linux-kernel - Re: [PATCH 1/2] mm: add per-order mTHP split counters

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Message-ID: <CAK1f24=rC+iEHkwAHmPBk=SUQ9HRHLA+Q7aKcADdO3uQSs9pnA@mail.gmail.com>
Date: Sun, 30 Jun 2024 19:34:17 +0800
From: Lance Yang <ioworker0@...il.com>
To: Barry Song <baohua@...nel.org>
Cc: Ryan Roberts <ryan.roberts@....com>, akpm@...ux-foundation.org, david@...hat.com, 
	baolin.wang@...ux.alibaba.com, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm: add per-order mTHP split counters

Hi Barry,

Thanks for following up!

On Sun, Jun 30, 2024 at 5:48 PM Barry Song <baohua@...nel.org> wrote:
>
> On Thu, Apr 25, 2024 at 3:41 AM Ryan Roberts <ryan.roberts@....com> wrote:
> >
> > + Barry
> >
> > On 24/04/2024 14:51, Lance Yang wrote:
> > > At present, the split counters in THP statistics no longer include
> > > PTE-mapped mTHP. Therefore, this commit introduces per-order mTHP split
> > > counters to monitor the frequency of mTHP splits. This will assist
> > > developers in better analyzing and optimizing system performance.
> > >
> > > /sys/kernel/mm/transparent_hugepage/hugepages-<size>/stats
> > >         split_page
> > >         split_page_failed
> > >         deferred_split_page
> > >
> > > Signed-off-by: Lance Yang <ioworker0@...il.com>
> > > ---
> > >  include/linux/huge_mm.h |  3 +++
> > >  mm/huge_memory.c        | 14 ++++++++++++--
> > >  2 files changed, 15 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > > index 56c7ea73090b..7b9c6590e1f7 100644
> > > --- a/include/linux/huge_mm.h
> > > +++ b/include/linux/huge_mm.h
> > > @@ -272,6 +272,9 @@ enum mthp_stat_item {
> > >       MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
> > >       MTHP_STAT_ANON_SWPOUT,
> > >       MTHP_STAT_ANON_SWPOUT_FALLBACK,
> > > +     MTHP_STAT_SPLIT_PAGE,
> > > +     MTHP_STAT_SPLIT_PAGE_FAILED,
> > > +     MTHP_STAT_DEFERRED_SPLIT_PAGE,
> > >       __MTHP_STAT_COUNT
> > >  };
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 055df5aac7c3..52db888e47a6 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -557,6 +557,9 @@ DEFINE_MTHP_STAT_ATTR(anon_fault_fallback, MTHP_STAT_ANON_FAULT_FALLBACK);
> > >  DEFINE_MTHP_STAT_ATTR(anon_fault_fallback_charge, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
> > >  DEFINE_MTHP_STAT_ATTR(anon_swpout, MTHP_STAT_ANON_SWPOUT);
> > >  DEFINE_MTHP_STAT_ATTR(anon_swpout_fallback, MTHP_STAT_ANON_SWPOUT_FALLBACK);
> > > +DEFINE_MTHP_STAT_ATTR(split_page, MTHP_STAT_SPLIT_PAGE);
> > > +DEFINE_MTHP_STAT_ATTR(split_page_failed, MTHP_STAT_SPLIT_PAGE_FAILED);
> > > +DEFINE_MTHP_STAT_ATTR(deferred_split_page, MTHP_STAT_DEFERRED_SPLIT_PAGE);
> > >
> > >  static struct attribute *stats_attrs[] = {
> > >       &anon_fault_alloc_attr.attr,
> > > @@ -564,6 +567,9 @@ static struct attribute *stats_attrs[] = {
> > >       &anon_fault_fallback_charge_attr.attr,
> > >       &anon_swpout_attr.attr,
> > >       &anon_swpout_fallback_attr.attr,
> > > +     &split_page_attr.attr,
> > > +     &split_page_failed_attr.attr,
> > > +     &deferred_split_page_attr.attr,
> > >       NULL,
> > >  };
> > >
> > > @@ -3083,7 +3089,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> > >       XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
> > >       struct anon_vma *anon_vma = NULL;
> > >       struct address_space *mapping = NULL;
> > > -     bool is_thp = folio_test_pmd_mappable(folio);
> > > +     int order = folio_order(folio);
> > >       int extra_pins, ret;
> > >       pgoff_t end;
> > >       bool is_hzp;
> > > @@ -3262,8 +3268,10 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> > >               i_mmap_unlock_read(mapping);
> > >  out:
> > >       xas_destroy(&xas);
> > > -     if (is_thp)
> > > +     if (order >= HPAGE_PMD_ORDER)
> > >               count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
> > > +     count_mthp_stat(order, !ret ? MTHP_STAT_SPLIT_PAGE :
> > > +                                   MTHP_STAT_SPLIT_PAGE_FAILED);
> > >       return ret;
> > >  }
> > >
> > > @@ -3327,6 +3335,8 @@ void deferred_split_folio(struct folio *folio)
> > >       if (list_empty(&folio->_deferred_list)) {
> > >               if (folio_test_pmd_mappable(folio))
> > >                       count_vm_event(THP_DEFERRED_SPLIT_PAGE);
> > > +             count_mthp_stat(folio_order(folio),
> > > +                             MTHP_STAT_DEFERRED_SPLIT_PAGE);
> >
> > There is a very long conversation with Barry about adding a 'global "mTHP became
> > partially mapped 1 or more processes" counter (inc only)', which terminates at
> > [1]. There is a lot of discussion about the required semantics around the need
> > for partial map to cover alignment and contiguity as well as whether all pages
> > are mapped, and to trigger once it becomes partial in at least 1 process.
> >
> > MTHP_STAT_DEFERRED_SPLIT_PAGE is giving much simpler semantics, but less
> > information as a result. Barry, what's your view here? I'm guessing this doesn't
> > quite solve what you are looking for?
>
> This doesn't quite solve what I am looking for but I still think the
> patch has its value.
>
> I'm looking for a solution that can:
>
>   *  Count the amount of memory in the system for each mTHP size.
>   *  Determine how much memory for each mTHP size is partially unmapped.
>
> For example, in a system with 16GB of memory, we might find that we have 3GB
> of 64KB mTHP, and within that, 512MB is partially unmapped, potentially wasting
> memory at this moment.  I'm uncertain whether Lance is interested in
> this job :-)

Nice, that's an interesting/valuable job for me ;)

Let's do it separately, as 'split' and friends probably can’t be the
solution you
mentioned above, IMHO.

Hmm... I don't have a good idea about the solution for now, but will
think it over
and come back to discuss it here.

>
> Counting deferred_split remains valuable as it can signal whether the system is
> experiencing significant partial unmapping.

Have a nice weekend!
Lance

>
> >
> > [1] https://lore.kernel.org/linux-mm/6cc7d781-884f-4d8f-a175-8609732b87eb@arm.com/
> >
> > Thanks,
> > Ryan
> >
> > >               list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
> > >               ds_queue->split_queue_len++;
> > >  #ifdef CONFIG_MEMCG
> >
>
> Thanks
> Barry