Re: [RFC 0/4] mm: zswap: add support for zswapin of large folios

Barry Song
2024-10-24T17:48:44Z

On Fri, Oct 25, 2024 at 3:29 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, Oct 24, 2024 at 12:35:48PM +1300, Barry Song wrote:
> > On Thu, Oct 24, 2024 at 9:36 AM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Thu, Oct 24, 2024 at 8:47 AM Usama Arif <usamaarif642@gmail.com> wrote:
> > > >
> > > >
> > > >
> > > > On 23/10/2024 19:52, Barry Song wrote:
> > > > > On Thu, Oct 24, 2024 at 7:31 AM Usama Arif <usamaarif642@gmail.com> wrote:
> > > > >>
> > > > >>
> > > > >>
> > > > >> On 23/10/2024 19:02, Yosry Ahmed wrote:
> > > > >>> [..]
> > > > >>>>>> I suspect the regression occurs because you're running an edge case
> > > > >>>>>> where the memory cgroup stays nearly full most of the time (this isn't
> > > > >>>>>> an inherent issue with large folio swap-in). As a result, swapping in
> > > > >>>>>> mTHP quickly triggers a memcg overflow, causing a swap-out. The
> > > > >>>>>> next swap-in then recreates the overflow, leading to a repeating
> > > > >>>>>> cycle.
> > > > >>>>>>
> > > > >>>>>
> > > > >>>>> Yes, agreed! Looking at the swap counters, I think this is what is going
> > > > >>>>> on as well.
> > > > >>>>>
> > > > >>>>>> We need a way to stop the cup from repeatedly filling to the brim and
> > > > >>>>>> overflowing. While not a definitive fix, the following change might help
> > > > >>>>>> improve the situation:
> > > > >>>>>>
> > > > >>>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > >>>>>>
> > > > >>>>>> index 17af08367c68..f2fa0eeb2d9a 100644
> > > > >>>>>> --- a/mm/memcontrol.c
> > > > >>>>>> +++ b/mm/memcontrol.c
> > > > >>>>>>
> > > > >>>>>> @@ -4559,7 +4559,10 @@ int mem_cgroup_swapin_charge_folio(struct folio
> > > > >>>>>> *folio, struct mm_struct *mm,
> > > > >>>>>> memcg = get_mem_cgroup_from_mm(mm);
> > > > >>>>>> rcu_read_unlock();
> > > > >>>>>>
> > > > >>>>>> - ret = charge_memcg(folio, memcg, gfp);
> > > > >>>>>> + if (folio_test_large(folio) && mem_cgroup_margin(memcg) <
> > > > >>>>>> MEMCG_CHARGE_BATCH)
> > > > >>>>>> + ret = -ENOMEM;
> > > > >>>>>> + else
> > > > >>>>>> + ret = charge_memcg(folio, memcg, gfp);
> > > > >>>>>>
> > > > >>>>>> css_put(&memcg->css);
> > > > >>>>>> return ret;
> > > > >>>>>> }
> > > > >>>>>>
> > > > >>>>>
> > > > >>>>> The diff makes sense to me. Let me test later today and get back to you.
> > > > >>>>>
> > > > >>>>> Thanks!
> > > > >>>>>
> > > > >>>>>> Please confirm if it makes the kernel build with memcg limitation
> > > > >>>>>> faster. If so, let's
> > > > >>>>>> work together to figure out an official patch :-) The above code hasn't consider
> > > > >>>>>> the parent memcg's overflow, so not an ideal fix.
> > > > >>>>>>
> > > > >>>>
> > > > >>>> Thanks Barry, I think this fixes the regression, and even gives an improvement!
> > > > >>>> I think the below might be better to do:
> > > > >>>>
> > > > >>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > >>>> index c098fd7f5c5e..0a1ec55cc079 100644
> > > > >>>> --- a/mm/memcontrol.c
> > > > >>>> +++ b/mm/memcontrol.c
> > > > >>>> @@ -4550,7 +4550,11 @@ int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
> > > > >>>> memcg = get_mem_cgroup_from_mm(mm);
> > > > >>>> rcu_read_unlock();
> > > > >>>>
> > > > >>>> - ret = charge_memcg(folio, memcg, gfp);
> > > > >>>> + if (folio_test_large(folio) &&
> > > > >>>> + mem_cgroup_margin(memcg) < max(MEMCG_CHARGE_BATCH, folio_nr_pages(folio)))
> > > > >>>> + ret = -ENOMEM;
> > > > >>>> + else
> > > > >>>> + ret = charge_memcg(folio, memcg, gfp);
> > > > >>>>
> > > > >>>> css_put(&memcg->css);
> > > > >>>> return ret;
> > > > >>>>
> > > > >>>>
> > > > >>>> AMD 16K+32K THP=always
> > > > >>>> metric mm-unstable mm-unstable + large folio zswapin series mm-unstable + large folio zswapin + no swap thrashing fix
> > > > >>>> real 1m23.038s 1m23.050s 1m22.704s
> > > > >>>> user 53m57.210s 53m53.437s 53m52.577s
> > > > >>>> sys 7m24.592s 7m48.843s 7m22.519s
> > > > >>>> zswpin 612070 999244 815934
> > > > >>>> zswpout 2226403 2347979 2054980
> > > > >>>> pgfault 20667366 20481728 20478690
> > > > >>>> pgmajfault 385887 269117 309702
> > > > >>>>
> > > > >>>> AMD 16K+32K+64K THP=always
> > > > >>>> metric mm-unstable mm-unstable + large folio zswapin series mm-unstable + large folio zswapin + no swap thrashing fix
> > > > >>>> real 1m22.975s 1m23.266s 1m22.549s
> > > > >>>> user 53m51.302s 53m51.069s 53m46.471s
> > > > >>>> sys 7m40.168s 7m57.104s 7m25.012s
> > > > >>>> zswpin 676492 1258573 1225703
> > > > >>>> zswpout 2449839 2714767 2899178
> > > > >>>> pgfault 17540746 17296555 17234663
> > > > >>>> pgmajfault 429629 307495 287859
> > > > >>>>
> > > > >>>
> > > > >>> Thanks Usama and Barry for looking into this. It seems like this would
> > > > >>> fix a regression with large folio swapin regardless of zswap. Can the
> > > > >>> same result be reproduced on zram without this series?
> > > > >>
> > > > >>
> > > > >> Yes, its a regression in large folio swapin support regardless of zswap/zram.
> > > > >>
> > > > >> Need to do 3 tests, one with probably the below diff to remove large folio support,
> > > > >> one with current upstream and one with upstream + swap thrashing fix.
> > > > >>
> > > > >> We only use zswap and dont have a zram setup (and I am a bit lazy to create one :)).
> > > > >> Any zram volunteers to try this?
> > > > >
> > > > > Hi Usama,
> > > > >
> > > > > I tried a quick experiment:
> > > > >
> > > > > echo 1 > /sys/module/zswap/parameters/enabled
> > > > > echo 0 > /sys/module/zswap/parameters/enabled
> > > > >
> > > > > This was to test the zRAM scenario. Enabling zswap even
> > > > > once disables mTHP swap-in. :)
> > > > >
> > > > > I noticed a similar regression with zRAM alone, but the change resolved
> > > > > the issue and even sped up the kernel build compared to the setup without
> > > > > mTHP swap-in.
> > > >
> > > > Thanks for trying, this is amazing!
> > > > >
> > > > > However, I’m still working on a proper patch to address this. The current
> > > > > approach:
> > > > >
> > > > > mem_cgroup_margin(memcg) < max(MEMCG_CHARGE_BATCH, folio_nr_pages(folio))
> > > > >
> > > > > isn’t sufficient, as it doesn’t cover cases where group A contains group B, and
> > > > > we’re operating within group B. The problem occurs not at the boundary of
> > > > > group B but at the boundary of group A.
> > > >
> > > > I am not sure I completely followed this. As MEMCG_CHARGE_BATCH=64, if we are
> > > > trying to swapin a 16kB page, we basically check if atleast 64/4 = 16 folios can be
> > > > charged to cgroup, which is reasonable. If we try to swapin a 1M folio, we just
> > > > check if we can charge atleast 1 folio. Are you saying that checking just 1 folio
> > > > is not enough in this case and can still cause thrashing, i.e we should check more?
> > >
> > > My understanding is that cgroups are hierarchical. Even if we don’t
> > > hit the memory
> > > limit of the folio’s direct memcg, we could still reach the limit of
> > > one of its parent
> > > memcgs. Imagine a structure like:
> > >
> > > /sys/fs/cgroup/a/b/c/d
> > >
> > > If we’re compiling the kernel in d, there’s a chance that while d
> > > isn’t at its limit, its
> > > parents (c, b, or a) could be. Currently, the check only applies to d.
> >
> > To clarify, I mean something like this:
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 17af08367c68..cc6d21848ee8 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4530,6 +4530,29 @@ int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
> > return 0;
> > }
> >
> > +/*
> > + * When the memory cgroup is nearly full, swapping in large folios can
> > + * easily lead to swap thrashing, as the memcg operates on the edge of
> > + * being full. We maintain a margin to allow for quick fallback to
> > + * smaller folios during the swap-in process.
> > + */
> > +static inline bool mem_cgroup_swapin_margin_protected(struct mem_cgroup *memcg,
> > + struct folio *folio)
> > +{
> > + unsigned int nr;
> > +
> > + if (!folio_test_large(folio))
> > + return false;
> > +
> > + nr = max_t(unsigned int, folio_nr_pages(folio), MEMCG_CHARGE_BATCH);
> > + for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
> > + if (mem_cgroup_margin(memcg) < nr)
> > + return true;
> > + }
> > +
> > + return false;
> > +}
> > +
> > /**
> > * mem_cgroup_swapin_charge_folio - Charge a newly allocated folio for swapin.
> > * @folio: folio to charge.
> > @@ -4547,7 +4570,8 @@ int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
> > {
> > struct mem_cgroup *memcg;
> > unsigned short id;
> > - int ret;
> > + int ret = -ENOMEM;
> > + bool margin_prot;
> >
> > if (mem_cgroup_disabled())
> > return 0;
> > @@ -4557,9 +4581,11 @@ int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
> > memcg = mem_cgroup_from_id(id);
> > if (!memcg || !css_tryget_online(&memcg->css))
> > memcg = get_mem_cgroup_from_mm(mm);
> > + margin_prot = mem_cgroup_swapin_margin_protected(memcg, folio);
> > rcu_read_unlock();
> >
> > - ret = charge_memcg(folio, memcg, gfp);
> > + if (!margin_prot)
> > + ret = charge_memcg(folio, memcg, gfp);
> >
> > css_put(&memcg->css);
> > return ret;
>
> I'm not quite following.
>
> The charging code DOES the margin check. If you just want to avoid
> reclaim, pass gfp without __GFP_DIRECT_RECLAIM, and it will return
> -ENOMEM if there is no margin.
>
> alloc_swap_folio() passes the THP mask, which should not include the
> reclaim flag per default (GFP_TRANSHUGE_LIGHT). Unless you run with
> defrag=always. Is that what's going on?

No, quite sure "defrag=never" can just achieve the same result. Imagine we only
have small folios—each time reclamation occurs, we have at least a
SWAP_CLUSTER_MAX buffer before the next reclamation is triggered.

.nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),

However, with large folios, we can quickly exhaust the SWAP_CLUSTER_MAX
buffer and reach the next reclamation point.
Once we consume SWAP_CLUSTER_MAX - 1, the mem_cgroup_swapin_charge_folio()
call for the final small folio with GFP_KERNEL will trigger reclamation.
if (mem_cgroup_swapin_charge_folio(folio, vma->vm_mm,
GFP_KERNEL, entry)) {

Thanks
Barry