Re: Advice on cgroup rstat lock

[Yosry Ahmed]

On Tue, Apr 9, 2024 at 8:37 AM Waiman Long <longman@xxxxxxxxxx> 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@xxxxxxxxxx/
> >>>>
> >>>> https://lore.kernel.org/lkml/20230421174020.2994750-1-yosryahmed@xxxxxxxxxx/
> >>>>
> >>>
> >>> 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.