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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. [hannes@cmpxchg.org: fix warning in __this_cpu_xchg() calls] Link: http://lkml.kernel.org/r/20171201135750.GB8097@cmpxchg.org Link: http://lkml.kernel.org/r/20171103153336.24044-3-hannes@cmpxchg.org Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com> Cc: Michal Hocko <mhocko@suse.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.