Hi Mel, Thanks for having a look at this. On Thu, 2022-03-03 at 11:45 +0000, Mel Gorman wrote: > On Tue, Feb 08, 2022 at 11:07:48AM +0100, Nicolas Saenz Julienne wrote: > > This series replaces mm/page_alloc's per-cpu page lists drain mechanism with > > one that allows accessing the lists remotely. Currently, only a local CPU is > > permitted to change its per-cpu lists, and it's expected to do so, on-demand, > > whenever a process demands it by means of queueing a drain task on the local > > CPU. This causes problems for NOHZ_FULL CPUs and real-time systems that can't > > take any sort of interruption and to some lesser extent inconveniences idle and > > virtualised systems. > > > > I know this has been sitting here for a long while. Last few weeks have > not been fun. No problem. > > Note that this is not the first attempt at fixing this per-cpu page lists: > > - The first attempt[1] tried to conditionally change the pagesets locking > > scheme based the NOHZ_FULL config. It was deemed hard to maintain as the > > NOHZ_FULL code path would be rarely tested. Also, this only solves the issue > > for NOHZ_FULL setups, which isn't ideal. > > - The second[2] unanimously switched the local_locks to per-cpu spinlocks. The > > performance degradation was too big. > > > > For unrelated reasons I looked at using llist to avoid locks entirely. It > turns out it's not possible and needs a lock. We know "local_locks to > per-cpu spinlocks" took a large penalty so I considered alternatives on > how a lock could be used. I found it's possible to both remote drain > the lists and avoid the disable/enable of IRQs entirely as long as a > preempting IRQ is willing to take the zone lock instead (should be very > rare). The IRQ part is a bit hairy though as softirqs are also a problem > and preempt-rt needs different rules and the llist has to sort PCP > refills which might be a loss in total. However, the remote draining may > still be interesting. The full series is at > https://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git/ mm-pcpllist-v1r2 I'll have a proper look at it soon. Regards, -- Nicolás Sáenz
