Re: [PATCH 09/10] mm, page_alloc: Reserve pageblocks for high-order atomic allocations on demand

[Johannes Weiner]

On Mon, Sep 21, 2015 at 11:52:41AM +0100, 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 an atomic high-order allocation
> 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>

Cool, this is much more obvious than trusting the MIGRATE_RESERVE
mechanism for higher order atomics.

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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