Messages in this thread | | | Date | Tue, 9 Apr 2024 11:37:31 -0400 | Subject | Re: Advice on cgroup rstat lock | From | Waiman Long <> |
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On 4/9/24 07:08, Jesper Dangaard Brouer wrote: > Let move this discussion upstream. > > On 22/03/2024 19.32, Yosry Ahmed wrote: >> [..] >>>> There was a couple of series that made all calls to >>>> cgroup_rstat_flush() sleepable, which allows the lock to be dropped >>>> (and IRQs enabled) in between CPU iterations. This fixed a similar >>>> problem that we used to face (except in our case, we saw hard lockups >>>> in extreme scenarios): >>>> https://lore.kernel.org/linux-mm/20230330191801.1967435-1-yosryahmed@google.com/ >>>> >>>> https://lore.kernel.org/lkml/20230421174020.2994750-1-yosryahmed@google.com/ >>>> >>> >>> I've only done the 6.6 backport, and these were in 6.5/6.6. > > Given I have these in my 6.6 kernel. You are basically saying I should > be able to avoid IRQ-disable for the lock, right? > > My main problem with the global cgroup_rstat_lock[3] is it disables IRQs > and (thereby also) BH/softirq (spin_lock_irq). This cause production > issues elsewhere, e.g. we are seeing network softirq "not-able-to-run" > latency issues (debug via softirq_net_latency.bt [5]). > > [3] > https://elixir.bootlin.com/linux/v6.9-rc3/source/kernel/cgroup/rstat.c#L10 > [5] > https://github.com/xdp-project/xdp-project/blob/master/areas/latency/softirq_net_latency.bt > > >>> And between 6.1 to 6.6 we did observe an improvement in this area. >>> (Maybe I don't have to do the 6.1 backport if the 6.6 release plan >>> progress) >>> >>> I've had a chance to get running in prod for 6.6 backport. >>> As you can see in attached grafana heatmap pictures, we do observe an >>> improved/reduced softirq wait time. >>> These softirq "not-able-to-run" outliers is *one* of the prod issues we >>> observed. As you can see, I still have other areas to improve/fix. >> >> I am not very familiar with such heatmaps, but I am glad there is an >> improvement with 6.6 and the backports. Let me know if there is >> anything I could do to help with your effort. > > The heatmaps give me an overview, but I needed a debugging tool, so I > developed some bpftrace scripts [1][2] I'm running on production. > To measure how long time we hold the cgroup rstat lock (results below). > Adding ACME and Daniel as I hope there is an easier way to measure lock > hold time and congestion. Notice tricky release/yield in > cgroup_rstat_flush_locked[4]. > > My production results on 6.6 with backported patches (below signature) > vs a our normal 6.6 kernel, with script [2]. The `@lock_time_hist_ns` > shows how long time the lock+IRQs were disabled (taking into account it > can be released in the loop [4]). > > Patched kernel: > > 21:49:02 time elapsed: 43200 sec > @lock_time_hist_ns: > [2K, 4K) 61 | | > [4K, 8K) 734 | | > [8K, 16K) 121500 |@@@@@@@@@@@@@@@@ | > [16K, 32K) 385714 > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| > [32K, 64K) 145600 |@@@@@@@@@@@@@@@@@@@ | > [64K, 128K) 156873 |@@@@@@@@@@@@@@@@@@@@@ | > [128K, 256K) 261027 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ | > [256K, 512K) 291986 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ | > [512K, 1M) 101859 |@@@@@@@@@@@@@ | > [1M, 2M) 19866 |@@ | > [2M, 4M) 10146 |@ | > [4M, 8M) 30633 |@@@@ | > [8M, 16M) 40365 |@@@@@ | > [16M, 32M) 21650 |@@ | > [32M, 64M) 5842 | | > [64M, 128M) 8 | | > > And normal 6.6 kernel: > > 21:48:32 time elapsed: 43200 sec > @lock_time_hist_ns: > [1K, 2K) 25 | | > [2K, 4K) 1146 | | > [4K, 8K) 59397 |@@@@ | > [8K, 16K) 571528 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ | > [16K, 32K) 542648 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ | > [32K, 64K) 202810 |@@@@@@@@@@@@@ | > [64K, 128K) 134564 |@@@@@@@@@ | > [128K, 256K) 72870 |@@@@@ | > [256K, 512K) 56914 |@@@ | > [512K, 1M) 83140 |@@@@@ | > [1M, 2M) 170514 |@@@@@@@@@@@ | > [2M, 4M) 396304 |@@@@@@@@@@@@@@@@@@@@@@@@@@@ | > [4M, 8M) 755537 > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| > [8M, 16M) 231222 |@@@@@@@@@@@@@@@ | > [16M, 32M) 76370 |@@@@@ | > [32M, 64M) 1043 | | > [64M, 128M) 12 | | > > > For the unpatched kernel we see more events in 4ms to 8ms bucket than > any other bucket. > For patched kernel, we clearly see a significant reduction of events in > the 4 ms to 64 ms area, but we still have some events in this area. I'm > very happy to see these patches improves the situation. But for network > processing I'm not happy to see events in area 16ms to 128ms area. If > we can just avoid disabling IRQs/softirq for the lock, I would be happy. > > How far can we go... could cgroup_rstat_lock be converted to a mutex?
The cgroup_rstat_lock was originally a mutex. It was converted to a spinlock in commit 0fa294fb1985 ("group: Replace cgroup_rstat_mutex with a spinlock"). Irq was disabled to enable calling from atomic context. Since commit 0a2dc6ac3329 ("cgroup: remove cgroup_rstat_flush_atomic()"), the rstat API hadn't been called from atomic context anymore. Theoretically, we could change it back to a mutex or not disabling interrupt. That will require that the API cannot be called from atomic context going forward.
Cheers, Longman
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