Messages in this thread | | | Date | Tue, 29 Aug 2023 11:05:28 -0400 | Subject | Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads | From | Waiman Long <> |
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On 8/29/23 03:27, Michal Hocko wrote: > On Mon 28-08-23 13:27:23, Waiman Long wrote: >> On 8/28/23 13:07, Yosry Ahmed wrote: >>>> Here I agree with you. Let's go with the approach which is easy to >>>> undo for now. Though I prefer the new explicit interface for flushing, >>>> that step would be very hard to undo. Let's reevaluate if the proposed >>>> approach shows negative impact on production traffic and I think >>>> Cloudflare folks can give us the results soon. >>> Do you prefer we also switch to using a mutex (with preemption >>> disabled) to avoid the scenario Michal described where flushers give >>> up the lock and sleep resulting in an unbounded wait time in the worst >>> case? >> Locking with mutex with preemption disabled is an oxymoron. > I believe Yosry wanted to disable preemption _after_ the lock is taken > to reduce the time spent while it is held. The idea to use the mutex is > to reduce spinning and more importantly to get rid of lock dropping > part. It is not really clear (but unlikely) we can drop it while > preserving the spinlock as the thing scales with O(#cgroups x #cpus) > in the worst case.
As I have said later in my email, I am not against disabling preemption selectively on some parts of the lock critical section where preemption is undesirable. However, I am against disabling preemption for the whole duration of the code where the mutex lock is held as it defeats the purpose of using mutex in the first place.
Cheers, Longman
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