[PATCH v3 19/21] gup: hint non-caching if clearing large regions

From: Ankur Arora
Date: Mon Jun 06 2022 - 16:47:06 EST

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When clearing a large region, or when the user explicitly hints
via FOLL_HINT_BULK that a call to get_user_pages() is part of a larger
region being gup'd, take the non-caching path.

One notable limitation is that this is only done when the underlying
pages are huge or gigantic, even if a large region composed of PAGE_SIZE
pages is being cleared. This is because non-caching stores are generally
weakly ordered and need some kind of store fence -- at PTE write
granularity -- to avoid data leakage. This is expensive enough to
negate any performance advantage.

Performance
==

System: Oracle X9-2c (2 nodes * 32 cores * 2 threads)
Processor: Intel Xeon(R) Platinum 8358 CPU @ 2.60GHz (Icelakex, 6:106:6)
Memory: 1024 GB evenly split between nodes
LLC-size: 48MB for each node (32-cores * 2-threads)
no_turbo: 1, Microcode: 0xd0002c1, scaling-governor: performance

System: Oracle E4-2c (2 nodes * 8 CCXes * 8 cores * 2 threads)
Processor: AMD EPYC 7J13 64-Core Processor (Milan, 25:1:1)
Memory: 512 GB evenly split between nodes
LLC-size: 32MB for each CCX (8-cores * 2-threads)
boost: 1, Microcode: 0xa00115d, scaling-governor: performance

Two workloads: qemu VM creation where that is the exclusive load
and, to probe the cache interference with unrelated processes aspect
of these changes, a kbuild with a background page clearing workload.

Workload: create a 192GB qemu-VM (backed by preallocated 2MB
pages on the local node)
==

Icelakex
--
Time (s) Delta (%)
clear_pages_erms() 16.49 ( +- 0.06s ) # 12.50 bytes/ns
clear_pages_movnt() 9.42 ( +- 0.20s ) -42.87% # 21.88 bytes/ns

It is easy enough to see where the improvement is coming from -- given
the non-caching stores, the CPU does not need to do any RFOs ending up
with way fewer L1-dcache-load-misses:

- 407,619,058 L1-dcache-loads # 24.746 M/sec ( +- 0.17% ) (69.20%)
- 3,245,399,461 L1-dcache-load-misses # 801.49% of all L1-dcache accesses ( +- 0.01% ) (69.22%)
+ 393,160,148 L1-dcache-loads # 41.786 M/sec ( +- 0.80% ) (69.22%)
+ 5,790,543 L1-dcache-load-misses # 1.50% of all L1-dcache accesses ( +- 1.55% ) (69.26%)

(Fuller perf stat output, at [1], [2].)

Milan
--
Time (s) Delta
clear_pages_erms() 11.83 ( +- 0.08s ) # 17.42 bytes/ns
clear_pages_clzero() 4.91 ( +- 0.27s ) -58.49% # 41.98 bytes/ns

Milan does significantly fewer RFO, as well.

- 6,882,968,897 L1-dcache-loads # 582.960 M/sec ( +- 0.03% ) (33.38%)
- 3,267,546,914 L1-dcache-load-misses # 47.45% of all L1-dcache accesses ( +- 0.02% ) (33.37%)
+ 418,489,450 L1-dcache-loads # 85.611 M/sec ( +- 1.19% ) (33.46%)
+ 5,406,557 L1-dcache-load-misses # 1.35% of all L1-dcache accesses ( +- 1.07% ) (33.45%)

(Fuller perf stat output, at [3], [4].)

Workload: Kbuild with background clear_huge_page()
==

Probe the cache-pollution aspect of this commit with a kbuild
(make -j 32 bzImage) alongside a background process doing
clear_huge_page() via mmap(length=64GB, flags=MAP_POPULATE|MAP_HUGE_2MB)
in a loop.

The expectation -- assuming kbuild performance is partly cache
limited -- is that the clear_huge_page() -> clear_pages_erms()
background load would show a greater slowdown than,
clear_huge_page() -> clear_pages_movnt(). The kbuild itself does not
use THP or similar, so any performance changes are due to the
background load.

Icelakex
--

# kbuild: 16 cores, 32 threads
# clear_huge_page() load: single thread bound to the same CPUset
# taskset -c 16-31,80-95 perf stat -r 5 -ddd \
make -C .. -j 32 O=b2 clean bzImage

- 8,226,884,900,694 instructions # 1.09 insn per cycle ( +- 0.02% ) (47.27%)
+ 8,223,413,950,371 instructions # 1.12 insn per cycle ( +- 0.03% ) (47.31%)

- 20,016,410,480,886 slots # 6.565 G/sec ( +- 0.01% ) (69.84%)
- 1,310,070,777,023 topdown-be-bound # 6.1% backend bound ( +- 0.28% ) (69.84%)
+ 19,328,950,611,944 slots # 6.494 G/sec ( +- 0.02% ) (69.87%)
+ 1,043,408,291,623 topdown-be-bound # 5.0% backend bound ( +- 0.33% ) (69.87%)

- 10,747,834,729 LLC-loads # 3.525 M/sec ( +- 0.05% ) (69.68%)
- 4,841,355,743 LLC-load-misses # 45.02% of all LL-cache accesses ( +- 0.06% ) (69.70%)
+ 10,466,865,056 LLC-loads # 3.517 M/sec ( +- 0.08% ) (69.68%)
+ 4,206,944,783 LLC-load-misses # 40.21% of all LL-cache accesses ( +- 0.06% ) (69.71%)

The LLC-load-misses show a significant improvement (-13.11%) which is
borne out in the (-20.35%) reduction in topdown-be-bound and a (2.7%)
improvement in IPC.

- 7,521,157,276,899 cycles # 2.467 GHz ( +- 0.02% ) (39.65%)
+ 7,348,971,235,549 cycles # 2.469 GHz ( +- 0.04% ) (39.68%)

The ends up with an overall improvement in cycles of (-2.28%).

(Fuller perf stat output, at [5], [6].)

Milan
--

# kbuild: 2 CCxes, 16 cores, 32 threads
# clear_huge_page() load: single thread bound to the same CPUset
# taskset -c 16-31,144-159 perf stat -r 5 -ddd \
make -C .. -j 32 O=b2 clean bzImage

- 302,739,130,717 stalled-cycles-backend # 3.82% backend cycles idle ( +- 0.10% ) (41.11%)
+ 287,703,667,307 stalled-cycles-backend # 3.74% backend cycles idle ( +- 0.04% ) (41.11%)

- 8,981,403,534,446 instructions # 1.13 insn per cycle
+ 8,969,062,192,998 instructions # 1.16 insn per cycle

Milan sees a (-4.96%) improvement in stalled-cycles-backend and
a (-2.65%) improvement in IPC.

- 7,930,842,057,103 cycles # 2.338 GHz ( +- 0.04% ) (41.09%)
+ 7,705,812,395,365 cycles # 2.339 GHz ( +- 0.01% ) (41.11%)

The ends up with an overall improvement in cycles of (-2.83%).

(Fuller perf stat output, at [7], [8].)

[1] Icelakex, clear_pages_erms()
# perf stat -r 5 --all-kernel -ddd ./qemu.sh

Performance counter stats for './qemu.sh' (5 runs):

16,329.41 msec task-clock # 0.990 CPUs utilized ( +- 0.42% )
143 context-switches # 8.681 /sec ( +- 0.93% )
1 cpu-migrations # 0.061 /sec ( +- 63.25% )
118 page-faults # 7.164 /sec ( +- 0.27% )
41,735,523,673 cycles # 2.534 GHz ( +- 0.42% ) (38.46%)
1,454,116,543 instructions # 0.03 insn per cycle ( +- 0.49% ) (46.16%)
266,749,920 branches # 16.194 M/sec ( +- 0.41% ) (53.86%)
928,726 branch-misses # 0.35% of all branches ( +- 0.38% ) (61.54%)
208,805,754,709 slots # 12.676 G/sec ( +- 0.41% ) (69.23%)
5,355,889,366 topdown-retiring # 2.5% retiring ( +- 0.50% ) (69.23%)
12,720,749,784 topdown-bad-spec # 6.1% bad speculation ( +- 1.38% ) (69.23%)
998,710,552 topdown-fe-bound # 0.5% frontend bound ( +- 0.85% ) (69.23%)
192,653,197,875 topdown-be-bound # 90.9% backend bound ( +- 0.38% ) (69.23%)
407,619,058 L1-dcache-loads # 24.746 M/sec ( +- 0.17% ) (69.20%)
3,245,399,461 L1-dcache-load-misses # 801.49% of all L1-dcache accesses ( +- 0.01% ) (69.22%)
10,805,747 LLC-loads # 656.009 K/sec ( +- 0.37% ) (69.25%)
804,475 LLC-load-misses # 7.44% of all LL-cache accesses ( +- 2.73% ) (69.26%)
<not supported> L1-icache-loads
18,134,527 L1-icache-load-misses ( +- 1.24% ) (30.80%)
435,474,462 dTLB-loads # 26.437 M/sec ( +- 0.28% ) (30.80%)
41,187 dTLB-load-misses # 0.01% of all dTLB cache accesses ( +- 4.06% ) (30.79%)
<not supported> iTLB-loads
440,135 iTLB-load-misses ( +- 1.07% ) (30.78%)
<not supported> L1-dcache-prefetches
<not supported> L1-dcache-prefetch-misses

16.4906 +- 0.0676 seconds time elapsed ( +- 0.41% )

[2] Icelakex, clear_pages_movnt()
# perf stat -r 5 --all-kernel -ddd ./qemu.sh

Performance counter stats for './qemu.sh' (5 runs):

9,896.77 msec task-clock # 1.050 CPUs utilized ( +- 2.08% )
135 context-switches # 14.348 /sec ( +- 0.74% )
0 cpu-migrations # 0.000 /sec
116 page-faults # 12.329 /sec ( +- 0.50% )
25,239,642,558 cycles # 2.683 GHz ( +- 2.11% ) (38.43%)
36,791,658,500 instructions # 1.54 insn per cycle ( +- 0.06% ) (46.12%)
3,475,279,229 branches # 369.361 M/sec ( +- 0.09% ) (53.82%)
1,987,098 branch-misses # 0.06% of all branches ( +- 0.71% ) (61.51%)
126,256,220,768 slots # 13.419 G/sec ( +- 2.10% ) (69.21%)
57,705,186,453 topdown-retiring # 47.8% retiring ( +- 0.28% ) (69.21%)
5,934,729,245 topdown-bad-spec # 4.3% bad speculation ( +- 5.91% ) (69.21%)
4,089,990,217 topdown-fe-bound # 3.1% frontend bound ( +- 2.11% ) (69.21%)
60,298,426,167 topdown-be-bound # 44.8% backend bound ( +- 4.21% ) (69.21%)
393,160,148 L1-dcache-loads # 41.786 M/sec ( +- 0.80% ) (69.22%)
5,790,543 L1-dcache-load-misses # 1.50% of all L1-dcache accesses ( +- 1.55% ) (69.26%)
1,069,049 LLC-loads # 113.621 K/sec ( +- 1.25% ) (69.27%)
728,260 LLC-load-misses # 70.65% of all LL-cache accesses ( +- 2.63% ) (69.30%)
<not supported> L1-icache-loads
14,620,549 L1-icache-load-misses ( +- 1.27% ) (30.80%)
404,962,421 dTLB-loads # 43.040 M/sec ( +- 1.13% ) (30.80%)
31,916 dTLB-load-misses # 0.01% of all dTLB cache accesses ( +- 4.61% ) (30.77%)
<not supported> iTLB-loads
396,984 iTLB-load-misses ( +- 2.23% ) (30.74%)
<not supported> L1-dcache-prefetches
<not supported> L1-dcache-prefetch-misses

9.428 +- 0.206 seconds time elapsed ( +- 2.18% )

[3] Milan, clear_pages_erms()
# perf stat -r 5 --all-kernel -ddd ./qemu.sh

Performance counter stats for './qemu.sh' (5 runs):

11,676.79 msec task-clock # 0.987 CPUs utilized ( +- 0.68% )
96 context-switches # 8.131 /sec ( +- 0.78% )
2 cpu-migrations # 0.169 /sec ( +- 18.71% )
106 page-faults # 8.978 /sec ( +- 0.23% )
28,161,726,414 cycles # 2.385 GHz ( +- 0.69% ) (33.33%)
141,032,827 stalled-cycles-frontend # 0.50% frontend cycles idle ( +- 52.44% ) (33.35%)
796,792,139 stalled-cycles-backend # 2.80% backend cycles idle ( +- 23.73% ) (33.35%)
1,140,172,646 instructions # 0.04 insn per cycle
# 0.50 stalled cycles per insn ( +- 0.89% ) (33.35%)
219,864,061 branches # 18.622 M/sec ( +- 1.06% ) (33.36%)
1,407,446 branch-misses # 0.63% of all branches ( +- 10.66% ) (33.40%)
6,882,968,897 L1-dcache-loads # 582.960 M/sec ( +- 0.03% ) (33.38%)
3,267,546,914 L1-dcache-load-misses # 47.45% of all L1-dcache accesses ( +- 0.02% ) (33.37%)
<not supported> LLC-loads
<not supported> LLC-load-misses
146,901,513 L1-icache-loads # 12.442 M/sec ( +- 0.78% ) (33.36%)
1,462,155 L1-icache-load-misses # 0.99% of all L1-icache accesses ( +- 0.83% ) (33.34%)
2,055,805 dTLB-loads # 174.118 K/sec ( +- 22.56% ) (33.33%)
136,260 dTLB-load-misses # 4.69% of all dTLB cache accesses ( +- 23.13% ) (33.35%)
941 iTLB-loads # 79.699 /sec ( +- 5.54% ) (33.35%)
115,444 iTLB-load-misses # 14051.12% of all iTLB cache accesses ( +- 21.17% ) (33.34%)
95,438,373 L1-dcache-prefetches # 8.083 M/sec ( +- 19.99% ) (33.34%)
<not supported> L1-dcache-prefetch-misses

11.8296 +- 0.0805 seconds time elapsed ( +- 0.68% )

[4] Milan, clear_pages_clzero()
# perf stat -r 5 --all-kernel -ddd ./qemu.sh

Performance counter stats for './qemu.sh' (5 runs):

4,599.00 msec task-clock # 0.937 CPUs utilized ( +- 5.93% )
91 context-switches # 18.616 /sec ( +- 0.92% )
0 cpu-migrations # 0.000 /sec
107 page-faults # 21.889 /sec ( +- 0.19% )
10,975,453,059 cycles # 2.245 GHz ( +- 6.02% ) (33.28%)
14,193,355 stalled-cycles-frontend # 0.12% frontend cycles idle ( +- 1.90% ) (33.35%)
38,969,144 stalled-cycles-backend # 0.33% backend cycles idle ( +- 23.92% ) (33.34%)
13,951,880,530 instructions # 1.20 insn per cycle
# 0.00 stalled cycles per insn ( +- 0.11% ) (33.33%)
3,426,708,418 branches # 701.003 M/sec ( +- 0.06% ) (33.36%)
2,350,619 branch-misses # 0.07% of all branches ( +- 0.61% ) (33.45%)
418,489,450 L1-dcache-loads # 85.611 M/sec ( +- 1.19% ) (33.46%)
5,406,557 L1-dcache-load-misses # 1.35% of all L1-dcache accesses ( +- 1.07% ) (33.45%)
<not supported> LLC-loads
<not supported> LLC-load-misses
90,088,059 L1-icache-loads # 18.429 M/sec ( +- 0.36% ) (33.44%)
1,081,035 L1-icache-load-misses # 1.20% of all L1-icache accesses ( +- 3.67% ) (33.42%)
4,017,464 dTLB-loads # 821.854 K/sec ( +- 1.02% ) (33.40%)
204,096 dTLB-load-misses # 5.22% of all dTLB cache accesses ( +- 9.77% ) (33.36%)
770 iTLB-loads # 157.519 /sec ( +- 5.12% ) (33.36%)
209,834 iTLB-load-misses # 29479.35% of all iTLB cache accesses ( +- 0.17% ) (33.34%)
1,596,265 L1-dcache-prefetches # 326.548 K/sec ( +- 1.55% ) (33.31%)
<not supported> L1-dcache-prefetch-misses

4.908 +- 0.272 seconds time elapsed ( +- 5.54% )

[5] Icelakex, kbuild + bg:clear_pages_erms() load.
# taskset -c 16-31,80-95 perf stat -r 5 -ddd \
make -C .. -j 32 O=b2 clean bzImage

Performance counter stats for 'make -C .. -j 32 O=b2 clean bzImage' (5 runs):

3,047,329.07 msec task-clock # 19.520 CPUs utilized ( +- 0.02% )
1,675,061 context-switches # 549.415 /sec ( +- 0.43% )
89,232 cpu-migrations # 29.268 /sec ( +- 2.34% )
85,752,972 page-faults # 28.127 K/sec ( +- 0.00% )
7,521,157,276,899 cycles # 2.467 GHz ( +- 0.02% ) (39.65%)
8,226,884,900,694 instructions # 1.09 insn per cycle ( +- 0.02% ) (47.27%)
1,744,557,848,503 branches # 572.209 M/sec ( +- 0.02% ) (54.83%)
36,252,079,075 branch-misses # 2.08% of all branches ( +- 0.02% ) (62.35%)
20,016,410,480,886 slots # 6.565 G/sec ( +- 0.01% ) (69.84%)
6,518,990,385,998 topdown-retiring # 30.5% retiring ( +- 0.02% ) (69.84%)
7,821,817,193,732 topdown-bad-spec # 36.7% bad speculation ( +- 0.29% ) (69.84%)
5,714,082,318,274 topdown-fe-bound # 26.7% frontend bound ( +- 0.10% ) (69.84%)
1,310,070,777,023 topdown-be-bound # 6.1% backend bound ( +- 0.28% ) (69.84%)
2,270,017,283,501 L1-dcache-loads # 744.558 M/sec ( +- 0.02% ) (69.60%)
103,295,556,544 L1-dcache-load-misses # 4.55% of all L1-dcache accesses ( +- 0.02% ) (69.64%)
10,747,834,729 LLC-loads # 3.525 M/sec ( +- 0.05% ) (69.68%)
4,841,355,743 LLC-load-misses # 45.02% of all LL-cache accesses ( +- 0.06% ) (69.70%)
<not supported> L1-icache-loads
180,672,238,145 L1-icache-load-misses ( +- 0.03% ) (31.18%)
2,216,149,664,522 dTLB-loads # 726.890 M/sec ( +- 0.03% ) (31.83%)
2,000,781,326 dTLB-load-misses # 0.09% of all dTLB cache accesses ( +- 0.08% ) (31.79%)
<not supported> iTLB-loads
1,938,124,234 iTLB-load-misses ( +- 0.04% ) (31.76%)
<not supported> L1-dcache-prefetches
<not supported> L1-dcache-prefetch-misses

156.1136 +- 0.0785 seconds time elapsed ( +- 0.05% )

[6] Icelakex, kbuild + bg:clear_pages_movnt() load.
# taskset -c 16-31,80-95 perf stat -r 5 -ddd \
make -C .. -j 32 O=b2 clean bzImage

Performance counter stats for 'make -C .. -j 32 O=b2 clean bzImage' (5 runs):

2,978,535.47 msec task-clock # 19.471 CPUs utilized ( +- 0.05% )
1,637,295 context-switches # 550.105 /sec ( +- 0.89% )
91,635 cpu-migrations # 30.788 /sec ( +- 1.88% )
85,754,138 page-faults # 28.812 K/sec ( +- 0.00% )
7,348,971,235,549 cycles # 2.469 GHz ( +- 0.04% ) (39.68%)
8,223,413,950,371 instructions # 1.12 insn per cycle ( +- 0.03% ) (47.31%)
1,743,914,970,674 branches # 585.928 M/sec ( +- 0.01% ) (54.87%)
36,188,623,655 branch-misses # 2.07% of all branches ( +- 0.05% ) (62.39%)
19,328,950,611,944 slots # 6.494 G/sec ( +- 0.02% ) (69.87%)
6,508,801,041,075 topdown-retiring # 31.7% retiring ( +- 0.35% ) (69.87%)
7,581,383,615,462 topdown-bad-spec # 36.4% bad speculation ( +- 0.43% ) (69.87%)
5,521,686,808,149 topdown-fe-bound # 26.8% frontend bound ( +- 0.14% ) (69.87%)
1,043,408,291,623 topdown-be-bound # 5.0% backend bound ( +- 0.33% ) (69.87%)
2,269,475,492,575 L1-dcache-loads # 762.507 M/sec ( +- 0.03% ) (69.63%)
101,544,979,642 L1-dcache-load-misses # 4.47% of all L1-dcache accesses ( +- 0.05% ) (69.66%)
10,466,865,056 LLC-loads # 3.517 M/sec ( +- 0.08% ) (69.68%)
4,206,944,783 LLC-load-misses # 40.21% of all LL-cache accesses ( +- 0.06% ) (69.71%)
<not supported> L1-icache-loads
180,267,126,923 L1-icache-load-misses ( +- 0.07% ) (31.17%)
2,216,010,317,050 dTLB-loads # 744.544 M/sec ( +- 0.03% ) (31.82%)
1,979,801,744 dTLB-load-misses # 0.09% of all dTLB cache accesses ( +- 0.10% ) (31.79%)
<not supported> iTLB-loads
1,925,390,304 iTLB-load-misses ( +- 0.08% ) (31.77%)
<not supported> L1-dcache-prefetches
<not supported> L1-dcache-prefetch-misses

152.972 +- 0.309 seconds time elapsed ( +- 0.20% )

[7] Milan, clear_pages_erms()
# taskset -c 16-31,144-159 perf stat -r 5 -ddd \
make -C .. -j 32 O=b2 clean bzImage

Performance counter stats for 'make -C .. -j 32 O=b2 clean bzImage' (5 runs):

3,390,130.53 msec task-clock # 18.241 CPUs utilized ( +- 0.04% )
1,720,283 context-switches # 507.160 /sec ( +- 0.27% )
96,694 cpu-migrations # 28.507 /sec ( +- 1.41% )
75,872,994 page-faults # 22.368 K/sec ( +- 0.00% )
7,930,842,057,103 cycles # 2.338 GHz ( +- 0.04% ) (41.09%)
39,974,518,172 stalled-cycles-frontend # 0.50% frontend cycles idle ( +- 0.05% ) (41.10%)
302,739,130,717 stalled-cycles-backend # 3.82% backend cycles idle ( +- 0.10% ) (41.11%)
8,981,403,534,446 instructions # 1.13 insn per cycle
# 0.03 stalled cycles per insn ( +- 0.03% ) (41.10%)
1,909,303,327,220 branches # 562.886 M/sec ( +- 0.02% ) (41.10%)
50,324,935,298 branch-misses # 2.64% of all branches ( +- 0.02% ) (41.09%)
3,563,297,595,796 L1-dcache-loads # 1.051 G/sec ( +- 0.03% ) (41.08%)
129,901,339,258 L1-dcache-load-misses # 3.65% of all L1-dcache accesses ( +- 0.10% ) (41.07%)
<not supported> LLC-loads
<not supported> LLC-load-misses
809,770,606,566 L1-icache-loads # 238.730 M/sec ( +- 0.03% ) (41.07%)
12,403,758,671 L1-icache-load-misses # 1.53% of all L1-icache accesses ( +- 0.08% ) (41.07%)
60,010,026,089 dTLB-loads # 17.692 M/sec ( +- 0.04% ) (41.07%)
3,254,066,681 dTLB-load-misses # 5.42% of all dTLB cache accesses ( +- 0.09% ) (41.07%)
5,195,070,952 iTLB-loads # 1.532 M/sec ( +- 0.03% ) (41.08%)
489,196,395 iTLB-load-misses # 9.42% of all iTLB cache accesses ( +- 0.10% ) (41.09%)
39,920,161,716 L1-dcache-prefetches # 11.769 M/sec ( +- 0.03% ) (41.09%)
<not supported> L1-dcache-prefetch-misses

185.852 +- 0.501 seconds time elapsed ( +- 0.27% )

[8] Milan, clear_pages_clzero()
# taskset -c 16-31,144-159 perf stat -r 5 -ddd \
make -C .. -j 32 O=b2 clean bzImage

Performance counter stats for 'make -C .. -j 32 O=b2 clean bzImage' (5 runs):

3,296,677.12 msec task-clock # 18.051 CPUs utilized ( +- 0.02% )
1,713,645 context-switches # 520.062 /sec ( +- 0.26% )
91,883 cpu-migrations # 27.885 /sec ( +- 0.83% )
75,877,740 page-faults # 23.028 K/sec ( +- 0.00% )
7,705,812,395,365 cycles # 2.339 GHz ( +- 0.01% ) (41.11%)
38,866,265,031 stalled-cycles-frontend # 0.50% frontend cycles idle ( +- 0.09% ) (41.10%)
287,703,667,307 stalled-cycles-backend # 3.74% backend cycles idle ( +- 0.04% ) (41.11%)
8,969,062,192,998 instructions # 1.16 insn per cycle
# 0.03 stalled cycles per insn ( +- 0.01% ) (41.11%)
1,906,857,866,689 branches # 578.699 M/sec ( +- 0.01% ) (41.10%)
50,155,411,444 branch-misses # 2.63% of all branches ( +- 0.03% ) (41.11%)
3,552,652,190,906 L1-dcache-loads # 1.078 G/sec ( +- 0.01% ) (41.13%)
127,238,478,917 L1-dcache-load-misses # 3.58% of all L1-dcache accesses ( +- 0.04% ) (41.13%)
<not supported> LLC-loads
<not supported> LLC-load-misses
808,024,730,682 L1-icache-loads # 245.222 M/sec ( +- 0.03% ) (41.13%)
7,773,178,107 L1-icache-load-misses # 0.96% of all L1-icache accesses ( +- 0.11% ) (41.13%)
59,684,355,294 dTLB-loads # 18.113 M/sec ( +- 0.04% ) (41.12%)
3,247,521,154 dTLB-load-misses # 5.44% of all dTLB cache accesses ( +- 0.04% ) (41.12%)
5,064,547,530 iTLB-loads # 1.537 M/sec ( +- 0.09% ) (41.12%)
462,977,175 iTLB-load-misses # 9.13% of all iTLB cache accesses ( +- 0.07% ) (41.12%)
39,307,810,241 L1-dcache-prefetches # 11.929 M/sec ( +- 0.06% ) (41.11%)
<not supported> L1-dcache-prefetch-misses

182.630 +- 0.365 seconds time elapsed ( +- 0.20% )

Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---

Notes:
Not sure if this wall of perf-stats (or indeed the whole kbuild test) is
warranted here.

To my eyes, there's no non-obvious information in the performance results
(reducing cache usage should and does lead to other processes getting a small
bump in performance), so is there any value in keeping this in the commit
message?

fs/hugetlbfs/inode.c | 7 ++++++-
mm/gup.c | 18 ++++++++++++++++++
mm/huge_memory.c | 2 +-
mm/hugetlb.c | 9 ++++++++-
4 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 62408047e8d7..993bb7227a2f 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -650,6 +650,7 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
loff_t hpage_size = huge_page_size(h);
unsigned long hpage_shift = huge_page_shift(h);
pgoff_t start, index, end;
+ bool hint_non_caching;
int error;
u32 hash;

@@ -667,6 +668,9 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
start = offset >> hpage_shift;
end = (offset + len + hpage_size - 1) >> hpage_shift;

+ /* Don't pollute the cache if we are fallocte'ing a large region. */
+ hint_non_caching = clear_page_prefer_non_caching((end - start) << hpage_shift);
+
inode_lock(inode);

/* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
@@ -745,7 +749,8 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
error = PTR_ERR(page);
goto out;
}
- clear_huge_page(page, addr, pages_per_huge_page(h));
+ clear_huge_page(page, addr, pages_per_huge_page(h),
+ hint_non_caching);
__SetPageUptodate(page);
error = huge_add_to_page_cache(page, mapping, index);
if (unlikely(error)) {
diff --git a/mm/gup.c b/mm/gup.c
index 551264407624..bceb6ff64687 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -944,6 +944,13 @@ static int faultin_page(struct vm_area_struct *vma,
*/
fault_flags |= FAULT_FLAG_TRIED;
}
+ if (*flags & FOLL_HINT_BULK) {
+ /*
+ * This page is part of a large region being faulted-in
+ * so attempt to minimize cache-pollution.
+ */
+ fault_flags |= FAULT_FLAG_NON_CACHING;
+ }
if (unshare) {
fault_flags |= FAULT_FLAG_UNSHARE;
/* FAULT_FLAG_WRITE and FAULT_FLAG_UNSHARE are incompatible */
@@ -1116,6 +1123,17 @@ static long __get_user_pages(struct mm_struct *mm,
if (!(gup_flags & FOLL_FORCE))
gup_flags |= FOLL_NUMA;

+ /*
+ * Non-cached page clearing is generally faster when clearing regions
+ * larger than O(LLC-size). So hint the non-caching path based on
+ * clear_page_prefer_non_caching().
+ *
+ * Note, however this check is optimistic -- nr_pages is the upper
+ * limit and we might be clearing less than that.
+ */
+ if (clear_page_prefer_non_caching(nr_pages * PAGE_SIZE))
+ gup_flags |= FOLL_HINT_BULK;
+
do {
struct page *page;
unsigned int foll_flags = gup_flags;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 73654db77a1c..c7294cffc384 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -594,7 +594,7 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
pgtable_t pgtable;
unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
vm_fault_t ret = 0;
- bool non_cached = false;
+ bool non_cached = vmf->flags & FAULT_FLAG_NON_CACHING;

VM_BUG_ON_PAGE(!PageCompound(page), page);

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0c4a31b5c1e9..d906c6558b15 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5481,7 +5481,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
spinlock_t *ptl;
unsigned long haddr = address & huge_page_mask(h);
bool new_page, new_pagecache_page = false;
- bool non_cached = false;
+ bool non_cached = flags & FAULT_FLAG_NON_CACHING;

/*
* Currently, we are forced to kill the process in the event the
@@ -6182,6 +6182,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
*/
fault_flags |= FAULT_FLAG_TRIED;
}
+ if (flags & FOLL_HINT_BULK) {
+ /*
+ * From the user hint, we might be faulting-in
+ * a large region so minimize cache-pollution.
+ */
+ fault_flags |= FAULT_FLAG_NON_CACHING;
+ }
ret = hugetlb_fault(mm, vma, vaddr, fault_flags);
if (ret & VM_FAULT_ERROR) {
err = vm_fault_to_errno(ret, flags);
--
2.31.1

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