Messages in this thread | | | Date | Tue, 9 Apr 2024 12:59:11 -0400 | Subject | Re: Advice on cgroup rstat lock | From | Waiman Long <> |
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On 4/9/24 12:45, Yosry Ahmed wrote: > On Tue, Apr 9, 2024 at 8:37 AM Waiman Long <longman@redhat.com> wrote: >> 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. > I think we should avoid flushing from atomic contexts going forward > anyway tbh. It's just too much work to do with IRQs disabled, and we > observed hard lockups before in worst case scenarios. > > I think one problem that was discussed before is that flushing is > exercised from multiple contexts and could have very high concurrency > (e.g. from reclaim when the system is under memory pressure). With a > mutex, the flusher could sleep with the mutex held and block other > threads for a while. > > I vaguely recall experimenting locally with changing that lock into a > mutex and not liking the results, but I can't remember much more. I > could be misremembering though. > > Currently, the lock is dropped in cgroup_rstat_flush_locked() between > CPU iterations if rescheduling is needed or the lock is being > contended (i.e. spin_needbreak() returns true). I had always wondered > if it's possible to introduce a similar primitive for IRQs? We could > also drop the lock (and re-enable IRQs) if IRQs are pending then.
I am not sure if there is a way to check if a hardirq is pending, but we do have a local_softirq_pending() helper.
Regards, Longman
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