| Message ID | 20230307065605.58209-1-zhengqi.arch@bytedance.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | make slab shrink lockless | expand |
On Tue, 7 Mar 2023 14:55:57 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote: > Hi all, > > This patch series aims to make slab shrink lockless. The v3 discussion did contain requests for some sort of measurements of real-world workloads. And Kirill did suggest a workload which could be used for this measurement. It's quite important that we have this info, please. I mean, speeding up real-world workloads is the entire point of the patchset and without measurements, we don't know if the patchset achieves its primary objective! > 3. Reproduction and testing > =========================== > > We can reproduce the down_read_trylock() hotspot through the following script: > > ``` > #!/bin/bash > > DIR="/root/shrinker/memcg/mnt" > > do_create() > { > mkdir -p /sys/fs/cgroup/memory/test > mkdir -p /sys/fs/cgroup/perf_event/test > echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes > for i in `seq 0 $1`; > do > mkdir -p /sys/fs/cgroup/memory/test/$i; > echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs; > echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs; > mkdir -p $DIR/$i; > done > } > > do_mount() > { > for i in `seq $1 $2`; > do > mount -t tmpfs $i $DIR/$i; > done > } > > do_touch() > { > for i in `seq $1 $2`; > do > echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs; > echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs; > dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 & > done > } > > case "$1" in > touch) > do_touch $2 $3 > ;; > test) > do_create 4000 > do_mount 0 4000 > do_touch 0 3000 > ;; > *) > exit 1 > ;; > esac > ``` > > Save the above script, then run test and touch commands. Then we can use the > following perf command to view hotspots: Well. Simply runnimg time that-script before and after and including the info in the changelog would be a start?
Hi Andrew, On 2023/3/8 06:20, Andrew Morton wrote: > On Tue, 7 Mar 2023 14:55:57 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote: > >> Hi all, >> >> This patch series aims to make slab shrink lockless. > > The v3 discussion did contain requests for some sort of measurements of > real-world workloads. And Kirill did suggest a workload which could be > used for this measurement. > > It's quite important that we have this info, please. I mean, speeding > up real-world workloads is the entire point of the patchset and without > measurements, we don't know if the patchset achieves its primary > objective! I agree with this. For the down_read_trylock() hotspot problem I encountered, I posted a reproduction program in the cover letter, and measured the change of IPC before and after applying the patchset. For the case mentioned by Kirill, theoretically there is no competition between slab shrink and register_shrinker() after applying this patchset. But I haven't found a way to reproduce it yet, I will continue to try to do it. > > >> 3. Reproduction and testing >> =========================== >> >> We can reproduce the down_read_trylock() hotspot through the following script: >> >> ``` >> #!/bin/bash >> >> DIR="/root/shrinker/memcg/mnt" >> >> do_create() >> { >> mkdir -p /sys/fs/cgroup/memory/test >> mkdir -p /sys/fs/cgroup/perf_event/test >> echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes >> for i in `seq 0 $1`; >> do >> mkdir -p /sys/fs/cgroup/memory/test/$i; >> echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs; >> echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs; >> mkdir -p $DIR/$i; >> done >> } >> >> do_mount() >> { >> for i in `seq $1 $2`; >> do >> mount -t tmpfs $i $DIR/$i; >> done >> } >> >> do_touch() >> { >> for i in `seq $1 $2`; >> do >> echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs; >> echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs; >> dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 & >> done >> } >> >> case "$1" in >> touch) >> do_touch $2 $3 >> ;; >> test) >> do_create 4000 >> do_mount 0 4000 >> do_touch 0 3000 >> ;; >> *) >> exit 1 >> ;; >> esac >> ``` >> >> Save the above script, then run test and touch commands. Then we can use the >> following perf command to view hotspots: > > Well. Simply runnimg > > time that-script I tried this, but the script process will be killed because of OOM, so the measured time is not accurate. I will continue to try to measure more data besides IPC. Thanks, Qi > > before and after and including the info in the changelog would be a start?
Hi all, This patch series aims to make slab shrink lockless. 1. Background ============= On our servers, we often find the following system cpu hotspots: 52.22% [kernel] [k] down_read_trylock 19.60% [kernel] [k] up_read 8.86% [kernel] [k] shrink_slab 2.44% [kernel] [k] idr_find 1.25% [kernel] [k] count_shadow_nodes 1.18% [kernel] [k] shrink lruvec 0.71% [kernel] [k] mem_cgroup_iter 0.71% [kernel] [k] shrink_node 0.55% [kernel] [k] find_next_bit And we used bpftrace to capture its calltrace as follows: @[ down_read_trylock+1 shrink_slab+128 shrink_node+371 do_try_to_free_pages+232 try_to_free_pages+243 _alloc_pages_slowpath+771 _alloc_pages_nodemask+702 pagecache_get_page+255 filemap_fault+1361 ext4_filemap_fault+44 __do_fault+76 handle_mm_fault+3543 do_user_addr_fault+442 do_page_fault+48 page_fault+62 ]: 1161690 @[ down_read_trylock+1 shrink_slab+128 shrink_node+371 balance_pgdat+690 kswapd+389 kthread+246 ret_from_fork+31 ]: 8424884 @[ down_read_trylock+1 shrink_slab+128 shrink_node+371 do_try_to_free_pages+232 try_to_free_pages+243 __alloc_pages_slowpath+771 __alloc_pages_nodemask+702 __do_page_cache_readahead+244 filemap_fault+1674 ext4_filemap_fault+44 __do_fault+76 handle_mm_fault+3543 do_user_addr_fault+442 do_page_fault+48 page_fault+62 ]: 20917631 We can see that down_read_trylock() of shrinker_rwsem is being called with high frequency at that time. Because of the poor multicore scalability of atomic operations, this can lead to a significant drop in IPC (instructions per cycle). And more, the shrinker_rwsem is a global read-write lock in shrinkers subsystem, which protects most operations such as slab shrink, registration and unregistration of shrinkers, etc. This can easily cause problems in the following cases. 1) When the memory pressure is high and there are many filesystems mounted or unmounted at the same time, slab shrink will be affected (down_read_trylock() failed). Such as the real workload mentioned by Kirill Tkhai: ``` One of the real workloads from my experience is start of an overcommitted node containing many starting containers after node crash (or many resuming containers after reboot for kernel update). In these cases memory pressure is huge, and the node goes round in long reclaim. ``` 2) If a shrinker is blocked (such as the case mentioned in [1]) and a writer comes in (such as mount a fs), then this writer will be blocked and cause all subsequent shrinker-related operations to be blocked. [1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@virtuozzo.com/ All the above cases can be solved by replacing the shrinker_rwsem trylocks with SRCU. 2. Survey ========= Before doing the code implementation, I found that there were many similar submissions in the community: a. Davidlohr Bueso submitted a patch in 2015. Subject: [PATCH -next v2] mm: srcu-ify shrinkers Link: https://lore.kernel.org/all/1437080113.3596.2.camel@stgolabs.net/ Result: It was finally merged into the linux-next branch, but failed on arm allnoconfig (without CONFIG_SRCU) b. Tetsuo Handa submitted a patchset in 2017. Subject: [PATCH 1/2] mm,vmscan: Kill global shrinker lock. Link: https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/ Result: Finally chose to use the current simple way (break when rwsem_is_contended()). And Christoph Hellwig suggested to using SRCU, but SRCU was not unconditionally enabled at the time. c. Kirill Tkhai submitted a patchset in 2018. Subject: [PATCH RFC 00/10] Introduce lockless shrink_slab() Link: https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/ Result: At that time, SRCU was not unconditionally enabled, and there were some objections to enabling SRCU. Later, because Kirill's focus was moved to other things, this patchset was not continued to be updated. d. Sultan Alsawaf submitted a patch in 2021. Subject: [PATCH] mm: vmscan: Replace shrinker_rwsem trylocks with SRCU protection Link: https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@kerneltoast.com/ Result: Rejected because SRCU was not unconditionally enabled. We can find that almost all these historical commits were abandoned because SRCU was not unconditionally enabled. But now SRCU has been unconditionally enable by Paul E. McKenney in 2023 [2], so it's time to replace shrinker_rwsem trylocks with SRCU. [2] https://lore.kernel.org/lkml/20230105003759.GA1769545@paulmck-ThinkPad-P17-Gen-1/ 3. Reproduction and testing =========================== We can reproduce the down_read_trylock() hotspot through the following script: ``` #!/bin/bash DIR="/root/shrinker/memcg/mnt" do_create() { mkdir -p /sys/fs/cgroup/memory/test mkdir -p /sys/fs/cgroup/perf_event/test echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes for i in `seq 0 $1`; do mkdir -p /sys/fs/cgroup/memory/test/$i; echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs; echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs; mkdir -p $DIR/$i; done } do_mount() { for i in `seq $1 $2`; do mount -t tmpfs $i $DIR/$i; done } do_touch() { for i in `seq $1 $2`; do echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs; echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs; dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 & done } case "$1" in touch) do_touch $2 $3 ;; test) do_create 4000 do_mount 0 4000 do_touch 0 3000 ;; *) exit 1 ;; esac ``` Save the above script, then run test and touch commands. Then we can use the following perf command to view hotspots: perf top -U -F 999 1) Before applying this patchset: 32.31% [kernel] [k] down_read_trylock 19.40% [kernel] [k] pv_native_safe_halt 16.24% [kernel] [k] up_read 15.70% [kernel] [k] shrink_slab 4.69% [kernel] [k] _find_next_bit 2.62% [kernel] [k] shrink_node 1.78% [kernel] [k] shrink_lruvec 0.76% [kernel] [k] do_shrink_slab 2) After applying this patchset: 27.83% [kernel] [k] _find_next_bit 16.97% [kernel] [k] shrink_slab 15.82% [kernel] [k] pv_native_safe_halt 9.58% [kernel] [k] shrink_node 8.31% [kernel] [k] shrink_lruvec 5.64% [kernel] [k] do_shrink_slab 3.88% [kernel] [k] mem_cgroup_iter At the same time, we use the following perf command to capture IPC information: perf stat -e cycles,instructions -G test -a --repeat 5 -- sleep 10 1) Before applying this patchset: Performance counter stats for 'system wide' (5 runs): 454187219766 cycles test ( +- 1.84% ) 78896433101 instructions test # 0.17 insn per cycle ( +- 0.44% ) 10.0020430 +- 0.0000366 seconds time elapsed ( +- 0.00% ) 2) After applying this patchset: Performance counter stats for 'system wide' (5 runs): 841954709443 cycles test ( +- 15.80% ) (98.69%) 527258677936 instructions test # 0.63 insn per cycle ( +- 15.11% ) (98.68%) 10.01064 +- 0.00831 seconds time elapsed ( +- 0.08% ) We can see that IPC drops very seriously when calling down_read_trylock() at high frequency. After using SRCU, the IPC is at a normal level. This series is based on next-20230306. Comments and suggestions are welcome. Thanks, Qi. Changelog in v3 -> v4: - fix bug in [PATCH v3 1/7] - revise commit messages - rebase onto the next-20230306 Changelog in v2 -> v3: - fix bug in [PATCH v2 1/7] (per Kirill) - add Kirill's pacth which restore a check similar to the rwsem_is_contendent() check by adding shrinker_srcu_generation Changelog in v1 -> v2: - add a map_nr_max field to shrinker_info (suggested by Kirill) - use shrinker_mutex in reparent_shrinker_deferred() (pointed by Kirill) Kirill Tkhai (1): mm: vmscan: add shrinker_srcu_generation Qi Zheng (7): mm: vmscan: add a map_nr_max field to shrinker_info mm: vmscan: make global slab shrink lockless mm: vmscan: make memcg slab shrink lockless mm: shrinkers: make count and scan in shrinker debugfs lockless mm: vmscan: hold write lock to reparent shrinker nr_deferred mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers() mm: shrinkers: convert shrinker_rwsem to mutex drivers/md/dm-cache-metadata.c | 2 +- drivers/md/dm-thin-metadata.c | 2 +- fs/super.c | 2 +- include/linux/memcontrol.h | 1 + mm/shrinker_debug.c | 38 +++----- mm/vmscan.c | 166 +++++++++++++++++++-------------- 6 files changed, 112 insertions(+), 99 deletions(-)