Messages in this thread | | | Date | Fri, 19 Apr 2024 15:15:01 +0200 | Subject | Re: [PATCH v1 2/3] cgroup/rstat: convert cgroup_rstat_lock back to mutex | From | Jesper Dangaard Brouer <> |
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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
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