On Tue 29-08-23 11:05:28, Waiman Long wrote: > 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. I certainly agree this is an antipattern. -- Michal Hocko SUSE Labs
