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From: Shakeel Butt To: Greg Thelen Cc: Tejun Heo , Johannes Weiner , Michal =?utf-8?Q?Koutn=C3=BD?= , Andrew Morton , Eric Dumazet , Yosry Ahmed , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Eric Dumazet Subject: Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num_cpu) Message-ID: <54wer7lbgg4mgxv7ky5zzsgjv2vi4diu7clvcklxgmrp2u4gvn@tr2twe5xdtgt> References: <20250319071330.898763-1-gthelen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250319071330.898763-1-gthelen@google.com> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: E211020006 X-Stat-Signature: obkedw1jphdr5fwxdmtk88hz46374dps X-HE-Tag: 1742406828-742504 X-HE-Meta: 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 cN+ObXaE 5i7O/OOMCZRb/em64YJxpsAB5QhEBDx2oNdRwDjI35jHUlKg+jX1TgXzh5R6N0IwxgSN5K4+/r696HEcpp3rtOQZCxSRyldUm6OJbOi4jt8RJi45PkP0F9ZKofDpNp0otEo6uT5UBf0nqW0KS/3cs+JK+NmQOHjj/pISQKnONZaECaXAKqr7no3H0y83+JlwwhwEyfcssW6kAxL5oYxEbvqgEZt3cJcktiD5ehHukzwmPPUI0tG5vGr+HgyPfkUFKGjrGIzCl4rnUty+10D2poCG0KUrNoGuULujlDnjeazZpFF1Nh8SjDYV3LQzkIvOgx4zsE/8kfNgwZJz3ZKySTzmfXw== 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: List-Subscribe: List-Unsubscribe: On Wed, Mar 19, 2025 at 12:13:30AM -0700, Greg Thelen wrote: > From: Eric Dumazet > > cgroup_rstat_flush_locked() grabs the irq safe cgroup_rstat_lock while > iterating all possible cpus. It only drops the lock if there is > scheduler or spin lock contention. If neither, then interrupts can be > disabled for a long time. On large machines this can disable interrupts > for a long enough time to drop network packets. On 400+ CPU machines > I've seen interrupt disabled for over 40 msec. Which kernel was this observed on in production? > > Prevent rstat from disabling interrupts while processing all possible > cpus. Instead drop and reacquire cgroup_rstat_lock for each cpu. Doing for each cpu might be too extreme. Have you tried with some batching? > This > approach was previously discussed in > https://lore.kernel.org/lkml/ZBz%2FV5a7%2F6PZeM7S@slm.duckdns.org/, > though this was in the context of an non-irq rstat spin lock. > > Benchmark this change with: > 1) a single stat_reader process with 400 threads, each reading a test > memcg's memory.stat repeatedly for 10 seconds. > 2) 400 memory hog processes running in the test memcg and repeatedly > charging memory until oom killed. Then they repeat charging and oom > killing. > Though this benchmark seems too extreme but userspace holding off irqs for that long time is bad. BTW are these memory hoggers, creating anon memory or file memory? Is [z]swap enabled? For the long term, I think we can use make this work without disabling irqs, similar to how networking manages sock lock. > v6.14-rc6 with CONFIG_IRQSOFF_TRACER with stat_reader and hogs, finds > interrupts are disabled by rstat for 45341 usec: > # => started at: _raw_spin_lock_irq > # => ended at: cgroup_rstat_flush > # > # > # _------=> CPU# > # / _-----=> irqs-off/BH-disabled > # | / _----=> need-resched > # || / _---=> hardirq/softirq > # ||| / _--=> preempt-depth > # |||| / _-=> migrate-disable > # ||||| / delay > # cmd pid |||||| time | caller > # \ / |||||| \ | / > stat_rea-96532 52d.... 0us*: _raw_spin_lock_irq > stat_rea-96532 52d.... 45342us : cgroup_rstat_flush > stat_rea-96532 52d.... 45342us : tracer_hardirqs_on <-cgroup_rstat_flush > stat_rea-96532 52d.... 45343us : > => memcg1_stat_format > => memory_stat_format > => memory_stat_show > => seq_read_iter > => vfs_read > => ksys_read > => do_syscall_64 > => entry_SYSCALL_64_after_hwframe > > With this patch the CONFIG_IRQSOFF_TRACER doesn't find rstat to be the > longest holder. The longest irqs-off holder has irqs disabled for > 4142 usec, a huge reduction from previous 45341 usec rstat finding. > > Running stat_reader memory.stat reader for 10 seconds: > - without memory hogs: 9.84M accesses => 12.7M accesses > - with memory hogs: 9.46M accesses => 11.1M accesses > The throughput of memory.stat access improves. > > The mode of memory.stat access latency after grouping by of 2 buckets: > - without memory hogs: 64 usec => 16 usec > - with memory hogs: 64 usec => 8 usec > The memory.stat latency improves. So, things are improving even without batching. I wonder if there are less readers then how will this look like. Can you try with single reader as well? > > Signed-off-by: Eric Dumazet > Signed-off-by: Greg Thelen > Tested-by: Greg Thelen Reviewed-by: Shakeel Butt