Re: [PATCH] writeback: sum memcg dirty counters as needed

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  7 Mar 2019 08:56:32 -0800 Greg Thelen <gthelen@google.com> 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.
> 
> ...
> 
> Make balance_dirty_pages() and wb_over_bg_thresh() work harder to
> collect exact per memcg counters when a memcg is close to the
> throttling/writeback threshold.  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 include exact dirty and writeback
> counters in memcg oom reports.  But that is saved for later.

Nice changelog, thanks.

> Signed-off-by: Greg Thelen <gthelen@google.com>

Did you consider cc:stable for this?  We may as well - the stablebots
backport everything which might look slightly like a fix anyway :(

> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -573,6 +573,22 @@ static inline unsigned long memcg_page_state(struct mem_cgroup *memcg,
>  	return x;
>  }
>  
> +/* idx can be of type enum memcg_stat_item or node_stat_item */
> +static inline unsigned long
> +memcg_exact_page_state(struct mem_cgroup *memcg, int idx)
> +{
> +	long x = atomic_long_read(&memcg->stat[idx]);
> +#ifdef CONFIG_SMP
> +	int cpu;
> +
> +	for_each_online_cpu(cpu)
> +		x += per_cpu_ptr(memcg->stat_cpu, cpu)->count[idx];
> +	if (x < 0)
> +		x = 0;
> +#endif
> +	return x;
> +}

This looks awfully heavyweight for an inline function.  Why not make it
a regular function and avoid the bloat and i-cache consumption?

Also, did you instead consider making this spill the percpu counters
into memcg->stat[idx]?  That might be more useful for potential future
callers.  It would become a little more expensive though.