Re: [rfc patch 2/6] vmscan: make distinction between memcg reclaim and LRU list selection

[Ying Han <yinghan@google.com>]

On Thu, May 12, 2011 at 9:03 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Thu, May 12, 2011 at 11:33:13AM -0400, Rik van Riel wrote:
>> On 05/12/2011 10:53 AM, Johannes Weiner wrote:
>> >The reclaim code has a single predicate for whether it currently
>> >reclaims on behalf of a memory cgroup, as well as whether it is
>> >reclaiming from the global LRU list or a memory cgroup LRU list.
>> >
>> >Up to now, both cases always coincide, but subsequent patches will
>> >change things such that global reclaim will scan memory cgroup lists.
>> >
>> >This patch adds a new predicate that tells global reclaim from memory
>> >cgroup reclaim, and then changes all callsites that are actually about
>> >global reclaim heuristics rather than strict LRU list selection.
>> >
>> >Signed-off-by: Johannes Weiner<hannes@cmpxchg.org>
>> >---
>> >  mm/vmscan.c |   96 ++++++++++++++++++++++++++++++++++------------------------
>> >  1 files changed, 56 insertions(+), 40 deletions(-)
>> >
>> >diff --git a/mm/vmscan.c b/mm/vmscan.c
>> >index f6b435c..ceeb2a5 100644
>> >--- a/mm/vmscan.c
>> >+++ b/mm/vmscan.c
>> >@@ -104,8 +104,12 @@ struct scan_control {
>> >      */
>> >     reclaim_mode_t reclaim_mode;
>> >
>> >-    /* Which cgroup do we reclaim from */
>> >-    struct mem_cgroup *mem_cgroup;
>> >+    /*
>> >+     * The memory cgroup we reclaim on behalf of, and the one we
>> >+     * are currently reclaiming from.
>> >+     */
>> >+    struct mem_cgroup *memcg;
>> >+    struct mem_cgroup *current_memcg;
>>
>> I can't say I'm fond of these names.  I had to read the
>> rest of the patch to figure out that the old mem_cgroup
>> got renamed to current_memcg.
>
> To clarify: sc->memcg will be the memcg that hit the hard limit and is
> the main target of this reclaim invocation.  current_memcg is the
> iterator over the hierarchy below the target.

I would assume the new variable memcg is a renaming of the
"mem_cgroup" which indicating which cgroup we reclaim on behalf of.
About the "current_memcg", i couldn't find where it is indicating to
be the current cgroup under the hierarchy below the "memcg".

Both mem_cgroup_shrink_node_zone() and try_to_free_mem_cgroup_pages()
are called within mem_cgroup_hierarchical_reclaim(), and the sc->memcg
is initialized w/ the victim passed down which is already the memcg
under hierarchy.

--Ying


> I realize this change in particular was placed a bit unfortunate in
> terms of understanding in the series, I just wanted to keep out the
> mem_cgroup to current_memcg renaming out of the next patch.  There is
> probably a better way, I'll fix it up and improve the comment.
>
>> Would it be better to call them my_memcg and reclaim_memcg?
>>
>> Maybe somebody else has better suggestions...
>
> Yes, suggestions welcome.  I'm not too fond of the naming, either.
>
>> Other than the naming, no objection.
>
> Thanks, Rik.
>
>        Hannes
>

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