[PATCH v2 0/3] mm/gup: Fix pin page write cache bouncing on has_pinned

From: Peter Xu
Date: Fri May 07 2021 - 11:06:25 EST

v2:
- patch 1: rename s/threads/nthreads/; assert() on pthread create/destroy [John]
- patch 2:
- rewrite commit message [John, Linus]
- use parentheses [Linus]
- patch 3:
- define mm_set_has_pinned_flag() helper and use it [John, Linus, Matthew]
- keep has_pinned comment but move to MMF_HAS_PINNED [John]

This series contains 3 patches, the 1st one enables threading for gup_benchmark
in the kselftest. The latter two patches are collected from Andrea's local
branch which can fix write cache bouncing issue with pinning fast-gup.

To be explicit on the latter two patches:

- the 2nd patch fixes the perf degrade when introducing has_pinned, then

- the last patch tries to remove the has_pinned with a bit in mm->flags

For patch 3: originally I think we had a plan to reuse has_pinned into a
counter very soon, however that's not happening at least until today, so maybe
it proves that we can remove it until we really want such a counter for
whatever reason. As the commit message stated, it saves 4 bytes for each mm
without observable regressions.

Regarding testing: we can reference to the commit message of patch 2 for some
detailed testing with will-is-scale. Meanwhile I did patch 1 just because then
we can even easily verify the patchset using the existing kselftest facilities
or even regress test it in the future with the repo if we want.

Below numbers are extra verification tests that I did besides commit message of
patch 2 using the new gup_benchmark and 256 cpus. Below test is done on 40
cpus host with Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz, and I can get similar
result (of course the write cache bouncing get severe with even more cores).

After patch 1 applied (only test patch, so using old kernel):

$ sudo chrt -f 1 ./gup_test -a -m 512 -j 40
PIN_FAST_BENCHMARK: Time: get:459632 put:5990 us
PIN_FAST_BENCHMARK: Time: get:461967 put:5840 us
PIN_FAST_BENCHMARK: Time: get:464521 put:6140 us
PIN_FAST_BENCHMARK: Time: get:465176 put:7100 us
PIN_FAST_BENCHMARK: Time: get:465960 put:6733 us
PIN_FAST_BENCHMARK: Time: get:465324 put:6781 us
PIN_FAST_BENCHMARK: Time: get:466018 put:7130 us
PIN_FAST_BENCHMARK: Time: get:466362 put:7118 us
PIN_FAST_BENCHMARK: Time: get:465118 put:6975 us
PIN_FAST_BENCHMARK: Time: get:466422 put:6602 us
PIN_FAST_BENCHMARK: Time: get:465791 put:6818 us
PIN_FAST_BENCHMARK: Time: get:467091 put:6298 us
PIN_FAST_BENCHMARK: Time: get:467694 put:5432 us
PIN_FAST_BENCHMARK: Time: get:469575 put:5581 us
PIN_FAST_BENCHMARK: Time: get:468124 put:6055 us
PIN_FAST_BENCHMARK: Time: get:468877 put:6720 us
PIN_FAST_BENCHMARK: Time: get:467212 put:4961 us
PIN_FAST_BENCHMARK: Time: get:467834 put:6697 us
PIN_FAST_BENCHMARK: Time: get:470778 put:6398 us
PIN_FAST_BENCHMARK: Time: get:469788 put:6310 us
PIN_FAST_BENCHMARK: Time: get:488277 put:7113 us
PIN_FAST_BENCHMARK: Time: get:486613 put:7085 us
PIN_FAST_BENCHMARK: Time: get:486940 put:7202 us
PIN_FAST_BENCHMARK: Time: get:488728 put:7101 us
PIN_FAST_BENCHMARK: Time: get:487570 put:7327 us
PIN_FAST_BENCHMARK: Time: get:489260 put:7027 us
PIN_FAST_BENCHMARK: Time: get:488846 put:6866 us
PIN_FAST_BENCHMARK: Time: get:488521 put:6745 us
PIN_FAST_BENCHMARK: Time: get:489950 put:6459 us
PIN_FAST_BENCHMARK: Time: get:489777 put:6617 us
PIN_FAST_BENCHMARK: Time: get:488224 put:6591 us
PIN_FAST_BENCHMARK: Time: get:488644 put:6477 us
PIN_FAST_BENCHMARK: Time: get:488754 put:6711 us
PIN_FAST_BENCHMARK: Time: get:488875 put:6743 us
PIN_FAST_BENCHMARK: Time: get:489290 put:6657 us
PIN_FAST_BENCHMARK: Time: get:490264 put:6684 us
PIN_FAST_BENCHMARK: Time: get:489631 put:6737 us
PIN_FAST_BENCHMARK: Time: get:488434 put:6655 us
PIN_FAST_BENCHMARK: Time: get:492213 put:6297 us
PIN_FAST_BENCHMARK: Time: get:491124 put:6173 us

After the whole series applied (new fixed kernel):

$ sudo chrt -f 1 ./gup_test -a -m 512 -j 40
PIN_FAST_BENCHMARK: Time: get:82038 put:7041 us
PIN_FAST_BENCHMARK: Time: get:82144 put:6817 us
PIN_FAST_BENCHMARK: Time: get:83417 put:6674 us
PIN_FAST_BENCHMARK: Time: get:82540 put:6594 us
PIN_FAST_BENCHMARK: Time: get:83214 put:6681 us
PIN_FAST_BENCHMARK: Time: get:83444 put:6889 us
PIN_FAST_BENCHMARK: Time: get:83194 put:7499 us
PIN_FAST_BENCHMARK: Time: get:84876 put:7369 us
PIN_FAST_BENCHMARK: Time: get:86092 put:10289 us
PIN_FAST_BENCHMARK: Time: get:86153 put:10415 us
PIN_FAST_BENCHMARK: Time: get:85026 put:7751 us
PIN_FAST_BENCHMARK: Time: get:85458 put:7944 us
PIN_FAST_BENCHMARK: Time: get:85735 put:8154 us
PIN_FAST_BENCHMARK: Time: get:85851 put:8299 us
PIN_FAST_BENCHMARK: Time: get:86323 put:9617 us
PIN_FAST_BENCHMARK: Time: get:86288 put:10496 us
PIN_FAST_BENCHMARK: Time: get:87697 put:9346 us
PIN_FAST_BENCHMARK: Time: get:87980 put:8382 us
PIN_FAST_BENCHMARK: Time: get:88719 put:8400 us
PIN_FAST_BENCHMARK: Time: get:87616 put:8588 us
PIN_FAST_BENCHMARK: Time: get:86730 put:9563 us
PIN_FAST_BENCHMARK: Time: get:88167 put:8673 us
PIN_FAST_BENCHMARK: Time: get:86844 put:9777 us
PIN_FAST_BENCHMARK: Time: get:88068 put:11774 us
PIN_FAST_BENCHMARK: Time: get:86170 put:15676 us
PIN_FAST_BENCHMARK: Time: get:87967 put:12827 us
PIN_FAST_BENCHMARK: Time: get:95773 put:7652 us
PIN_FAST_BENCHMARK: Time: get:87734 put:13650 us
PIN_FAST_BENCHMARK: Time: get:89833 put:14237 us
PIN_FAST_BENCHMARK: Time: get:96186 put:8029 us
PIN_FAST_BENCHMARK: Time: get:95532 put:8886 us
PIN_FAST_BENCHMARK: Time: get:95351 put:5826 us
PIN_FAST_BENCHMARK: Time: get:96401 put:8407 us
PIN_FAST_BENCHMARK: Time: get:96473 put:8287 us
PIN_FAST_BENCHMARK: Time: get:97177 put:8430 us
PIN_FAST_BENCHMARK: Time: get:98120 put:5263 us
PIN_FAST_BENCHMARK: Time: get:96271 put:7757 us
PIN_FAST_BENCHMARK: Time: get:99628 put:10467 us
PIN_FAST_BENCHMARK: Time: get:99344 put:10045 us
PIN_FAST_BENCHMARK: Time: get:94212 put:15485 us

Summary:

Old kernel: 477729.97 (+-3.79%)
New kernel: 89144.65 (+-11.76%)

I'm not sure whether I should add Fixes for patch 2. If to add it'll be:

Fixes: 008cfe4418b3d ("mm: Introduce mm_struct.has_pinned")

Then cc stable for 5.9+. However I'll skip adding it if no one asks, as this
is a perf fix, and frequent+concurrent pinning should not really happen that much.

Please review, thanks.

Andrea Arcangeli (2):
mm: gup: allow FOLL_PIN to scale in SMP
mm: gup: pack has_pinned in MMF_HAS_PINNED

Peter Xu (1):
mm/gup_benchmark: Support threading

fs/proc/task_mmu.c | 2 +-
include/linux/mm.h | 2 +-
include/linux/mm_types.h | 10 ---
include/linux/sched/coredump.h | 8 +++
kernel/fork.c | 1 -
mm/gup.c | 15 ++++-
tools/testing/selftests/vm/gup_test.c | 96 ++++++++++++++++++---------
7 files changed, 88 insertions(+), 46 deletions(-)

--
2.31.1