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From: Roman Gushchin To: Kirill Tkhai Cc: Mike Rapoport , Qi Zheng , Andrew Morton , hannes@cmpxchg.org, shakeelb@google.com, mhocko@kernel.org, muchun.song@linux.dev, david@redhat.com, shy828301@gmail.com, sultan@kerneltoast.com, dave@stgolabs.net, penguin-kernel@i-love.sakura.ne.jp, paulmck@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 0/8] make slab shrink lockless Message-ID: References: <20230226144655.79778-1-zhengqi.arch@bytedance.com> <20230226115100.7e12bda7931dd65dbabcebe3@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 70D4D10000D X-Rspam-User: X-Stat-Signature: qzgiw61dj1ic6mkwxz8cz8hhr7yyzhqp X-HE-Tag: 1677526362-81400 X-HE-Meta: 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 4IfhH5Yy B6PoJB5+Yf9opYcHL46HkD9wM1Su0UqqHfsA8/x5bdFtda5DgoymCehASoSUq0DVpourKfgeGyppJjTEjX101cxosIHQ6jyv9DX/oLVQAUQ3gPAGG0UMye++6Gzn+PHYVUbfvvSYuacmOeNsiDd83TILgoL5YZJitWrPN X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Feb 27, 2023 at 10:20:59PM +0300, Kirill Tkhai wrote: > On 27.02.2023 18:08, Mike Rapoport wrote: > > Hi, > > > > On Mon, Feb 27, 2023 at 09:31:51PM +0800, Qi Zheng wrote: > >> > >> > >> On 2023/2/27 03:51, Andrew Morton wrote: > >>> On Sun, 26 Feb 2023 22:46:47 +0800 Qi Zheng wrote: > >>> > >>>> Hi all, > >>>> > >>>> This patch series aims to make slab shrink lockless. > >>> > >>> What an awesome changelog. > >>> > >>>> 2. Survey > >>>> ========= > >>> > >>> Especially this part. > >>> > >>> Looking through all the prior efforts and at this patchset I am not > >>> immediately seeing any statements about the overall effect upon > >>> real-world workloads. For a good example, does this patchset > >>> measurably improve throughput or energy consumption on your servers? > >> > >> Hi Andrew, > >> > >> I re-tested with the following physical machines: > >> > >> Architecture: x86_64 > >> CPU(s): 96 > >> On-line CPU(s) list: 0-95 > >> Model name: Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz > >> > >> I found that the reason for the hotspot I described in cover letter is > >> wrong. The reason for the down_read_trylock() hotspot is not because of > >> the failure to trylock, but simply because of the atomic operation > >> (cmpxchg). And this will lead to a significant reduction in IPC (insn > >> per cycle). > > > > ... > > > >> 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 > >> > >> 2. 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. > > > > The results you present do show improvement in IPC for an artificial test > > script. But more interesting would be to see how a real world workloads > > benefit from your changes. > > 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. > > This patch patchset makes prealloc_memcg_shrinker() independent of do_shrink_slab(), > so prealloc_memcg_shrinker() won't have to wait till shrink_slab_memcg() completes its > current bit iteration, sees rwsem_is_contended() and the iteration breaks. > > Also, it's important to mention that currently we have the strange behavior: > > prealloc_memcg_shrinker() > down_write(&shrinker_rwsem) > idr_alloc() > reclaim > for each child memcg > shrink_slab_memcg() > down_read_trylock(&shrinker_rwsem) -> fail But this can happen only if we get -ENOMEM in idr_alloc()? Doesn't seem to be a very hot path. Thanks!