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. > 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 It's still waiting on tests to complete and not all the changelogs are complete which is why it's not posted. This is a comparison of vanilla vs "local_locks to per-cpu spinlocks" versus the git series up to "mm/page_alloc: Remotely drain per-cpu lists" for the page faulting microbench I originally complained about. The test machine is a 2-socket CascadeLake machine. pft timings 5.17.0-rc5 5.17.0-rc5 5.17.0-rc5 vanilla mm-remotedrain-v2r1 mm-pcpdrain-v1r1 Amean elapsed-1 32.54 ( 0.00%) 33.08 * -1.66%* 32.82 * -0.86%* Amean elapsed-4 8.66 ( 0.00%) 9.24 * -6.72%* 8.69 * -0.38%* Amean elapsed-7 5.02 ( 0.00%) 5.43 * -8.16%* 5.05 * -0.55%* Amean elapsed-12 3.07 ( 0.00%) 3.38 * -10.00%* 3.09 * -0.72%* Amean elapsed-21 2.36 ( 0.00%) 2.38 * -0.89%* 2.19 * 7.39%* Amean elapsed-30 1.75 ( 0.00%) 1.87 * -6.50%* 1.62 * 7.59%* Amean elapsed-48 1.71 ( 0.00%) 2.00 * -17.32%* 1.71 ( -0.08%) Amean elapsed-79 1.56 ( 0.00%) 1.62 * -3.84%* 1.56 ( -0.02%) Amean elapsed-80 1.57 ( 0.00%) 1.65 * -5.31%* 1.57 ( -0.04%) Note the local_lock conversion took 1 1-17% penalty while the git tree takes a negligile penalty while still allowing remote drains. It might have some potential while being less complex than the RCU approach. -- Mel Gorman SUSE Labs
