On Thu, Oct 24, 2024 at 9:36 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Thu, Oct 24, 2024 at 8:47 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
> >
> >
> >
> > On 23/10/2024 19:52, Barry Song wrote:
> > > On Thu, Oct 24, 2024 at 7:31 AM Usama Arif <usamaarif642@xxxxxxxxx> 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;
>
> >
> > If we want to maintain consitency for all folios another option is
> > mem_cgroup_margin(memcg) < MEMCG_CHARGE_BATCH * folio_nr_pages(folio)
> > but I think this is too extreme, we would be checking if 64M can be charged to
> > cgroup just to swapin 1M.
> >
> > >
> > > I believe there’s still room for improvement. For example, if a 64KB charge
> > > attempt fails, there’s no need to waste time trying 32KB or 16KB. We can
> > > directly fall back to 4KB, as 32KB and 16KB will also fail based on our
> > > margin detection logic.
> > >
> >
> > Yes that makes sense. Would something like below work to fix that:
> >
> > 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;
> > diff --git a/mm/memory.c b/mm/memory.c
> > index fecdd044bc0b..b6ce6605dc63 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4123,6 +4123,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
> > pte_t *pte;
> > gfp_t gfp;
> > int order;
> > + int ret;
> >
> > /*
> > * If uffd is active for the vma we need per-page fault fidelity to
> > @@ -4170,9 +4171,13 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
> > addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> > folio = vma_alloc_folio(gfp, order, vma, addr, true);
> > if (folio) {
> > - if (!mem_cgroup_swapin_charge_folio(folio, vma->vm_mm,
> > - gfp, entry))
> > + ret = mem_cgroup_swapin_charge_folio(folio, vma->vm_mm, gfp, entry);
> > + if (!ret) {
> > return folio;
> > + } else if (ret == -ENOMEM) {
> > + folio_put(folio);
> > + goto fallback;
> > + }
> > folio_put(folio);
> > }
> > order = next_order(&orders, order);
> >
>
> Yes, does it make your kernel build even faster?
Thanks
Barry
