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@xxxxxxxxx> 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@xxxxxxxxxxxxxxx/ > > > > 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?
