Re: [PATCH 3/3] mm: memcontrol: fix excessive complexity in memory.stat reporting

[Vladimir Davydov <vdavydov.dev@gmail.com>]

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@cmpxchg.org>
> ---
>  include/linux/memcontrol.h |  96 +++++++++++++++++++++++++++---------------
>  mm/memcontrol.c            | 101 +++++++++++++++++++++++----------------------
>  2 files changed, 113 insertions(+), 84 deletions(-)

Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>

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