Ingo, Thomas or Peter, is there any chance one of you could take a look at patch "[PATCH 2/6] mm/page_alloc: Convert per-cpu list protection to local_lock" from this series? It's partially motivated by PREEMPT_RT. More details below.
On Mon, Mar 29, 2021 at 01:06:42PM +0100, Mel Gorman wrote: > This series requires patches in Andrew's tree so the series is also > available at > > git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git > mm-percpu-local_lock-v1r15 > > The PCP (per-cpu page allocator in page_alloc.c) share locking requirements > with vmstat which is inconvenient and causes some issues. Possibly because > of that, the PCP list and vmstat share the same per-cpu space meaning that > it's possible that vmstat updates dirty cache lines holding per-cpu lists > across CPUs unless padding is used. The series splits that structure and > separates the locking. > The bulk page allocation series that the local_lock work had an additional fix so I've rebased this onto git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-percpu-local_lock-v1r16 > Second, PREEMPT_RT considers the following sequence to be unsafe > as documented in Documentation/locking/locktypes.rst > > local_irq_disable(); > spin_lock(&lock); > > The pcp allocator has this sequence for rmqueue_pcplist (local_irq_save) > -> __rmqueue_pcplist -> rmqueue_bulk (spin_lock). This series explicitly > separates the locking requirements for the PCP list (local_lock) and stat > updates (irqs disabled). Once that is done, the length of time IRQs are > disabled can be reduced and in some cases, IRQ disabling can be replaced > with preempt_disable. > It's this part I'm interested in even though it only partially addresses the preempt-rt tree concerns. More legwork is needed for preempt-rt which is outside the context of this series. At minimum, it involves 1. Split locking of pcp and buddy allocator instead of using spin_lock() when it's "known" that IRQs are disabled (not necessarily a valid assumption on PREEMPT_RT) 2. Split the zone lock into what protects the zone metadata and what protects the free lists This looks straight-forward but it involves audit work and it may be difficult to avoid regressing non-PREEMPT_RT kernels by disabling/enabling IRQs when switching between the pcp allocator and the buddy allocator. > After that, it was very obvious that zone_statistics in particular has way > too much overhead and leaves IRQs disabled for longer than necessary. It > has perfectly accurate counters requiring IRQs be disabled for parallel > RMW sequences when inaccurate ones like vm_events would do. The series > makes the NUMA statistics (NUMA_HIT and friends) inaccurate counters that > only require preempt be disabled. > > Finally the bulk page allocator can then do all the stat updates in bulk > with IRQs enabled which should improve the efficiency of the bulk page > allocator. Technically, this could have been done without the local_lock > and vmstat conversion work and the order simply reflects the timing of > when different series were implemented. > > No performance data is included because despite the overhead of the > stats, it's within the noise for most workloads but Jesper and Chuck may > observe a significant different with the same tests used for the bulk > page allocator. The series is more likely to be interesting to the RT > folk in terms of slowing getting the PREEMPT tree into mainline. > > drivers/base/node.c | 18 +-- > include/linux/mmzone.h | 29 +++-- > include/linux/vmstat.h | 65 ++++++----- > mm/mempolicy.c | 2 +- > mm/page_alloc.c | 173 ++++++++++++++++------------ > mm/vmstat.c | 254 +++++++++++++++-------------------------- > 6 files changed, 254 insertions(+), 287 deletions(-) > > -- > 2.26.2 > -- Mel Gorman SUSE Labs
