Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads

[Waiman Long]

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