On Wed 23-08-23 07:55:40, Yosry Ahmed wrote: > On Wed, Aug 23, 2023 at 12:33 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Tue 22-08-23 08:30:05, Yosry Ahmed wrote: > > > On Tue, Aug 22, 2023 at 2:06 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > On Mon 21-08-23 20:54:58, Yosry Ahmed wrote: > > [...] > > > So to answer your question, I don't think a random user can really > > > affect the system in a significant way by constantly flushing. In > > > fact, in the test script (which I am now attaching, in case you're > > > interested), there are hundreds of threads that are reading stats of > > > different cgroups every 1s, and I don't see any negative effects on > > > in-kernel flushers in this case (reclaimers). > > > > I suspect you have missed my point. > > I suspect you are right :) > > > > Maybe I am just misunderstanding > > the code but it seems to me that the lock dropping inside > > cgroup_rstat_flush_locked effectivelly allows unbounded number of > > contenders which is really dangerous when it is triggerable from the > > userspace. The number of spinners at a moment is always bound by the > > number CPUs but depending on timing many potential spinners might be > > cond_rescheded and the worst time latency to complete can be really > > high. Makes more sense? > > I think I understand better now. So basically because we might drop > the lock and resched, there can be nr_cpus spinners + other spinners > that are currently scheduled away, so these will need to wait to be > scheduled and then start spinning on the lock. This may happen for one > reader multiple times during its read, which is what can cause a high > worst case latency. > > I hope I understood you correctly this time. Did I? Yes. I would just add that this could also influence the worst case latency for a different reader - so an adversary user can stall others. Exposing a shared global lock in uncontrolable way over generally available user interface is not really a great idea IMHO. -- Michal Hocko SUSE Labs
