Re: [PATCH v1 2/3] cgroup/rstat: convert cgroup_rstat_lock back to mutex

Shakeel Butt
2024-04-19T16:11:42Z

On Fri, Apr 19, 2024 at 03:15:01PM +0200, Jesper Dangaard Brouer wrote:
>
>
> On 18/04/2024 22.39, Yosry Ahmed wrote:
> > On Thu, Apr 18, 2024 at 7:49 AM Shakeel Butt <shakeel.butt@linux.dev> 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
>

Can you simply replace mem_cgroup_flush_stats() in
prepare_scan_control() with the ratelimited version and see if the issue
still persists for your production traffic?

Also were you able to get which specific stats are getting the most
updates?