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([2620:10d:c090:500::5:58db]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-22410a7f700sm543275ad.144.2025.03.05.17.07.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 05 Mar 2025 17:07:06 -0800 (PST) Message-ID: Date: Wed, 5 Mar 2025 17:07:04 -0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: JP Kobryn Subject: Re: [PATCH 0/4 v2] cgroup: separate rstat trees To: =?UTF-8?Q?Michal_Koutn=C3=BD?= Cc: tj@kernel.org, shakeel.butt@linux.dev, yosryahmed@google.com, mhocko@kernel.org, hannes@cmpxchg.org, akpm@linux-foundation.org, linux-mm@kvack.org, cgroups@vger.kernel.org, kernel-team@meta.com References: <20250227215543.49928-1-inwardvessel@gmail.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 3/3/25 7:19 AM, Michal Koutný wrote: > Hello JP. > > On Thu, Feb 27, 2025 at 01:55:39PM -0800, inwardvessel wrote: >> From: JP Kobryn >> >> The current design of rstat takes the approach that if one subsystem is >> to be flushed, all other subsystems with pending updates should also be >> flushed. It seems that over time, the stat-keeping of some subsystems >> has grown in size to the extent that they are noticeably slowing down >> others. This has been most observable in situations where the memory >> controller is enabled. One big area where the issue comes up is system >> telemetry, where programs periodically sample cpu stats. It would be a >> benefit for programs like this if the overhead of having to flush memory >> stats (and others) could be eliminated. It would save cpu cycles for >> existing cpu-based telemetry programs and improve scalability in terms >> of sampling frequency and volume of hosts. > >> This series changes the approach of "flush all subsystems" to "flush >> only the requested subsystem". > ... > >> before: >> sizeof(struct cgroup_rstat_cpu) =~ 176 bytes /* can vary based on config */ >> >> nr_cgroups * sizeof(struct cgroup_rstat_cpu) >> nr_cgroups * 176 bytes >> >> after: > ... >> nr_cgroups * (176 + 16 * 2) >> nr_cgroups * 208 bytes > > ~ 32B/cgroup/cpu Thanks. I'll make this clear in the cover letter next rev. > >> With regard to validation, there is a measurable benefit when reading >> stats with this series. A test program was made to loop 1M times while >> reading all four of the files cgroup.stat, cpu.stat, io.stat, >> memory.stat of a given parent cgroup each iteration. This test program >> has been run in the experiments that follow. > > Thanks for looking into this and running experiments on the behavior of > split rstat trees. And thank you for reviewing along with the good questions. > >> The first experiment consisted of a parent cgroup with memory.swap.max=0 >> and memory.max=1G. On a 52-cpu machine, 26 child cgroups were created >> and within each child cgroup a process was spawned to frequently update >> the memory cgroup stats by creating and then reading a file of size 1T >> (encouraging reclaim). The test program was run alongside these 26 tasks >> in parallel. The results showed a benefit in both time elapsed and perf >> data of the test program. >> >> time before: >> real 0m44.612s >> user 0m0.567s >> sys 0m43.887s >> >> perf before: >> 27.02% mem_cgroup_css_rstat_flush >> 6.35% __blkcg_rstat_flush >> 0.06% cgroup_base_stat_cputime_show >> >> time after: >> real 0m27.125s >> user 0m0.544s >> sys 0m26.491s > > So this shows that flushing rstat trees one by one (as the test program > reads *.stat) is quicker than flushing all at once (+idle reads of > *.stat). > Interesting, I'd not bet on that at first but that is convincing to > favor the separate trees approach. > >> perf after:mem_cgroup_css_rstat_flush >> 6.03% mem_cgroup_css_rstat_flush >> 0.37% blkcg_print_stat >> 0.11% cgroup_base_stat_cputime_show > > I'd understand why the series reduces time spent in > mem_cgroup_flush_stats() but what does the lower proportion of > mem_cgroup_css_rstat_flush() show? When the entry point for flushing is reading the file memory.stat, memory_stat_show() is called which leads to __mem_cgroup_flush_stats(). In this function, there is an early return when (!force && !needs_flush) is true. This opportunity to "skip" a flush is not reached when another subsystem has initiated the flush and entry point for flushing memory is css->css_rstat_flush(). To verify above, I made use of a tracepoint previously added [0] to get info info on the number of memcg flushes performed vs skipped. In a comparison between reading only the memory.stat file vs reading {memory,io,cpu}.stat files under the same test, the flush count increased by about the same value the skip count decreased. Reading memory.stat non-forced flushes: 5781 flushes skipped: 995826 Reading {memory,io.cpu}.stat non-forced flushes: 12047 flushes skipped: 990857 If the flushes were not skipped, I think we would see similar proportion of mem_cgroup_css_rstat_flush() when reading memory.stat. [0] https://lore.kernel.org/all/20241029021106.25587-1-inwardvessel@gmail.com/ > > >> Another experiment was setup on the same host using a parent cgroup with >> two child cgroups. The same swap and memory max were used as the >> previous experiment. In the two child cgroups, kernel builds were done >> in parallel, each using "-j 20". The perf comparison of the test program >> was very similar to the values in the previous experiment. The time >> comparison is shown below. >> >> before: >> real 1m2.077s >> user 0m0.784s >> sys 1m0.895s > > This is 1M loops of stats reading program like before? I.e. if this > should be analogous to 0m44.612s above why isn't it same? (I'm thinking > of more frequent updates in the latter test.) Yes. One notable difference on this test is there are more threads in the workload (40 vs 26) which are doing the updates. > >> after: >> real 0m32.216s >> user 0m0.709s >> sys 0m31.256s > > What was impact on the kernel build workloads (cgroup_rstat_updated)? You can now find some workload timing results further down. If you're asking specifically about time spent in cgroup_rstat_updated(), perf reports show fractional values on both sides. > > (Perhaps the saved 30s of CPU work (if potentially moved from readers to > writers) would be spread too thin in all of two 20-parallel kernel > builds, right?) Are you suggesting a workload with fewer threads? > > ... >> For the final experiment, perf events were recorded during a kernel >> build with the same host and cgroup setup. The builds took place in the >> child node. Control and experimental sides both showed similar in cycles >> spent on cgroup_rstat_updated() and appeard insignificant compared among >> the events recorded with the workload. > > What's the change between control vs experiment? Runnning in root cg vs > nested? Or running without *.stat readers vs with them against the > kernel build? > (This clarification would likely answer my question above.) > workload control with no readers: real 6m54.818s user 117m3.122s sys 5m4.996s workload experiment with no readers: real 6m54.862s user 117m12.812s sys 5m0.943s workload control with constant readers {memory,io,cpu,cgroup}.stat: real 6m59.468s user 118m26.981s sys 5m20.163s workload experiment with constant readers {memory,io,cpu,cgroup}.stat: real 6m57.031s user 118m13.833s sys 5m3.454s These tests were done in a child (nested) cgroup. Were you also asking for a root vs nested experiment or were you just needing clarification on the test details? > > Michal