On Fri, Feb 24, 2023 at 10:34:10AM +0800, Hillf Danton wrote: > On Thu, Feb 09, 2023 at 12:01:50PM -0300, Marcelo Tosatti wrote: > > This patch series addresses the following two problems: > > > > 1. A customer provided some evidence which indicates that > > the idle tick was stopped; albeit, CPU-specific vmstat > > counters still remained populated. > > > > Thus one can only assume quiet_vmstat() was not > > invoked on return to the idle loop. If I understand > > correctly, I suspect this divergence might erroneously > > prevent a reclaim attempt by kswapd. If the number of > > zone specific free pages are below their per-cpu drift > > value then zone_page_state_snapshot() is used to > > compute a more accurate view of the aforementioned > > statistic. Thus any task blocked on the NUMA node > > specific pfmemalloc_wait queue will be unable to make > > significant progress via direct reclaim unless it is > > killed after being woken up by kswapd > > (see throttle_direct_reclaim()) > > > > 2. With a SCHED_FIFO task that busy loops on a given CPU, > > and kworker for that CPU at SCHED_OTHER priority, > > queuing work to sync per-vmstats will either cause that > > work to never execute, or stalld (i.e. stall daemon) > > boosts kworker priority which causes a latency > > violation > > > > By having vmstat_shepherd flush the per-CPU counters to the > > global counters from remote CPUs. > > > > This is done using cmpxchg to manipulate the counters, > > both CPU locally (via the account functions), > > and remotely (via cpu_vm_stats_fold). > > Frankly another case of bandaid[1] ? > > [1] https://lore.kernel.org/lkml/20230223150624.GA29739@xxxxxx/ Only if you disable per-CPU vmstat counters for isolated CPUs (then maintenance of the data structures in isolated CPUs is not necessary). Which would be terrible for performance, however.
