On Fri, Nov 03, 2017 at 11:33:36AM -0400, Johannes Weiner wrote: > We've seen memory.stat reads in top-level cgroups take up to fourteen > seconds during a userspace bug that created tens of thousands of ghost > cgroups pinned by lingering page cache. > > Even with a more reasonable number of cgroups, aggregating memory.stat > is unnecessarily heavy. The complexity is this: > > nr_cgroups * nr_stat_items * nr_possible_cpus > > where the stat items are ~70 at this point. With 128 cgroups and 128 > CPUs - decent, not enormous setups - reading the top-level memory.stat > has to aggregate over a million per-cpu counters. This doesn't scale. > > Instead of spreading the source of truth across all CPUs, use the > per-cpu counters merely to batch updates to shared atomic counters. > > This is the same as the per-cpu stocks we use for charging memory to > the shared atomic page_counters, and also the way the global vmstat > counters are implemented. > > Vmstat has elaborate spilling thresholds that depend on the number of > CPUs, amount of memory, and memory pressure - carefully balancing the > cost of counter updates with the amount of per-cpu error. That's > because the vmstat counters are system-wide, but also used for > decisions inside the kernel (e.g. NR_FREE_PAGES in the > allocator). Neither is true for the memory controller. > > Use the same static batch size we already use for page_counter updates > during charging. The per-cpu error in the stats will be 128k, which is > an acceptable ratio of cores to memory accounting granularity. > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> > --- > include/linux/memcontrol.h | 96 +++++++++++++++++++++++++++--------------- > mm/memcontrol.c | 101 +++++++++++++++++++++++---------------------- > 2 files changed, 113 insertions(+), 84 deletions(-) Acked-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html