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b=QSBgerpIj8TeFS3NY4mN5Ig1k5xpEmiGHvhpHEUyW4FLs5rjK5xn6cubu5TXhAI9M e5a0mNnqSPlzP+wYNxUzLae0E9sWPC9/JC92I56naUDCSjrvOpAsMx3OorqHip+jmW COfTa5yCrPTxGWduszOM8WDrdOLefxBinc3VapKh8TlLbGgRdn6beiVH+R/RDpPyD5 dn4Wu+TGuHkwh0PVBOA39aIHrrAbzGbDA6Tdd+Ny0Tz2vXphlpPuzGutJGlEkgBTve V+rQMbULFii+BBakZdR5ZOELSUqD9fLtkbjTInPKPS1aqOGSHeUbqd/xxCDfZQTlgX p2IdxONIexR2A== Message-ID: <6392f7e8-d14c-40f4-8a19-110dfffb9707@kernel.org> Date: Fri, 19 Apr 2024 15:15:01 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/3] cgroup/rstat: convert cgroup_rstat_lock back to mutex To: Yosry Ahmed , Shakeel Butt Cc: tj@kernel.org, hannes@cmpxchg.org, lizefan.x@bytedance.com, cgroups@vger.kernel.org, longman@redhat.com, netdev@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@cloudflare.com, Arnaldo Carvalho de Melo , Sebastian Andrzej Siewior , mhocko@kernel.org References: <171328983017.3930751.9484082608778623495.stgit@firesoul> <171328989335.3930751.3091577850420501533.stgit@firesoul> <651a52ac-b545-4b25-b82f-ad3a2a57bf69@kernel.org> Content-Language: en-US From: Jesper Dangaard Brouer In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 2502514002B X-Rspam-User: X-Stat-Signature: 6kne8ke4f3bp6zfhd7k67y77q19cikw8 X-HE-Tag: 1713532506-420288 X-HE-Meta: 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 dSIG0gc7 3tlelOPnZIHzTO4ytfAZJNlY/3gtQAnNHtqZUxth5bZ9M6DimE4GjZVv5dZZPH3Dc2P8hLrJSbj9AkGRkhNZLK5O2p/CpKx2LdAjzzCixTkBTNz+Fi79yUr7Q+sRTxRTIp4D/FfS08okRcKky7bTwBpl1hmBxVojGHW09kpWUUiuyVIQtkknzYGrvguBsGQ7ZqkS3rSzsFeQUPfBJuiuKkUsmSZ+7hmYTIaJMTgRZdpAfIUcVNGqBcRqxd1AsSHbUT2QfAMBHS56rwIX9L8g44hxxATuvplPJBDzgSWczKhgQsEGwsLyPcUAp3A/wLk+tbLg8vSNFl5Rqp5AI85an8Kr98LKxumYpXhf4VOf0QTo81bh+jbVyelMGgwUQioIbZzVi9bTaDHETv5jdhIuynoPFHm03RdY6Ia34MDdwM6uMWKVCMrQPIVr6VxGv5Fy6LULN 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 18/04/2024 22.39, Yosry Ahmed wrote: > On Thu, Apr 18, 2024 at 7:49 AM Shakeel Butt wrote: >> >> On Thu, Apr 18, 2024 at 11:02:06AM +0200, Jesper Dangaard Brouer wrote: >>> >>> >>> On 18/04/2024 04.19, Yosry Ahmed wrote: >> [...] >>>> >>>> I will keep the high-level conversation about using the mutex here in >>>> the cover letter thread, but I am wondering why we are keeping the >>>> lock dropping logic here with the mutex? >>>> >>> >>> I agree that yielding the mutex in the loop makes less sense. >>> Especially since the raw_spin_unlock_irqrestore(cpu_lock, flags) call >>> will be a preemption point for my softirq. But I kept it because, we >>> are running a CONFIG_PREEMPT_VOLUNTARY kernel, so I still worried that >>> there was no sched point for other userspace processes while holding the >>> mutex, but I don't fully know the sched implication when holding a mutex. >>> >> >> Are the softirqs you are interested in, raised from the same cpu or >> remote cpu? What about local_softirq_pending() check in addition to >> need_resched() and spin_needbreak() checks? If softirq can only be >> raised on local cpu then convert the spin_lock to non-irq one (Please >> correct me if I am wrong but on return from hard irq and not within bh >> or irq disabled spin_lock, the kernel will run the pending softirqs, >> right?). Did you get the chance to test these two changes or something >> similar in your prod environment? > > I tried making the spinlock a non-irq lock before, but Tejun objected [1]. > [1] https://lore.kernel.org/lkml/ZBz%2FV5a7%2F6PZeM7S@slm.duckdns.org/ > After reading [1], I think using a mutex is a better approach (than non-irq spinlock). > Perhaps we could experiment with always dropping the lock at CPU > boundaries instead? > I don't think this will be enough (always dropping the lock at CPU boundaries). My measured "lock-hold" times that is blocking IRQ (and softirq) for too long. When looking at prod with my new cgroup tracepoint script[2]. When contention occurs, I see many Yields happening and with same magnitude as Contended. But still see events with long "lock-hold" times, even-though yields are high. [2] https://github.com/xdp-project/xdp-project/blob/master/areas/latency/cgroup_rstat_tracepoint.bt Example output: 12:46:56 High Lock-contention: wait: 739 usec (0 ms) on CPU:56 comm:kswapd7 12:46:56 Long lock-hold time: 6381 usec (6 ms) on CPU:27 comm:kswapd3 12:46:56 Long lock-hold time: 18905 usec (18 ms) on CPU:100 comm:kworker/u261:12 12:46:56 time elapsed: 36 sec (interval = 1 sec) Flushes(2051) 15/interval (avg 56/sec) Locks(44464) 1340/interval (avg 1235/sec) Yields(42413) 1325/interval (avg 1178/sec) Contended(42112) 1322/interval (avg 1169/sec) There is reported 15 flushes/sec, but locks are yielded quickly. More problematically (for softirq latency) we see a Long lock-hold time reaching 18 ms. For network RX softirq I need lower than 0.5ms latency, to avoid RX-ring HW queue overflows. --Jesper p.s. I'm seeing a pattern with kswapdN contending on this lock. @stack[697, kswapd3]: __cgroup_rstat_lock+107 __cgroup_rstat_lock+107 cgroup_rstat_flush_locked+851 cgroup_rstat_flush+35 shrink_node+226 balance_pgdat+807 kswapd+521 kthread+228 ret_from_fork+48 ret_from_fork_asm+27 @stack[698, kswapd4]: __cgroup_rstat_lock+107 __cgroup_rstat_lock+107 cgroup_rstat_flush_locked+851 cgroup_rstat_flush+35 shrink_node+226 balance_pgdat+807 kswapd+521 kthread+228 ret_from_fork+48 ret_from_fork_asm+27 @stack[699, kswapd5]: __cgroup_rstat_lock+107 __cgroup_rstat_lock+107 cgroup_rstat_flush_locked+851 cgroup_rstat_flush+35 shrink_node+226 balance_pgdat+807 kswapd+521 kthread+228 ret_from_fork+48 ret_from_fork_asm+27