Re: [PATCH v1] [mm-unstable] mm: Fix memcg reclaim on memory tiered systems

[Huang, Ying]

Mina Almasry <almasrymina@xxxxxxxxxx> writes:

> On Sun, Dec 4, 2022 at 6:39 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Mina Almasry <almasrymina@xxxxxxxxxx> writes:
>>
>> > commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
>> > reclaim"") enabled demotion in memcg reclaim, which is the right thing
>> > to do, however, I suspect it introduced a regression in the behavior of
>> > try_to_free_mem_cgroup_pages().
>> >
>> > The callers of try_to_free_mem_cgroup_pages() expect it to attempt to
>> > reclaim - not demote - nr_pages from the cgroup. I.e. the memory usage
>> > of the cgroup should reduce by nr_pages. The callers expect
>> > try_to_free_mem_cgroup_pages() to also return the number of pages
>> > reclaimed, not demoted.
>> >
>> > However, what try_to_free_mem_cgroup_pages() actually does is it
>> > unconditionally counts demoted pages as reclaimed pages. So in practice
>> > when it is called it will often demote nr_pages and return the number of
>> > demoted pages to the caller. Demoted pages don't lower the memcg usage,
>> > and so I think try_to_free_mem_cgroup_pages() is not actually doing what
>> > the callers want it to do.
>> >
>> > I suspect various things work suboptimally on memory systems or don't
>> > work at all due to this:
>> >
>> > - memory.high enforcement likely doesn't work (it just demotes nr_pages
>> >   instead of lowering the memcg usage by nr_pages).
>> > - try_charge_memcg() will keep retrying the charge while
>> >   try_to_free_mem_cgroup_pages() is just demoting pages and not actually
>> >   making any room for the charge.
>> > - memory.reclaim has a wonky interface. It advertises to the user it
>> >   reclaims the provided amount but it will actually demote that amount.
>> >
>> > There may be more effects to this issue.
>> >
>> > To fix these issues I propose shrink_folio_list() to only count pages
>> > demoted from inside of sc->nodemask to outside of sc->nodemask as
>> > 'reclaimed'.
>> >
>> > For callers such as reclaim_high() or try_charge_memcg() that set
>> > sc->nodemask to NULL, try_to_free_mem_cgroup_pages() will try to
>> > actually reclaim nr_pages and return the number of pages reclaimed. No
>> > demoted pages would count towards the nr_pages requirement.
>> >
>> > For callers such as memory_reclaim() that set sc->nodemask,
>> > try_to_free_mem_cgroup_pages() will free nr_pages from that nodemask
>> > with either reclaim or demotion.
>>
>> Have you checked all callers?  For example, IIUC, in
>> reclaim_clean_pages_from_list(), although sc.nodemask == NULL, the
>> demoted pages should be counted as reclaimed.
>
> I checked all call stacks leading to shrink_folio_list() now (at least
> I hope). Here is what I think they do and how I propose to handle
> them:
>
> - reclaim_clean_pages_from_list() & __node_reclaim() & balance_pgdat()
> These try to free memory from a specific node, and both demotion and
> reclaim from that node should be counted. I propose these calls set
> sc>nodemask = pgdat.node_id to signal to shrink_folio_list() that both
> demotion and reclaim from this node should be counted.
>
> - try_to_free_pages()
> Tries to free pages from a specific nodemask. It sets sc->nodemask to
> ac->nodemask. In this case pages demoted within the nodemask should
> not count. Pages demoted outside of the nodemask should count, which
> this patch already tries to do.
>
> - mem_cgroup_shrink_node()
> This is memcg soft limit reclaim. AFAIU only reclaim should be
> counted. It already sets sc->nodemask=NULL to indicate that it
> requires reclaim from all nodes and that only reclaimed memory should
> be counted, which this patch already tries to do.
>
> - try_to_free_mem_cgroup_pages()
> This is covered in the commit message. Many callers set nodemask=NULL
> indicating they want reclaim and demotion should not count.
> memory.reclaim sets nodemask depending on the 'nodes=' arg and wants
> demotion and reclaim from that nodemask.
>
> - reclaim_folio_list()
> Sets no_demotion = 1. No ambiguity here, only reclaims and counts
> reclaimed pages.
>
> If agreeable I can fix reclaim_clean_pages_from_list() &
> __node_reclaim() & balance_pgdat() call sites in v3.

Looks good to me, Thanks!

>> How about count both
>> "demoted" and "reclaimed" in struct scan_control, and let callers to
>> determine how to use the number?
>>
>
> I don't think this is by itself enough. Pages demoted between 2 nodes
> that are both in sc->nodemask should not count, I think. So 'demoted'
> needs to be specifically pages demoted outside of the nodemask.

Yes.  Maybe we can do that when we need it.  I suggest to change the
return value description in the comments of shrink_folio_list().

> We can do 2 things:
>
> 1. Only allow the kernel to demote outside the nodemask (which you
> don't prefer).
> 2. Allow the kernel to demote inside the nodemask but not count them.
>
> I will see if I can implement #2.

Thanks!

>> > Tested this change using memory.reclaim interface. With this change,
>> >
>> >       echo "1m" > memory.reclaim
>> >
>> > Will cause freeing of 1m of memory from the cgroup regardless of the
>> > demotions happening inside.
>> >
>> >       echo "1m nodes=0" > memory.reclaim
>>
>> Have you tested these tests in the original kernel?  If so, whether does
>> the issue you suspected above occurs during testing?
>>
>
> Yes. I set up a test case where I allocate 500m in a cgroup, and then do:
>
>     echo "50m" > memory.reclaim
>
> Without my fix, my kernel demotes 70mb and reclaims 4 mb.
>
> With my v1 fix, my kernel demotes all memory possible and reclaims 60mb.
>
> I will add this to the commit message in the next version.

Good!  Thanks!

Best Regards,
Huang, Ying

>>
>> > Will cause freeing of 1m of node 0 by demotion if a demotion target is
>> > available, and by reclaim if no demotion target is available.
>> >
>> > Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
>> >
>> > ---
>> >
>> > This is developed on top of mm-unstable largely because I need the
>> > memory.reclaim nodes= arg to test it properly.
>> > ---
>> >  mm/vmscan.c | 13 ++++++++++++-
>> >  1 file changed, 12 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > index 2b42ac9ad755..8f6e993b870d 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -1653,6 +1653,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>> >       LIST_HEAD(free_folios);
>> >       LIST_HEAD(demote_folios);
>> >       unsigned int nr_reclaimed = 0;
>> > +     unsigned int nr_demoted = 0;
>> >       unsigned int pgactivate = 0;
>> >       bool do_demote_pass;
>> >       struct swap_iocb *plug = NULL;
>> > @@ -2085,7 +2086,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>> >       /* 'folio_list' is always empty here */
>> >
>> >       /* Migrate folios selected for demotion */
>> > -     nr_reclaimed += demote_folio_list(&demote_folios, pgdat);
>> > +     nr_demoted = demote_folio_list(&demote_folios, pgdat);
>> > +
>> > +     /*
>> > +      * Only count demoted folios as reclaimed if we demoted them from
>> > +      * inside of the nodemask to outside of the nodemask, hence reclaiming
>> > +      * pages in the nodemask.
>> > +      */
>> > +     if (sc->nodemask && node_isset(pgdat->node_id, *sc->nodemask) &&
>> > +         !node_isset(next_demotion_node(pgdat->node_id), *sc->nodemask))
>> > +             nr_reclaimed += nr_demoted;
>> > +
>> >       /* Folios that could not be demoted are still in @demote_folios */
>> >       if (!list_empty(&demote_folios)) {
>> >               /* Folios which weren't demoted go back on @folio_list */
>> > --
>> > 2.39.0.rc0.267.gcb52ba06e7-goog
>>