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Date:   Fri, 10 Jul 2020 14:29:17 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org,
        Domas Mituzas <domas@fb.com>, Tejun Heo <tj@kernel.org>,
        Chris Down <chris@chrisdown.name>
Subject: Re: [PATCH] mm: memcontrol: avoid workload stalls when lowering
 memory.high
Message-ID: <20200710122917.GB3022@dhcp22.suse.cz>
References: <20200709194718.189231-1-guro@fb.com>
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On Thu 09-07-20 12:47:18, Roman Gushchin wrote:
> Memory.high limit is implemented in a way such that the kernel
> penalizes all threads which are allocating a memory over the limit.
> Forcing all threads into the synchronous reclaim and adding some
> artificial delays allows to slow down the memory consumption and
> potentially give some time for userspace oom handlers/resource control
> agents to react.
> 
> It works nicely if the memory usage is hitting the limit from below,
> however it works sub-optimal if a user adjusts memory.high to a value
> way below the current memory usage. It basically forces all workload
> threads (doing any memory allocations) into the synchronous reclaim
> and sleep. This makes the workload completely unresponsive for
> a long period of time and can also lead to a system-wide contention on
> lru locks. It can happen even if the workload is not actually tight on
> memory and has, for example, a ton of cold pagecache.
> 
> In the current implementation writing to memory.high causes an atomic
> update of page counter's high value followed by an attempt to reclaim
> enough memory to fit into the new limit. To fix the problem described
> above, all we need is to change the order of execution: try to push
> the memory usage under the limit first, and only then set the new
> high limit.

Shakeel would this help with your pro-active reclaim usecase? It would
require to reset the high limit right after the reclaim returns which is
quite ugly but it would at least not require a completely new interface.
You would simply do
	high = current - to_reclaim
	echo $high > memory.high
	echo infinity > memory.high # To prevent direct reclaim
				    # allocation stalls

The primary reason to set the high limit in advance was to catch
potential runaways more effectively because they would just get
throttled while memory_high_write is reclaiming. With this change
the reclaim here might be just playing never ending catch up. On the
plus side a break out from the reclaim loop would just enforce the limit
so if the operation takes too long then the reclaim burden will move
over to consumers eventually. So I do not see any real danger.

> Signed-off-by: Roman Gushchin <guro@fb.com>
> Reported-by: Domas Mituzas <domas@fb.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Chris Down <chris@chrisdown.name>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memcontrol.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b8424aa56e14..4b71feee7c42 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6203,8 +6203,6 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>  	if (err)
>  		return err;
>  
> -	page_counter_set_high(&memcg->memory, high);
> -
>  	for (;;) {
>  		unsigned long nr_pages = page_counter_read(&memcg->memory);
>  		unsigned long reclaimed;
> @@ -6228,6 +6226,8 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
>  			break;
>  	}
>  
> +	page_counter_set_high(&memcg->memory, high);
> +
>  	return nbytes;
>  }
>  
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs