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

[Waiman Long]

On 8/28/23 13:35, Waiman Long wrote:
> On 8/28/23 13:28, Yosry Ahmed wrote:
> > On Mon, Aug 28, 2023 at 10:27 AM Waiman Long <longman@xxxxxxxxxx> 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. Use 
> > > spinlock
> > > if you want to have preemption disabled. The purpose of usiing mutex is
> > > to allow the lock owner to sleep, but you can't sleep with preemption
> > > disabled. You need to enable preemption first. You can disable
> > > preemption for a short time in a non-sleeping section of the lock
> > > critical section, but I would not recommend disabling preemption for 
> > > the
> > > whole critical section.
> > I thought using a mutex with preemption disabled would at least allow
> > waiters to sleep rather than spin, is this not correct (or doesn't
> > matter) ?
>
> Because of optimistic spinning, a mutex lock waiter will only sleep if 
> the lock holder sleep or when its time slice run out. So the waiters 
> are likely to spin for quite a while before they go to sleep.

Perhaps you can add a mutex at the read side so that only 1 reader can 
contend with the global rstat spinlock at any time if this is a concern.

Regards,
Longman