[PATCH -V3 4/9] mm: restrict the pcp batch scale factor to avoid too long latency

Huang Ying
2023-10-16T05:30:50Z

In page allocator, PCP (Per-CPU Pageset) is refilled and drained in
batches to increase page allocation throughput, reduce page
allocation/freeing latency per page, and reduce zone lock contention.
But too large batch size will cause too long maximal
allocation/freeing latency, which may punish arbitrary users. So the
default batch size is chosen carefully (in zone_batchsize(), the value
is 63 for zone > 1GB) to avoid that.

In commit 3b12e7e97938 ("mm/page_alloc: scale the number of pages that
are batch freed"), the batch size will be scaled for large number of
page freeing to improve page freeing performance and reduce zone lock
contention. Similar optimization can be used for large number of
pages allocation too.

To find out a suitable max batch scale factor (that is, max effective
batch size), some tests and measurement on some machines were done as
follows.

A set of debug patches are implemented as follows,

- Set PCP high to be 2 * batch to reduce the effect of PCP high

- Disable free batch size scaling to get the raw performance.

- The code with zone lock held is extracted from rmqueue_bulk() and
free_pcppages_bulk() to 2 separate functions to make it easy to
measure the function run time with ftrace function_graph tracer.

- The batch size is hard coded to be 63 (default), 127, 255, 511,
1023, 2047, 4095.

Then will-it-scale/page_fault1 is used to generate the page
allocation/freeing workload. The page allocation/freeing throughput
(page/s) is measured via will-it-scale. The page allocation/freeing
average latency (alloc/free latency avg, in us) and allocation/freeing
latency at 99 percentile (alloc/free latency 99%, in us) are measured
with ftrace function_graph tracer.

The test results are as follows,

Sapphire Rapids Server
======================
Batch throughput free latency free latency alloc latency alloc latency
page/s avg / us 99% / us avg / us 99% / us
----- ---------- ------------ ------------ ------------- -------------
63 513633.4 2.33 3.57 2.67 6.83
127 517616.7 4.35 6.65 4.22 13.03
255 520822.8 8.29 13.32 7.52 25.24
511 524122.0 15.79 23.42 14.02 49.35
1023 525980.5 30.25 44.19 25.36 94.88
2047 526793.6 59.39 84.50 45.22 140.81

Ice Lake Server
===============
Batch throughput free latency free latency alloc latency alloc latency
page/s avg / us 99% / us avg / us 99% / us
----- ---------- ------------ ------------ ------------- -------------
63 620210.3 2.21 3.68 2.02 4.35
127 627003.0 4.09 6.86 3.51 8.28
255 630777.5 7.70 13.50 6.17 15.97
511 633651.5 14.85 22.62 11.66 31.08
1023 637071.1 28.55 42.02 20.81 54.36
2047 638089.7 56.54 84.06 39.28 91.68

Cascade Lake Server
===================
Batch throughput free latency free latency alloc latency alloc latency
page/s avg / us 99% / us avg / us 99% / us
----- ---------- ------------ ------------ ------------- -------------
63 404706.7 3.29 5.03 3.53 4.75
127 422475.2 6.12 9.09 6.36 8.76
255 411522.2 11.68 16.97 10.90 16.39
511 428124.1 22.54 31.28 19.86 32.25
1023 414718.4 43.39 62.52 40.00 66.33
2047 429848.7 86.64 120.34 71.14 106.08

Commet Lake Desktop
===================
Batch throughput free latency free latency alloc latency alloc latency
page/s avg / us 99% / us avg / us 99% / us
----- ---------- ------------ ------------ ------------- -------------

63 795183.13 2.18 3.55 2.03 3.05
127 803067.85 3.91 6.56 3.85 5.52
255 812771.10 7.35 10.80 7.14 10.20
511 817723.48 14.17 27.54 13.43 30.31
1023 818870.19 27.72 40.10 27.89 46.28

Coffee Lake Desktop
===================
Batch throughput free latency free latency alloc latency alloc latency
page/s avg / us 99% / us avg / us 99% / us
----- ---------- ------------ ------------ ------------- -------------
63 510542.8 3.13 4.40 2.48 3.43
127 514288.6 5.97 7.89 4.65 6.04
255 516889.7 11.86 15.58 8.96 12.55
511 519802.4 23.10 28.81 16.95 26.19
1023 520802.7 45.30 52.51 33.19 45.95
2047 519997.1 90.63 104.00 65.26 81.74

From the above data, to restrict the allocation/freeing latency to be
less than 100 us in most times, the max batch scale factor needs to be
less than or equal to 5.

Although it is reasonable to use 5 as max batch scale factor for the
systems tested, there are also slower systems. Where smaller value
should be used to constrain the page allocation/freeing latency.

So, in this patch, a new kconfig option (PCP_BATCH_SCALE_MAX) is added
to set the max batch scale factor. Whose default value is 5, and
users can reduce it when necessary.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Lameter <cl@linux.com>
---
mm/Kconfig | 11 +++++++++++
mm/page_alloc.c | 2 +-
2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 264a2df5ecf5..ece4f2847e2b 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -704,6 +704,17 @@ config HUGETLB_PAGE_SIZE_VARIABLE
config CONTIG_ALLOC
def_bool (MEMORY_ISOLATION && COMPACTION) || CMA

+config PCP_BATCH_SCALE_MAX
+ int "Maximum scale factor of PCP (Per-CPU pageset) batch allocate/free"
+ default 5
+ range 0 6
+ help
+ In page allocator, PCP (Per-CPU pageset) is refilled and drained in
+ batches. The batch number is scaled automatically to improve page
+ allocation/free throughput. But too large scale factor may hurt
+ latency. This option sets the upper limit of scale factor to limit
+ the maximum latency.
+
config PHYS_ADDR_T_64BIT
def_bool 64BIT

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ba2d8f06523e..a5a5a4c3cd2b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2340,7 +2340,7 @@ static int nr_pcp_free(struct per_cpu_pages *pcp, int high, bool free_high)
* freeing of pages without any allocation.
*/
batch <<= pcp->free_factor;
- if (batch < max_nr_free)
+ if (batch < max_nr_free && pcp->free_factor < CONFIG_PCP_BATCH_SCALE_MAX)
pcp->free_factor++;
batch = clamp(batch, min_nr_free, max_nr_free);

--
2.39.2