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SubjectRe: [patch 05/11] mm: move memcg hierarchy reclaim to generic reclaim code
On Tue 20-09-11 15:29:28, Johannes Weiner wrote:
> On Tue, Sep 20, 2011 at 03:09:15PM +0200, Michal Hocko wrote:
> > On Mon 12-09-11 12:57:22, Johannes Weiner wrote:
> > > Memory cgroup limit reclaim and traditional global pressure reclaim
> > > will soon share the same code to reclaim from a hierarchical tree of
> > > memory cgroups.
> > >
> > > In preparation of this, move the two right next to each other in
> > > shrink_zone().
> >
> > I like the way how you've split mem_cgroup_hierarchical_reclaim into
> > mem_cgroup_reclaim and mem_cgroup_soft_reclaim and I guess this deserves
> > a note in the patch description. Especially that mem_cgroup_reclaim is
> > hierarchical even though it doesn't use mem_cgroup_iter directly but
> > rather via do_try_to_free_pages and shrink_zone.
> >
> > I am not sure I see how shrink_mem_cgroup_zone works. See comments and
> > questions bellow:
> >
> > >
> > > Signed-off-by: Johannes Weiner <jweiner@redhat.com>
> > > ---
> > > include/linux/memcontrol.h | 25 ++++++-
> > > mm/memcontrol.c | 167 ++++++++++++++++++++++----------------------
> > > mm/vmscan.c | 43 ++++++++++-
> > > 3 files changed, 147 insertions(+), 88 deletions(-)
> > >
> > [...]
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index f4b404e..413e1f8 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > [...]
> > > @@ -783,19 +781,33 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
> > > return memcg;
> > > }
> > >
> > > -struct mem_cgroup_iter {
> > > - struct zone *zone;
> > > - int priority;
> > > - unsigned int generation;
> > > -};
> > > -
> > > -static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> > > - struct mem_cgroup *prev,
> > > - struct mem_cgroup_iter *iter)
> > > +/**
> > > + * mem_cgroup_iter - iterate over memory cgroup hierarchy
> > > + * @root: hierarchy root
> > > + * @prev: previously returned memcg, NULL on first invocation
> > > + * @iter: token for partial walks, NULL for full walks
> > > + *
> > > + * Returns references to children of the hierarchy starting at @root,
> >
> > I guess you meant "starting at @prev"
>
> Nope, although it is a bit ambiguous, both the hierarchy as well as
> the iteration start at @root. Unless @iter is specified, but then the
> hierarchy still starts at @root.

OK I guess I see where my misundestanding comes from. You are describing
the function in the way how is it supposed to be used (with prev=NULL
and then reusing the return value). I was thinking about general
description where someone wants to provide a prev and expects that the
function would return a next group under the hierarchy. In that case we
would have to go into ids so this is probably better.

>
> I attached a patch below that fixes the phrasing.

Makes more sense.

[...]
> > > @@ -2104,12 +2104,43 @@ restart:
> > > static void shrink_zone(int priority, struct zone *zone,
> > > struct scan_control *sc)
> > > {
> > > - struct mem_cgroup_zone mz = {
> > > - .mem_cgroup = sc->target_mem_cgroup,
> > > + struct mem_cgroup *root = sc->target_mem_cgroup;
> > > + struct mem_cgroup_iter iter = {
> > > .zone = zone,
> > > + .priority = priority,
> > > };
> > > + struct mem_cgroup *mem;
> > > +
> > > + if (global_reclaim(sc)) {
> > > + struct mem_cgroup_zone mz = {
> > > + .mem_cgroup = NULL,
> > > + .zone = zone,
> > > + };
> > > +
> > > + shrink_mem_cgroup_zone(priority, &mz, sc);
> > > + return;
> > > + }
> > > +
> > > + mem = mem_cgroup_iter(root, NULL, &iter);
> > > + do {
> > > + struct mem_cgroup_zone mz = {
> > > + .mem_cgroup = mem,
> > > + .zone = zone,
> > > + };
> > >
> > > - shrink_mem_cgroup_zone(priority, &mz, sc);
> > > + shrink_mem_cgroup_zone(priority, &mz, sc);
> > > + /*
> > > + * Limit reclaim has historically picked one memcg and
> > > + * scanned it with decreasing priority levels until
> > > + * nr_to_reclaim had been reclaimed. This priority
> > > + * cycle is thus over after a single memcg.
> > > + */
> > > + if (!global_reclaim(sc)) {
> >
> > How can we have global_reclaim(sc) == true here?
>
> We can't yet, but will when global reclaim and limit reclaim use that
> same loop a patch or two later.

OK, I see. Still a bit tricky for review, though.

> I added it early to have an anchor for the comment above it, so that
> it's obvious how limit reclaim behaves, and that I don't have to
> change limit reclaim-specific conditions when changing how global
> reclaim works.
>
> > Shouldn't we just check how much have we reclaimed from that group and
> > iterate only if it wasn't sufficient (at least SWAP_CLUSTER_MAX)?
>
> I played with various exit conditions and in the end just left the
> behaviour exactly like we had it before this patch, just that the
> algorithm is now inside out. If you want to make such a fundamental
> change, you have to prove it works :-)

OK, I see.

>
>
> Signed-off-by: Johannes Weiner <jweiner@redhat.com>

For the whole patch.
Reviewed-by: Michal Hocko <mhocko@suse.cz>

> ---
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a7d14a5..349620c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -784,8 +784,8 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
> * @prev: previously returned memcg, NULL on first invocation
> * @iter: token for partial walks, NULL for full walks
> *
> - * Returns references to children of the hierarchy starting at @root,
> - * or @root itself, or %NULL after a full round-trip.
> + * Returns references to children of the hierarchy below @root, or
> + * @root itself, or %NULL after a full round-trip.
> *
> * Caller must pass the return value in @prev on subsequent
> * invocations for reference counting, or use mem_cgroup_iter_break()
> @@ -1493,7 +1493,8 @@ static unsigned long mem_cgroup_reclaim(struct mem_cgroup *mem,
> total += try_to_free_mem_cgroup_pages(mem, gfp_mask, noswap);
> /*
> * Avoid freeing too much when shrinking to resize the
> - * limit. XXX: Shouldn't the margin check be enough?
> + * limit, and bail easily so that signals have a
> + * chance to kill the resizing task.
> */
> if (total && (flags & MEM_CGROUP_RECLAIM_SHRINK))
> break;
>
--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic


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