Re: [RFC][PATCH 4/4] memcg: make oom less frequently

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> [2009-01-09 17:52:15]:

> On Fri, 9 Jan 2009 11:28:04 +0530, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > * Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> [2009-01-08 19:15:20]:
> > 
> > > In previous implementation, mem_cgroup_try_charge checked the return
> > > value of mem_cgroup_try_to_free_pages, and just retried if some pages
> > > had been reclaimed.
> > > But now, try_charge(and mem_cgroup_hierarchical_reclaim called from it)
> > > only checks whether the usage is less than the limit.
> > > 
> > > This patch tries to change the behavior as before to cause oom less frequently.
> > > 
> > > To prevent try_charge from getting stuck in infinite loop,
> > > MEM_CGROUP_RECLAIM_RETRIES_MAX is defined.
> > > 
> > > 
> > > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > > ---
> > >  mm/memcontrol.c |   16 ++++++++++++----
> > >  1 files changed, 12 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 804c054..fedd76b 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -42,6 +42,7 @@
> > > 
> > >  struct cgroup_subsys mem_cgroup_subsys __read_mostly;
> > >  #define MEM_CGROUP_RECLAIM_RETRIES	5
> > > +#define MEM_CGROUP_RECLAIM_RETRIES_MAX	32
> > 
> > Why 32 are you seeing frequent OOMs? I had 5 iterations to allow
> > 
> > 1. pages to move to swap cache, which added back pressure to memcg in
> > the original implementation, since the pages came back
> > 2. It look longer to move, recalim those pages.
> > 
> > Ideally 3 would suffice, but I added an additional 2 retries for
> > safety.
> > 
> Before this patch, try_charge doesn't check the return value of
> try_to_free_page, i.e. how many pages has been reclaimed, and
> only checks whether the usage has become less than the limit.
> So, oom can be caused if the group is too busy.
> 
> IIRC memory-cgroup-hierarchical-reclaim patch introduced this behavior,
> and, I don't remember in detail, some tests which had not caused oom
> started to cause oom after it.
> That was the motivation of my first version of this patch(*1).
> 
> *1 http://lkml.org/lkml/2008/11/28/35
> 
> Anyway, this is the updated version.
> I removed RETRIES_MAX.
> 
> 
> Thanks,
> Daisuke Nishimura.
> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> In previous implementation, mem_cgroup_try_charge checked the return
> value of mem_cgroup_try_to_free_pages, and just retried if some pages
> had been reclaimed.
> But now, try_charge(and mem_cgroup_hierarchical_reclaim called from it)
> only checks whether the usage is less than the limit.
> 
> This patch tries to change the behavior as before to cause oom less frequently.
> 
> ChangeLog: RFC->v1
> - removed RETRIES_MAX.
> 
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> ---
>  mm/memcontrol.c |   10 ++++++----
>  1 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7ba5c61..fb0e9eb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -781,10 +781,10 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
>  	 * but there might be left over accounting, even after children
>  	 * have left.
>  	 */
> -	ret = try_to_free_mem_cgroup_pages(root_mem, gfp_mask, noswap,
> +	ret += try_to_free_mem_cgroup_pages(root_mem, gfp_mask, noswap,
>  					   get_swappiness(root_mem));
>  	if (mem_cgroup_check_under_limit(root_mem))
> -		return 0;
> +		return 1;	/* indicate reclaim has succeeded */
>  	if (!root_mem->use_hierarchy)
>  		return ret;
> 
> @@ -795,10 +795,10 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
>  			next_mem = mem_cgroup_get_next_node(root_mem);
>  			continue;
>  		}
> -		ret = try_to_free_mem_cgroup_pages(next_mem, gfp_mask, noswap,
> +		ret += try_to_free_mem_cgroup_pages(next_mem, gfp_mask, noswap,
>  						   get_swappiness(next_mem));
>  		if (mem_cgroup_check_under_limit(root_mem))
> -			return 0;
> +			return 1;	/* indicate reclaim has succeeded */
>  		next_mem = mem_cgroup_get_next_node(root_mem);
>  	}
>  	return ret;
> @@ -883,6 +883,8 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
> 
>  		ret = mem_cgroup_hierarchical_reclaim(mem_over_limit, gfp_mask,
>  							noswap);
> +		if (ret)
> +			continue;
> 
>  		/*
>  		 * try_to_free_mem_cgroup_pages() might not give us a full
> 

This makes sense

Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>


-- 
	Balbir

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