The patch titled
Subject: mm/page_alloc: limit number of high-order pages on PCP during bulk free
has been added to the -mm tree. Its filename is
mm-page_alloc-limit-number-of-high-order-pages-on-pcp-during-bulk-free.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-page_alloc-limit-number-of-high-order-pages-on-pcp-during-bulk-free.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-page_alloc-limit-number-of-high-order-pages-on-pcp-during-bulk-free.patch
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From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Subject: mm/page_alloc: limit number of high-order pages on PCP during bulk free
When a PCP is mostly used for frees then high-order pages can exist on PCP
lists for some time. This is problematic when the allocation pattern is
all allocations from one CPU and all frees from another resulting in
colder pages being used. When bulk freeing pages, limit the number of
high-order pages that are stored on the PCP lists.
Netperf running on localhost exhibits this pattern and while it does not
matter for some machines, it does matter for others with smaller caches
where cache misses cause problems due to reduced page reuse. Pages freed
directly to the buddy list may be reused quickly while still cache hot
where as storing on the PCP lists may be cold by the time
free_pcppages_bulk() is called.
Using perf kmem:mm_page_alloc, the 5 most used page frames were
5.17-rc3
13041 pfn=0x111a30
13081 pfn=0x5814d0
13097 pfn=0x108258
13121 pfn=0x689598
13128 pfn=0x5814d8
5.17-revert-highpcp
192009 pfn=0x54c140
195426 pfn=0x1081d0
200908 pfn=0x61c808
243515 pfn=0xa9dc20
402523 pfn=0x222bb8
5.17-full-series
142693 pfn=0x346208
162227 pfn=0x13bf08
166413 pfn=0x2711e0
166950 pfn=0x2702f8
The spread is wider as there is still time before pages freed to one PCP
get released with a tradeoff between fast reuse and reduced zone lock
acquisition.
On the machine used to gather the traces, the headline performance was
equivalent.
netperf-tcp
5.17.0-rc3 5.17.0-rc3 5.17.0-rc3
vanilla mm-reverthighpcp-v1r1 mm-highpcplimit-v2
Hmean 64 839.93 ( 0.00%) 840.77 ( 0.10%) 841.02 ( 0.13%)
Hmean 128 1614.22 ( 0.00%) 1622.07 * 0.49%* 1636.41 * 1.37%*
Hmean 256 2952.00 ( 0.00%) 2953.19 ( 0.04%) 2977.76 * 0.87%*
Hmean 1024 10291.67 ( 0.00%) 10239.17 ( -0.51%) 10434.41 * 1.39%*
Hmean 2048 17335.08 ( 0.00%) 17399.97 ( 0.37%) 17134.81 * -1.16%*
Hmean 3312 22628.15 ( 0.00%) 22471.97 ( -0.69%) 22422.78 ( -0.91%)
Hmean 4096 25009.50 ( 0.00%) 24752.83 * -1.03%* 24740.41 ( -1.08%)
Hmean 8192 32745.01 ( 0.00%) 31682.63 * -3.24%* 32153.50 * -1.81%*
Hmean 16384 39759.59 ( 0.00%) 36805.78 * -7.43%* 38948.13 * -2.04%*
On a 1-socket skylake machine with a small CPU cache that suffers more if
cache misses are too high
netperf-tcp
5.17.0-rc3 5.17.0-rc3 5.17.0-rc3
vanilla mm-reverthighpcp-v1 mm-highpcplimit-v2
Hmean 64 938.95 ( 0.00%) 941.50 * 0.27%* 943.61 * 0.50%*
Hmean 128 1843.10 ( 0.00%) 1857.58 * 0.79%* 1861.09 * 0.98%*
Hmean 256 3573.07 ( 0.00%) 3667.45 * 2.64%* 3674.91 * 2.85%*
Hmean 1024 13206.52 ( 0.00%) 13487.80 * 2.13%* 13393.21 * 1.41%*
Hmean 2048 22870.23 ( 0.00%) 23337.96 * 2.05%* 23188.41 * 1.39%*
Hmean 3312 31001.99 ( 0.00%) 32206.50 * 3.89%* 31863.62 * 2.78%*
Hmean 4096 35364.59 ( 0.00%) 36490.96 * 3.19%* 36112.54 * 2.11%*
Hmean 8192 48497.71 ( 0.00%) 49954.05 * 3.00%* 49588.26 * 2.25%*
Hmean 16384 58410.86 ( 0.00%) 60839.80 * 4.16%* 62282.96 * 6.63%*
Note that this was a machine that did not benefit from caching high-order
pages and performance is almost restored with the series applied. It's
not fully restored as cache misses are still higher. This is a trade-off
between optimising for a workload that does all allocs on one CPU and
frees on another or more general workloads that need high-order pages for
SLUB and benefit from avoiding zone->lock for every SLUB refill/drain.
Link: https://lkml.kernel.org/r/20220217002227.5739-7-mgorman@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 26 +++++++++++++++++++++-----
1 file changed, 21 insertions(+), 5 deletions(-)
--- a/mm/page_alloc.c~mm-page_alloc-limit-number-of-high-order-pages-on-pcp-during-bulk-free
+++ a/mm/page_alloc.c
@@ -3355,10 +3355,15 @@ static bool free_unref_page_prepare(stru
return true;
}
-static int nr_pcp_free(struct per_cpu_pages *pcp, int high, int batch)
+static int nr_pcp_free(struct per_cpu_pages *pcp, int high, int batch,
+ bool free_high)
{
int min_nr_free, max_nr_free;
+ /* Free everything if batch freeing high-order pages. */
+ if (unlikely(free_high))
+ return pcp->count;
+
/* Check for PCP disabled or boot pageset */
if (unlikely(high < batch))
return 1;
@@ -3379,11 +3384,12 @@ static int nr_pcp_free(struct per_cpu_pa
return batch;
}
-static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone)
+static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
+ bool free_high)
{
int high = READ_ONCE(pcp->high);
- if (unlikely(!high))
+ if (unlikely(!high || free_high))
return 0;
if (!test_bit(ZONE_RECLAIM_ACTIVE, &zone->flags))
@@ -3403,17 +3409,27 @@ static void free_unref_page_commit(struc
struct per_cpu_pages *pcp;
int high;
int pindex;
+ bool free_high;
__count_vm_event(PGFREE);
pcp = this_cpu_ptr(zone->per_cpu_pageset);
pindex = order_to_pindex(migratetype, order);
list_add(&page->lru, &pcp->lists[pindex]);
pcp->count += 1 << order;
- high = nr_pcp_high(pcp, zone);
+
+ /*
+ * As high-order pages other than THP's stored on PCP can contribute
+ * to fragmentation, limit the number stored when PCP is heavily
+ * freeing without allocation. The remainder after bulk freeing
+ * stops will be drained from vmstat refresh context.
+ */
+ free_high = (pcp->free_factor && order && order <= PAGE_ALLOC_COSTLY_ORDER);
+
+ high = nr_pcp_high(pcp, zone, free_high);
if (pcp->count >= high) {
int batch = READ_ONCE(pcp->batch);
- free_pcppages_bulk(zone, nr_pcp_free(pcp, high, batch), pcp, pindex);
+ free_pcppages_bulk(zone, nr_pcp_free(pcp, high, batch, free_high), pcp, pindex);
}
}
_
Patches currently in -mm which might be from mgorman@xxxxxxxxxxxxxxxxxxx are
mm-page_alloc-fetch-the-correct-pcp-buddy-during-bulk-free.patch
mm-page_alloc-track-range-of-active-pcp-lists-during-bulk-free.patch
mm-page_alloc-simplify-how-many-pages-are-selected-per-pcp-list-during-bulk-free.patch
mm-page_alloc-drain-the-requested-list-first-during-bulk-free.patch
mm-page_alloc-free-pages-in-a-single-pass-during-bulk-free.patch
mm-page_alloc-limit-number-of-high-order-pages-on-pcp-during-bulk-free.patch
