Re: [PATCH V7 1/2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes

[Waiman Long <longman@redhat.com>]

On 7/16/24 20:35, Yosry Ahmed wrote:
> [..]
>>
>> This is a clean (meaning no cadvisor interference) example of kswapd
>> starting simultaniously on many NUMA nodes, that in 27 out of 98 cases
>> hit the race (which is handled in V6 and V7).
>>
>> The BPF "cnt" maps are getting cleared every second, so this
>> approximates per sec numbers.  This patch reduce pressure on the lock,
>> but we are still seeing (kfunc:vmlinux:cgroup_rstat_flush_locked) full
>> flushes approx 37 per sec (every 27 ms). On the positive side
>> ongoing_flusher mitigation stopped 98 per sec of these.
>>
>> In this clean kswapd case the patch removes the lock contention issue
>> for kswapd. The lock_contended cases 27 seems to be all related to
>> handled_race cases 27.
>>
>> The remaning high flush rate should also be addressed, and we should
>> also work on aproaches to limit this like my ealier proposal[1].
> I honestly don't think a high number of flushes is a problem on its
> own as long as we are not spending too much time flushing, especially
> when we have magnitude-based thresholding so we know there is
> something to flush (although it may not be relevant to what we are
> doing).
>
> If we keep observing a lot of lock contention, one thing that I
> thought about is to have a variant of spin_lock with a timeout. This
> limits the flushing latency, instead of limiting the number of flushes
> (which I believe is the wrong metric to optimize).

Except for semaphore, none of our locking primitives allow for a timeout 
parameter. For sleeping locks, I don't think it is hard to add variants 
with timeout parameter, but not the spinning locks.

Cheers,
Longman