -- kswapd high-order
awareness and protection for high-order atomic requests. Historically the
kernel depended on MIGRATE_RESERVE to preserve min_free_kbytes as high-order
free pages for as long as possible. This patch introduces MIGRATE_HIGHATOMIC
that reserves pageblocks for high-order atomic allocations on demand and
avoids using those blocks for order-0 allocations. This is more flexible
and reliable than MIGRATE_RESERVE was.

A MIGRATE_HIGHORDER pageblock is created when a high-order allocation

^ atomic ...

request steals a pageblock but limits the total number to 1% of the zone.
Callers that speculatively abuse atomic allocations for long-lived
high-order allocations to access the reserve will quickly fail. Note that
SLUB is currently not such an abuser as it reclaims at least once. It is
possible that the pageblock stolen has few suitable high-order pages and
will need to steal again in the near future but there would need to be
strong justification to search all pageblocks for an ideal candidate.

The pageblocks are unreserved if an allocation fails after a direct
reclaim attempt.

The watermark checks account for the reserved pageblocks when the allocation
request is not a high-order atomic allocation.

The reserved pageblocks can not be used for order-0 allocations. This may
allow temporary wastage until a failed reclaim reassigns the pageblock. This
is deliberate as the intent of the reservation is to satisfy a limited
number of atomic high-order short-lived requests if the system requires them.

The stutter benchmark was used to evaluate this but while it was running
there was a systemtap script that randomly allocated between 1 high-order
page and 12.5% of memory's worth of order-3 pages using GFP_ATOMIC. This
is much larger than the potential reserve and it does not attempt to be
realistic. It is intended to stress random high-order allocations from
an unknown source, show that there is a reduction in failures without
introducing an anomaly where atomic allocations are more reliable than
regular allocations. The amount of memory reserved varied throughout the
workload as reserves were created and reclaimed under memory pressure. The
allocation failures once the workload warmed up were as follows;

4.2-rc5-vanilla 70%
4.2-rc5-atomic-reserve 56%

The failure rate was also measured while building multiple kernels. The
failure rate was 14% but is 6% with this patch applied.

Overall, this is a small reduction but the reserves are small relative to the
number of allocation requests. In early versions of the patch, the failure
rate reduced by a much larger amount but that required much larger reserves
and perversely made atomic allocations seem more reliable than regular allocations.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

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