Re: [RFC PATCH] vmscan: memcg: Always use swappiness of the reclaimed memcg swappiness and oom_control

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Apr 24, 2014 at 04:27:04PM +0200, Michal Hocko wrote:
> On Thu 24-04-14 08:19:17, Johannes Weiner wrote:
> > On Fri, Apr 18, 2014 at 01:36:11PM +0200, Michal Hocko wrote:
> > > On Wed 16-04-14 17:13:18, Johannes Weiner wrote:
> > > > Per-memcg swappiness and oom killing can currently not be tweaked on a
> > > > memcg that is part of a hierarchy, but not the root of that hierarchy.
> > > > Users have complained that they can't configure this when they turned
> > > > on hierarchy mode.  In fact, with hierarchy mode becoming the default,
> > > > this restriction disables the tunables entirely.
> > > 
> > > Except when we would handle the first level under root differently,
> > > which is ugly.
> > > 
> > > > But there is no good reason for this restriction. 
> > > 
> > > I had a patch for this somewhere on the think_more pile. I wasn't
> > > particularly happy about the semantic so I haven't posted it.
> > > 
> > > > The settings for
> > > > swappiness and OOM killing are taken from whatever memcg whose limit
> > > > triggered reclaim and OOM invocation, regardless of its position in
> > > > the hierarchy tree.
> > > 
> > > This is OK for the OOM knob because the memory pressure cannot be
> > > handled at that level in hierarchy and that is where the OOM happens.
> > > 
> > > I am not so sure about the swappiness though. The swappiness tells us
> > > how to proportionally scan anon vs. file LRUs and those are per-memcg,
> > > not per-hierarchy (unlike the charge) so it makes sense to use it
> > > per-memcg IMO.
> > > 
> > > Besides that using the reclaim target value might be quite confusing.
> > > Say, somebody wants to prevent from swapping in a certain group and
> > > yet the pages find their way to swap depending on where the reclaim is
> > > triggered from.
> > > Another thing would be that setting swappiness on an unlimited group has
> > > no effect although I would argue it makes some sense in configuration
> > > when parent is controlled by somebody else. I would like to tell how
> > > to reclaim me when I cannot say how much memory I can have. 
> > > 
> > > It is true that we have a different behavior for the global reclaim
> > > already but I am not entirely happy about that. Having a different
> > > behavior for the global vs. limit reclaims just calls for troubles and
> > > should be avoided as much as possible.
> > > 
> > > So let's think what is the best semantic before we merge this. I would
> > > be more inclined for using per-memcg swappiness all the time (root using
> > > the global knob) for all reclaims.
> > 
> > Yeah, we've always used the triggering group's swappiness value but at
> > the same time forced the whole hierarchy to have the same setting as
> > the root.
> > 
> > I don't really feel strongly about this.  If you prefer the per-memcg
> > swappiness I can send a followup patch - or you can.
> 
> OK, I originally thought this would be in the same patch but now that I
> think about it some more it would be better to have it separate in case
> it turns out this will cause some issues (at least
> global_reclaim-always-use-global-vm_swappiness is a behavior change).
> So what do you think about this?
> ---
> >From 3a865b7b53aed96d93bbcf865028e63fd6f582ab Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Thu, 24 Apr 2014 15:28:05 +0200
> Subject: [RFC PATCH] vmscan: memcg: Always use swappiness of the reclaimed memcg
> 
> The memory reclaim always uses swappiness of the reclaim target memcg
> (origin of the memory pressure) or vm_swappiness for the global memory
> reclaim. This behavior was consistent (except for difference between
> global and hard limit reclaim) because swappiness was enforced to be
> consistent within each memcg hierarchy.
> 
> After "mm: memcontrol: remove hierarchy restrictions for swappiness
> and oom_control" each memcg can have its own swappiness independent on
> hierarchical parents, though, so the consistency guarantee is gone.
> This can lead to an unexpected behavior. Say that a group is explicitly
> configured to not swapout by memory.swappiness=0 but its memory gets
> swapped out anyway when the memory pressure comes from its parent with a
> different swapping policy.
> It is also unexpected that the knob is meaningless without setting the
> hard limit which would trigger the reclaim and enforce the swappiness.
> There are setups where the hard limit is configured higher in the
> hierarchy by an administrator and children groups are under control of
> somebody else who is interested in the swapout behavior but not
> necessarily about the memory limit.
> 
> >From a semantic point of view swappiness is an attribute defining
> anon vs. file proportional scanning of LRU which is memcg specific
> (unlike charges which are propagated up the hierarchy) so it should be
> applied to the particular memcg's LRU regardless where the memory
> pressure comes from.
> 
> This patch removes vmscan_swappiness() and stores the swappiness into
> the scan_control structure. mem_cgroup_swappiness is then used to
> provide the correct value before shrink_lruvec is called.  The global
> vm_swappiness is used for the root memcg.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

> @@ -2221,6 +2217,7 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
>  
>  			lruvec = mem_cgroup_zone_lruvec(zone, memcg);
>  
> +			sc->swappiness = mem_cgroup_swappiness(memcg);
>  			shrink_lruvec(lruvec, sc);

This is a little nasty, but oh well...

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