Re: [PATCH v2] writeback: use exact memcg dirty counts

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Mar 29, 2019 at 10:46:09AM -0700, Greg Thelen wrote:
> Since commit a983b5ebee57 ("mm: memcontrol: fix excessive complexity in
> memory.stat reporting") memcg dirty and writeback counters are managed
> as:
> 1) per-memcg per-cpu values in range of [-32..32]
> 2) per-memcg atomic counter
> When a per-cpu counter cannot fit in [-32..32] it's flushed to the
> atomic.  Stat readers only check the atomic.
> Thus readers such as balance_dirty_pages() may see a nontrivial error
> margin: 32 pages per cpu.
> Assuming 100 cpus:
>    4k x86 page_size:  13 MiB error per memcg
>   64k ppc page_size: 200 MiB error per memcg
> Considering that dirty+writeback are used together for some decisions
> the errors double.
> 
> This inaccuracy can lead to undeserved oom kills.  One nasty case is
> when all per-cpu counters hold positive values offsetting an atomic
> negative value (i.e. per_cpu[*]=32, atomic=n_cpu*-32).
> balance_dirty_pages() only consults the atomic and does not consider
> throttling the next n_cpu*32 dirty pages.  If the file_lru is in the
> 13..200 MiB range then there's absolutely no dirty throttling, which
> burdens vmscan with only dirty+writeback pages thus resorting to oom
> kill.
> 
> It could be argued that tiny containers are not supported, but it's more
> subtle.  It's the amount the space available for file lru that matters.
> If a container has memory.max-200MiB of non reclaimable memory, then it
> will also suffer such oom kills on a 100 cpu machine.
> 
> The following test reliably ooms without this patch.  This patch avoids
> oom kills.
> 
>   $ cat test
>   mount -t cgroup2 none /dev/cgroup
>   cd /dev/cgroup
>   echo +io +memory > cgroup.subtree_control
>   mkdir test
>   cd test
>   echo 10M > memory.max
>   (echo $BASHPID > cgroup.procs && exec /memcg-writeback-stress /foo)
>   (echo $BASHPID > cgroup.procs && exec dd if=/dev/zero of=/foo bs=2M count=100)
> 
>   $ cat memcg-writeback-stress.c
>   /*
>    * Dirty pages from all but one cpu.
>    * Clean pages from the non dirtying cpu.
>    * This is to stress per cpu counter imbalance.
>    * On a 100 cpu machine:
>    * - per memcg per cpu dirty count is 32 pages for each of 99 cpus
>    * - per memcg atomic is -99*32 pages
>    * - thus the complete dirty limit: sum of all counters 0
>    * - balance_dirty_pages() only sees atomic count -99*32 pages, which
>    *   it max()s to 0.
>    * - So a workload can dirty -99*32 pages before balance_dirty_pages()
>    *   cares.
>    */
>   #define _GNU_SOURCE
>   #include <err.h>
>   #include <fcntl.h>
>   #include <sched.h>
>   #include <stdlib.h>
>   #include <stdio.h>
>   #include <sys/stat.h>
>   #include <sys/sysinfo.h>
>   #include <sys/types.h>
>   #include <unistd.h>
> 
>   static char *buf;
>   static int bufSize;
> 
>   static void set_affinity(int cpu)
>   {
>   	cpu_set_t affinity;
> 
>   	CPU_ZERO(&affinity);
>   	CPU_SET(cpu, &affinity);
>   	if (sched_setaffinity(0, sizeof(affinity), &affinity))
>   		err(1, "sched_setaffinity");
>   }
> 
>   static void dirty_on(int output_fd, int cpu)
>   {
>   	int i, wrote;
> 
>   	set_affinity(cpu);
>   	for (i = 0; i < 32; i++) {
>   		for (wrote = 0; wrote < bufSize; ) {
>   			int ret = write(output_fd, buf+wrote, bufSize-wrote);
>   			if (ret == -1)
>   				err(1, "write");
>   			wrote += ret;
>   		}
>   	}
>   }
> 
>   int main(int argc, char **argv)
>   {
>   	int cpu, flush_cpu = 1, output_fd;
>   	const char *output;
> 
>   	if (argc != 2)
>   		errx(1, "usage: output_file");
> 
>   	output = argv[1];
>   	bufSize = getpagesize();
>   	buf = malloc(getpagesize());
>   	if (buf == NULL)
>   		errx(1, "malloc failed");
> 
>   	output_fd = open(output, O_CREAT|O_RDWR);
>   	if (output_fd == -1)
>   		err(1, "open(%s)", output);
> 
>   	for (cpu = 0; cpu < get_nprocs(); cpu++) {
>   		if (cpu != flush_cpu)
>   			dirty_on(output_fd, cpu);
>   	}
> 
>   	set_affinity(flush_cpu);
>   	if (fsync(output_fd))
>   		err(1, "fsync(%s)", output);
>   	if (close(output_fd))
>   		err(1, "close(%s)", output);
>   	free(buf);
>   }
> 
> Make balance_dirty_pages() and wb_over_bg_thresh() work harder to
> collect exact per memcg counters.  This avoids the aforementioned oom
> kills.
> 
> This does not affect the overhead of memory.stat, which still reads the
> single atomic counter.
> 
> Why not use percpu_counter?  memcg already handles cpus going offline,
> so no need for that overhead from percpu_counter.  And the
> percpu_counter spinlocks are more heavyweight than is required.
> 
> It probably also makes sense to use exact dirty and writeback counters
> in memcg oom reports.  But that is saved for later.
> 
> Cc: stable@vger.kernel.org # v4.16+
> Signed-off-by: Greg Thelen <gthelen@google.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks Greg!