Re: [PATCH v3] memcg: add nr_pages argument for hierarchical reclaim

[Michal Hocko]

On Thu 18-08-11 15:58:21, Johannes Weiner wrote:
> On Thu, Aug 18, 2011 at 02:57:54PM +0200, Michal Hocko wrote:
> > I have just realized that num_online_nodes should be much better than
> > MAX_NUMNODES. 
> > Just for reference, the patch is based on top of
> > https://lkml.org/lkml/2011/8/9/82 (it doesn't depend on it but it also
> > doesn't make much sense without it)
> > 
> > Changes since v2:
> > - use num_online_nodes rather than MAX_NUMNODES
> > Changes since v1:
> > - reclaim nr_nodes * SWAP_CLUSTER_MAX in mem_cgroup_force_empty
> > ---
> > From: Michal Hocko <mhocko@xxxxxxx>
> > Subject: memcg: add nr_pages argument for hierarchical reclaim
> > 
> > Now that we are doing memcg direct reclaim limited to nr_to_reclaim
> > pages (introduced by "memcg: stop vmscan when enough done.") we have to
> > be more careful. Currently we are using SWAP_CLUSTER_MAX which is OK for
> > most callers but it might cause failures for limit resize or force_empty
> > code paths on big NUMA machines.
> 
> The limit resizing path retries as long as reclaim makes progress, so
> this is just handwaving.

limit resizing paths do not check the return value of
mem_cgroup_hierarchical_reclaim so the number of retries is not
affected. It is true that fixing that would be much easier.

> 
> After Kame's patch, the force-empty path has an increased risk of
> failing to move huge pages to the parent, because it tries reclaim
> only once.  This could need further evaluation, and possibly a fix.

Agreed

> But instead:
> 
> > @@ -2331,8 +2331,14 @@ static int mem_cgroup_do_charge(struct m
> >  	if (!(gfp_mask & __GFP_WAIT))
> >  		return CHARGE_WOULDBLOCK;
> >  
> > +	/*
> > +	 * We are lying about nr_pages because we do not want to
> > +	 * reclaim too much for THP pages which should rather fallback
> > +	 * to small pages.
> > +	 */
> >  	ret = mem_cgroup_hierarchical_reclaim(mem_over_limit, NULL,
> > -					      gfp_mask, flags, NULL);
> > +					      gfp_mask, flags, NULL,
> > +					      1);
> >  	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
> >  		return CHARGE_RETRY;
> >  	/*
> 
> You tell it to reclaim _less_ than before, further increasing the risk
> of failure...
> 
> > @@ -2350,7 +2351,7 @@ unsigned long try_to_free_mem_cgroup_pag
> >  		.may_writepage = !laptop_mode,
> >  		.may_unmap = 1,
> >  		.may_swap = !noswap,
> > -		.nr_to_reclaim = SWAP_CLUSTER_MAX,
> > +		.nr_to_reclaim = max_t(unsigned long, nr_pages, SWAP_CLUSTER_MAX),
> 
> ...but wait, this transparently fixes it up and ignores the caller's
> request.
> 
> Sorry, but this is just horrible!

Yes, I do not like it as well and tried to point it out in the comment.
Anyway I do agree that this doesn't solve the problem you are describing
above and the limit resizing paths can be fixed much easier so the patch
is pointless.

> 
> For the past weeks I have been chasing memcg bugs that came in with
> sloppy and untested code, that was merged for handwavy reasons.

Yes, I feel big responsibility about that.

> 
> Changes to algorithms need to be tested and optimizations need to be
> quantified in other parts of the VM and the kernel, too.  I have no
> idea why this doesn't seem to apply to the memory cgroup subsystem.

Yes, we should definitely do better during review process.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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