On Wed, Oct 23, 2024 at 10:27 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Hi, Kairui, Hi Ying, > > Kairui Song <ryncsn@xxxxxxxxx> writes: > > > From: Kairui Song <kasong@xxxxxxxxxxx> > > > > This series improved the swap allocator performance greatly by reworking > > the locking design and simplify a lot of code path. > > > > This is follow up of previous swap cluster allocator series: > > https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-0-cb9c148b9297@xxxxxxxxxx/ > > > > And this series is based on an follow up fix of the swap cluster > > allocator: > > https://lore.kernel.org/linux-mm/20241022175512.10398-1-ryncsn@xxxxxxxxx/ > > > > This is part of the new swap allocator work item discussed in > > Chris's "Swap Abstraction" discussion at LSF/MM 2024, and > > "mTHP and swap allocator" discussion at LPC 2024. > > > > Previous series introduced a fully cluster based allocation algorithm, > > this series completely get rid of the old allocation path and makes the > > allocator avoid grabbing the si->lock unless needed. This bring huge > > performance gain and get rid of slot cache on freeing path. > > Great! > > > Currently, swap locking is mainly composed of two locks, cluster lock > > (ci->lock) and device lock (si->lock). The device lock is widely used > > to protect many things, causing it to be the main bottleneck for SWAP. > > Device lock can be confusing with another device lock for struct device. > Better to call it swap device lock? Good idea, I'll use the term swap device lock then. > > > Cluster lock is much more fine-grained, so it will be best to use > > ci->lock instead of si->lock as much as possible. > > > > `perf lock` indicates this issue clearly. Doing linux kernel build > > using tmpfs and ZRAM with limited memory (make -j64 with 1G memcg and 4k > > pages), result of "perf lock contention -ab sleep 3": > > > > contended total wait max wait avg wait type caller > > > > 34948 53.63 s 7.11 ms 1.53 ms spinlock free_swap_and_cache_nr+0x350 > > 16569 40.05 s 6.45 ms 2.42 ms spinlock get_swap_pages+0x231 > > 11191 28.41 s 7.03 ms 2.54 ms spinlock swapcache_free_entries+0x59 > > 4147 22.78 s 122.66 ms 5.49 ms spinlock page_vma_mapped_walk+0x6f3 > > 4595 7.17 s 6.79 ms 1.56 ms spinlock swapcache_free_entries+0x59 > > 406027 2.74 s 2.59 ms 6.74 us spinlock list_lru_add+0x39 > > ...snip... > > > > The top 5 caller are all users of si->lock, total wait time up sums to > > several minutes in the 3 seconds time window. > > Can you show results of `perf record -g`, `perf report -g` too? I have > interest to check hot spot shifting too. Sure. I think `perf lock` result is already good enough and cleaner. My test environment are mostly VM based so spinlock slow path may get offloaded to host, and can't be see by perf record, I collected following data after disabled paravirt spinlock: The time consumption and stack trace of a page fault before: - 78.45% 0.17% cc1 [kernel.kallsyms] [k] asm_exc_page_fault - 78.28% asm_exc_page_fault - 78.18% exc_page_fault - 78.17% do_user_addr_fault - 78.09% handle_mm_fault - 78.06% __handle_mm_fault - 69.69% do_swap_page - 55.87% alloc_swap_folio - 55.60% mem_cgroup_swapin_charge_folio - 55.48% charge_memcg - 55.45% try_charge_memcg - 55.36% try_to_free_mem_cgroup_pages - do_try_to_free_pages - 55.35% shrink_node - 55.27% shrink_lruvec - 55.13% try_to_shrink_lruvec - 54.79% evict_folios - 54.35% shrink_folio_list - 30.01% add_to_swap - 29.77% folio_alloc_swap - 29.50% get_swap_pages 25.03% queued_spin_lock_slowpath - 2.71% alloc_swap_scan_cluster 1.80% queued_spin_lock_slowpath + 0.89% __try_to_reclaim_swap - 1.74% swap_reclaim_full_clusters 1.74% queued_spin_lock_slowpath - 10.88% try_to_unmap_flush_dirty - 10.87% arch_tlbbatch_flush - 10.85% on_each_cpu_cond_mask smp_call_function_many_cond + 7.45% pageout + 2.71% try_to_unmap_flush + 1.90% try_to_unmap + 0.78% folio_referenced - 9.41% cluster_swap_free_nr - 9.39% free_swap_slot - 9.35% swapcache_free_entries 8.40% queued_spin_lock_slowpath 0.93% swap_entry_range_free - 3.61% swap_read_folio_bdev_sync - 3.55% submit_bio_wait - 3.51% submit_bio_noacct_nocheck + 3.46% __submit_bio + 7.71% do_pte_missing + 0.61% wp_page_copy The queued_spin_lock_slowpath above is the si->lock, and there are multiple users of it so the total overhead is higher than shown. After: - 75.05% 0.43% cc1 [kernel.kallsyms] [k] asm_exc_page_fault - 74.62% asm_exc_page_fault - 74.36% exc_page_fault - 74.34% do_user_addr_fault - 74.10% handle_mm_fault - 73.96% __handle_mm_fault - 67.55% do_swap_page - 45.92% alloc_swap_folio - 45.03% mem_cgroup_swapin_charge_folio - 44.58% charge_memcg - 44.44% try_charge_memcg - 44.12% try_to_free_mem_cgroup_pages - do_try_to_free_pages - 44.10% shrink_node - 43.86% shrink_lruvec - 41.92% try_to_shrink_lruvec - 40.67% evict_folios - 37.12% shrink_folio_list - 20.88% pageout + 20.02% swap_writepage + 0.72% shmem_writepage - 4.08% add_to_swap - 2.48% folio_alloc_swap - 2.12% __mem_cgroup_try_charge_swap - 1.47% swap_cgroup_record + 1.32% _raw_spin_lock_irqsave - 1.56% add_to_swap_cache - 1.04% xas_store + 1.01% workingset_update_node + 3.97% try_to_unmap_flush_dirty + 3.51% folio_referenced + 2.24% __remove_mapping + 1.16% try_to_unmap + 0.52% try_to_unmap_flush 2.50% queued_spin_lock_slowpath 0.79% scan_folios + 1.20% try_to_inc_max_seq + 1.92% lru_add_drain + 0.73% vma_alloc_folio_noprof - 9.81% swap_read_folio_bdev_sync - 9.61% submit_bio_wait + 9.49% submit_bio_noacct_nocheck - 8.06% cluster_swap_free_nr - 8.02% swap_entry_range_free + 3.92% __mem_cgroup_uncharge_swap + 2.90% zram_slot_free_notify 0.58% clear_shadow_from_swap_cache - 1.32% __folio_batch_add_and_move - 1.30% folio_batch_move_lru + 1.10% folio_lruvec_lock_irqsave spin_lock usage is much lower. I prefer the perf lock output as it shows the exact time and user of locks. > > > Following the new allocator design, many operation doesn't need to touch > > si->lock at all. We only need to take si->lock when doing operations > > across multiple clusters (eg. changing the cluster list), other > > operations only need to take ci->lock. So ideally allocator should > > always take ci->lock first, then, if needed, take si->lock. But due > > to historical reasons, ci->lock is used inside si->lock by design, > > causing lock inversion if we simply try to acquire si->lock after > > acquiring ci->lock. > > > > This series audited all si->lock usage, simplify legacy codes, eliminate > > usage of si->lock as much as possible by introducing new designs based > > on the new cluster allocator. > > > > Old HDD allocation codes are removed, cluster allocator is adapted > > with small changes for HDD usage, test is looking OK. > > I think that it's a good idea to remove HDD allocation specific code. > Can you check the performance of swapping to HDD? However, I understand > that many people have no HDD in hand. It's not hard to make cluster allocator work well with HDD in theory, see the commit "mm, swap: use a global swap cluster for non-rotation device". The testing is not very reliable though, I found HDD swap performance is very unstable because of the IO pattern of HDD, so it's just a best effort try. > > And this also removed slot cache for freeing path. The performance is > > better without it, and this enables other clean up and optimizations > > as discussed before: > > https://lore.kernel.org/all/CAMgjq7ACohT_uerSz8E_994ZZCv709Zor+43hdmesW_59W1BWw@xxxxxxxxxxxxxx/ > > > > After this series, lock contention on si->lock is nearly unobservable > > with `perf lock` with the same test above : > > > > contended total wait max wait avg wait type caller > > ... snip ... > > 91 204.62 us 4.51 us 2.25 us spinlock cluster_move+0x2e > > ... snip ... > > 47 125.62 us 4.47 us 2.67 us spinlock cluster_move+0x2e > > ... snip ... > > 23 63.15 us 3.95 us 2.74 us spinlock cluster_move+0x2e > > ... snip ... > > 17 41.26 us 4.58 us 2.43 us spinlock cluster_isolate_lock+0x1d > > ... snip ... > > > > cluster_move and cluster_isolate_lock are basically the only users > > of si->lock now, performance gain is huge with reduced LOC. > > > > Tests > > === > > > > Build kernel with defconfig on tmpfs with ZRAM as swap: > > --- > > > > Running a test matrix which is scaled up progressive for a intuitive result. > > The test are ran on top of tmpfs, using memory cgroup for memory limitation, > > on a 48c96t system. > > > > 12 test run for each case, it can be seen clearly that as concurrent job > > number goes higher the performance gain is higher, the performance is > > higher even with low concurrency. > > > > make -j<NR> | System Time (seconds) | Total Time (seconds) > > (NR / Mem / ZRAM) | (Before / After / Delta) | (Before / After / Delta) > > With 4k pages only: > > 6 / 192M / 3G | 5258 / 5235 / -0.3% | 1420 / 1414 / -0.3% > > 12 / 256M / 4G | 5518 / 5337 / -3.3% | 758 / 742 / -2.1% > > 24 / 384M / 5G | 7091 / 5766 / -18.7% | 476 / 422 / -11.3% > > 48 / 768M / 7G | 11139 / 5831 / -47.7% | 330 / 221 / -33.0% > > 96 / 1.5G / 10G | 21303 / 11353 / -46.7% | 283 / 180 / -36.4% > > With 64k mTHP: > > 24 / 512M / 5G | 5104 / 4641 / -18.7% | 376 / 358 / -4.8% > > 48 / 1G / 7G | 8693 / 4662 / -18.7% | 257 / 176 / -31.5% > > 96 / 2G / 10G | 17056 / 10263 / -39.8% | 234 / 169 / -27.8% > > How much is the swap in/out throughput before/after the change? This may not be too beneficial for typical throughput measurement: - For example doing the same test with brd will only show a ~20% performance improvement, still a big gain though. I think the si->lock spinlock wasting CPU cycles may effect CPU sensitive things like ZRAM even more. - And simple benchmarks which just do multiple sequential swaps in/out in multiple thread hardly stress the allocator. I haven't found a good benchmark to simulate random parallel IOs on SWAP yet, I can write one later. A more close to real word benchmark like build kernel test, or mysql/sysbench all showed great improment. > > When I worked on swap in/out performance before, the hot spot shifts from > swap related code to LRU lock and zone lock. Things may change a lot > now. > > If zram is used as swap device, the hot spot may become > compression/decompression after solving the swap lock contention. To > stress swap subsystem further, we may use a ram disk as swap. > Previously, we have used a simulated pmem device (backed by DRAM). That > can be setup as in, > > https://pmem.io/blog/2016/02/how-to-emulate-persistent-memory/ > > After creating the raw block device: /dev/pmem0, we can do > > $ mkswap /dev/pmem0 > $ swapon /dev/pmem0 > > Can you use something similar if necessary? I used to test with brd, as described above, I think using ZRAM with test simulating real workload is more useful. And I did include a Sequential SWAP test, the result is looking OK (no regression, minor to none improvement). I can have a try with the pmem setup later, I guess the result will be similar to brd test. > > > With more aggressive setup, it shows clearly both the performance and > > fragmentation are better: > > > > tiem make -j96 / 768M memcg, 4K pages, 10G ZRAM, on Intel 8255C * 2: > > (avg of 4 test run) > > Before: > > Sys time: 73578.30, Real time: 864.05 > > tiem make -j96 / 1G memcg, 4K pages, 10G ZRAM: > > After: (-54.7% sys time, -49.3% real time) > > Sys time: 33314.76, Real time: 437.67 > > > > time make -j96 / 1152M memcg, 64K mTHP, 10G ZRAM, on Intel 8255C * 2: > > (avg of 4 test run) > > Before: > > Sys time: 74044.85, Real time: 846.51 > > hugepages-64kB/stats/swpout: 1735216 > > hugepages-64kB/stats/swpout_fallback: 430333 > > After: (-51.4% sys time, -47.7% real time, -63.2% mTHP failure) > > Sys time: 35958.87, Real time: 442.69 > > hugepages-64kB/stats/swpout: 1866267 > > hugepages-64kB/stats/swpout_fallback: 158330 > > > > There is a up to 54.7% improvement for build kernel test, and lower > > fragmentation rate. Performance improvement should be even larger for > > micro benchmarks > > Very good result! > > > Build kernel with tinyconfig on tmpfs with HDD as swap: > > --- > > > > This test is similar to above, but HDD test is very noisy and slow, the > > deviation is huge, so just use tinyconfig instead and take the median test > > result of 3 test run, which looks OK: > > > > Before this series: > > 114.44user 29.11system 39:42.90elapsed 6%CPU > > 2901232inputs+0outputs (238877major+4227640minor)pagefaults > > > > After this commit: > > 113.90user 23.81system 38:11.77elapsed 6%CPU > > 2548728inputs+0outputs (235471major+4238110minor)pagefaults > > > > Single thread SWAP: > > --- > > > > Sequential SWAP should also be slightly faster as we removed a lot of > > unnecessary parts. Test using micro benchmark for swapout/in 4G > > zero memory using ZRAM, 10 test runs: > > > > Swapout Before (avg. 3359304): > > 3353796 3358551 3371305 3356043 3367524 3355303 3355924 3354513 3360776 > > > > Swapin Before (avg. 1928698): > > 1920283 1927183 1934105 1921373 1926562 1938261 1927726 1928636 1934155 > > > > Swapout After (avg. 3347511, -0.4%): > > 3337863 3347948 3355235 3339081 3333134 3353006 3354917 3346055 3360359 > > > > Swapin After (avg. 1922290, -0.3%): > > 1919101 1925743 1916810 1917007 1923930 1935152 1917403 1923549 1921913 > > > > Worth noticing the patch "mm, swap: use a global swap cluster for > > non-rotation device" introduced minor overhead for certain tests (see > > the test results in commit message), but the gain from later commit > > covered that, it can be further improved later. > > > > Suggested-by: Chris Li <chrisl@xxxxxxxxxx> > > Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx> > > > > Kairui Song (13): > > mm, swap: minor clean up for swap entry allocation > > mm, swap: fold swap_info_get_cont in the only caller > > mm, swap: remove old allocation path for HDD > > mm, swap: use cluster lock for HDD > > mm, swap: clean up device availability check > > mm, swap: clean up plist removal and adding > > mm, swap: hold a reference of si during scan and clean up flags > > mm, swap: use an enum to define all cluster flags and wrap flags > > changes > > mm, swap: reduce contention on device lock > > mm, swap: simplify percpu cluster updating > > mm, swap: introduce a helper for retrieving cluster from offset > > mm, swap: use a global swap cluster for non-rotation device > > mm, swap_slots: remove slot cache for freeing path > > > > fs/btrfs/inode.c | 1 - > > fs/iomap/swapfile.c | 1 - > > include/linux/swap.h | 36 +- > > include/linux/swap_slots.h | 3 - > > mm/page_io.c | 1 - > > mm/swap_slots.c | 78 +-- > > mm/swapfile.c | 1198 ++++++++++++++++-------------------- > > 7 files changed, 558 insertions(+), 760 deletions(-) > > -- > Best Regards, > Huang, Ying
