linux-kernel - Re: [PATCH] mm, memcg: unify reclaim retry limits with page allocator

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Message-Id: <20200520164037.e3598bc902e39415f4c263e7@linux-foundation.org>
Date:   Wed, 20 May 2020 16:40:37 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] mm, memcg: unify reclaim retry limits with page
 allocator

On Wed, 20 May 2020 17:31:42 +0100 Chris Down <chris@...isdown.name> wrote:

> Reclaim retries have been set to 5 since the beginning of time in
> 66e1707bc346 ("Memory controller: add per cgroup LRU and reclaim").
> However, we now have a generally agreed-upon standard for page reclaim:
> MAX_RECLAIM_RETRIES (currently 16), added many years later in
> 0a0337e0d1d1 ("mm, oom: rework oom detection").
> 
> In the absence of a compelling reason to declare an OOM earlier in memcg
> context than page allocator context, it seems reasonable to supplant
> MEM_CGROUP_RECLAIM_RETRIES with MAX_RECLAIM_RETRIES, making the page
> allocator and memcg internals more similar in semantics when reclaim
> fails to produce results, avoiding premature OOMs or throttling.
> 
> ...
>
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -73,9 +73,6 @@ EXPORT_SYMBOL(memory_cgrp_subsys);
>  
>  struct mem_cgroup *root_mem_cgroup __read_mostly;
>  
> -/* The number of times we should retry reclaim failures before giving up. */

hm, what tree is this against?

> -#define MEM_CGROUP_RECLAIM_RETRIES	5
> -
>  /* Socket memory accounting disabled? */
>  static bool cgroup_memory_nosocket;
>  
> @@ -2386,7 +2383,7 @@ void mem_cgroup_handle_over_high(void)
>  	unsigned long pflags;
>  	unsigned long nr_reclaimed;
>  	unsigned int nr_pages = current->memcg_nr_pages_over_high;
> -	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
> +	int nr_retries = MAX_RECLAIM_RETRIES;

I can't seem to find a tree in which mem_cgroup_handle_over_high() has
a local `nr_retries'.