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

Jesper Dangaard Brouer
2024-04-19T13:15:05Z

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