Re: Advice on cgroup rstat lock

[Waiman Long]

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.

Cheers,
Longman