Re: [PATCH v4 1/2] mm: add per-order mTHP anon_alloc and anon_alloc_fallback counters

[Barry Song]

On Fri, Apr 12, 2024 at 4:38 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 05/04/2024 11:27, Barry Song wrote:
> > From: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > Profiling a system blindly with mTHP has become challenging due to the
> > lack of visibility into its operations. Presenting the success rate of
> > mTHP allocations appears to be pressing need.
> >
> > Recently, I've been experiencing significant difficulty debugging
> > performance improvements and regressions without these figures.
> > It's crucial for us to understand the true effectiveness of mTHP in
> > real-world scenarios, especially in systems with fragmented memory.
> >
> > This patch sets up the framework for per-order mTHP counters, starting
> > with the introduction of anon_alloc and anon_alloc_fallback counters.
> > Incorporating additional counters should now be straightforward as well.
> >
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > ---
> >  include/linux/huge_mm.h | 19 ++++++++++++++++
> >  mm/huge_memory.c        | 48 +++++++++++++++++++++++++++++++++++++++++
> >  mm/memory.c             |  2 ++
> >  3 files changed, 69 insertions(+)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index e896ca4760f6..c5d33017a4dd 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -264,6 +264,25 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
> >                                         enforce_sysfs, orders);
> >  }
> >
> > +enum mthp_stat_item {
> > +     MTHP_STAT_ANON_ALLOC,
> > +     MTHP_STAT_ANON_ALLOC_FALLBACK,
> > +     __MTHP_STAT_COUNT
> > +};
> > +
> > +struct mthp_stat {
> > +     unsigned long stats[PMD_ORDER + 1][__MTHP_STAT_COUNT];
>
> I saw a fix for this allocation dynamically due to powerpc PMD_ORDER not being
> constant. I wonder if ilog2(MAX_PTRS_PER_PTE) would help here?
>

It's a possibility. However, since we've passed all the build tests
using dynamic
allocation, it might not be worth the effort to attempt static
allocation again. Who
knows what will happen next :-)

> > +};
> > +
> > +DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
> > +
> > +static inline void count_mthp_stat(int order, enum mthp_stat_item item)
>
> I thought we were going to call this always counting up type of stat and event?
> "count_mthp_event"? But I'm happy with it as is, personally.
>
> > +{
> > +     if (unlikely(order > PMD_ORDER))
> > +             return;
>
> I'm wondering if it also makes sense to ignore order == 0? Although I guess if
> called for order-0 its safe since the storage exists and sum_mthp_stat() is
> never be called for 0. Ignore this comment :)

Agreed. I'd like to change it to ignore oder 0;

>
> > +     this_cpu_inc(mthp_stats.stats[order][item]);
> > +}
> > +
> >  #define transparent_hugepage_use_zero_page()                         \
> >       (transparent_hugepage_flags &                                   \
> >        (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG))
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 9d4b2fbf6872..5b875f0fc923 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -526,6 +526,46 @@ static const struct kobj_type thpsize_ktype = {
> >       .sysfs_ops = &kobj_sysfs_ops,
> >  };
> >
> > +DEFINE_PER_CPU(struct mthp_stat, mthp_stats) = {{{0}}};
> > +
> > +static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)
> > +{
> > +     unsigned long sum = 0;
> > +     int cpu;
> > +
> > +     for_each_online_cpu(cpu) {
>
> What happens if a cpu that was online and collected a bunch of stats gets
> offlined? The user will see stats get smaller?
>
> Perhaps this should be for_each_possible_cpu()? Although I'm not sure what
> happens to percpu data when a cpu goes offline? Is the data preserved? Or wiped,
> or unmapped? dunno. Might we need to rescue stats into a global counter at
> offline-time?

Good catch. I see /proc/vmstat is always using the  for_each_online_cpu() but it
doesn't have the issue, but mTHP counters do have the problem.

* step 1: cat  the current thp_swpout value before running a test
program which does
swpout;

/ # cat /proc/vmstat | grep thp_swpout
thp_swpout 0
/ # cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/stats/anon_swpout
0

* step 2: run the test program on cpu2;

/ # taskset -c 2 /home/barry/develop/linux/swpcache-2m

* step 3: cat the current thp_swpout value after running a test
program which does
swpout;

/ # cat /proc/vmstat | grep thp_swpout
thp_swpout 98
/ # cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/stats/anon_swpout
98

*step 4: offline cpu2 and read thp_swpout;

/ # echo 0 > /sys/devices/system/cpu/cpu2/online
[  339.058661] psci: CPU2 killed (polled 0 ms)

/ # cat /proc/vmstat | grep thp_swpout
thp_swpout 98
/ # cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/stats/anon_swpout
0

*step 5: online cpu2 and read thp_swpout

/ # echo 1 > /sys/devices/system/cpu/cpu2/online
[  791.642058] CPU2: Booted secondary processor 0x0000000002 [0x000f0510]

/ # cat /proc/vmstat | grep thp_swpout
thp_swpout 98
/ # cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/stats/anon_swpout
98



As you can see, in step 4, /proc/vmstat is all good but mTHP counters become
zero.

The reason is /proc/vmstat will fold the offline cpu to an online cpu
but mthp counters lack
it:

/*
 * Fold the foreign cpu events into our own.
 *
 * This is adding to the events on one processor
 * but keeps the global counts constant.
 */
void vm_events_fold_cpu(int cpu)
{
        struct vm_event_state *fold_state = &per_cpu(vm_event_states, cpu);
        int i;

        for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
                count_vm_events(i, fold_state->event[i]);
                fold_state->event[i] = 0;
        }
}

static int page_alloc_cpu_dead(unsigned int cpu)
{
        ...
        /*
         * Spill the event counters of the dead processor
         * into the current processors event counters.
         * This artificially elevates the count of the current
         * processor.
         */
        vm_events_fold_cpu(cpu);
        ...

        return 0;
}

So I will do the same thing for mTHP counters - fold offline cpu
counters to online one.

>
> > +             struct mthp_stat *this = &per_cpu(mthp_stats, cpu);
> > +
> > +             sum += this->stats[order][item];
> > +     }
> > +
> > +     return sum;
> > +}
> > +
> > +#define DEFINE_MTHP_STAT_ATTR(_name, _index)                                 \
> > +static ssize_t _name##_show(struct kobject *kobj,                    \
> > +                     struct kobj_attribute *attr, char *buf)         \
> > +{                                                                    \
> > +     int order = to_thpsize(kobj)->order;                            \
> > +                                                                     \
> > +     return sysfs_emit(buf, "%lu\n", sum_mthp_stat(order, _index));  \
> > +}                                                                    \
> > +static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
>
> Very nice!

Right. I got duplicated copy-paste and bad small in code so I wrote this macro.

>
> > +
> > +DEFINE_MTHP_STAT_ATTR(anon_alloc, MTHP_STAT_ANON_ALLOC);
> > +DEFINE_MTHP_STAT_ATTR(anon_alloc_fallback, MTHP_STAT_ANON_ALLOC_FALLBACK);
> > +
> > +static struct attribute *stats_attrs[] = {
> > +     &anon_alloc_attr.attr,
> > +     &anon_alloc_fallback_attr.attr,
> > +     NULL,
> > +};
> > +
> > +static struct attribute_group stats_attr_group = {
> > +     .name = "stats",
> > +     .attrs = stats_attrs,
> > +};
> > +
> >  static struct thpsize *thpsize_create(int order, struct kobject *parent)
> >  {
> >       unsigned long size = (PAGE_SIZE << order) / SZ_1K;
> > @@ -549,6 +589,12 @@ static struct thpsize *thpsize_create(int order, struct kobject *parent)
> >               return ERR_PTR(ret);
> >       }
> >
> > +     ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group);
> > +     if (ret) {
> > +             kobject_put(&thpsize->kobj);
> > +             return ERR_PTR(ret);
> > +     }
> > +
> >       thpsize->order = order;
> >       return thpsize;
> >  }
> > @@ -1050,8 +1096,10 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> >       folio = vma_alloc_folio(gfp, HPAGE_PMD_ORDER, vma, haddr, true);
> >       if (unlikely(!folio)) {
> >               count_vm_event(THP_FAULT_FALLBACK);
> > +             count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_ALLOC_FALLBACK);
>
> I think we should aim for the PMD-oder MTHP_STAT_ANON_ALLOC and
> MTHP_STAT_ANON_ALLOC_FALLBACK to match THP_FAULT_ALLOC and THP_FAULT_FALLBACK.
> Its not currently setup this way...

right. I also realized this and asked for your comments on this in another
thread.

>
>
> >               return VM_FAULT_FALLBACK;
> >       }
> > +     count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_ALLOC);
> >       return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp);
> >  }
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 649e3ed94487..1723c8ddf9cb 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4374,8 +4374,10 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> >                       }
> >                       folio_throttle_swaprate(folio, gfp);
> >                       clear_huge_page(&folio->page, vmf->address, 1 << order);
> > +                     count_mthp_stat(order, MTHP_STAT_ANON_ALLOC);
> >                       return folio;
> >               }
> > +             count_mthp_stat(order, MTHP_STAT_ANON_ALLOC_FALLBACK);
>
> ...And we should follow the usage same pattern for the smaller mTHP here too.
> Which means MTHP_STAT_ANON_ALLOC_FALLBACK should be after the next: label We

The only difference is the case

if (mem_cgroup_charge(folio, vma->vm_mm, gfp))
      goto next;

but vmstat is counting this as fallback so i feel good to move after next,

        if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
                folio_put(folio);
                count_vm_event(THP_FAULT_FALLBACK);
                count_vm_event(THP_FAULT_FALLBACK_CHARGE);
                return VM_FAULT_FALLBACK;
        }

> could introduce a MTHP_STAT_ANON_ALLOC_FALLBACK_CHARGE which would only trigger
> on a fallback due to charge failure, just like THP_FAULT_FALLBACK_CHARGE?

it is fine to add this THP_FAULT_FALLBACK_CHARGE though it is not that
useful for profiling buddy fragmentation.

>
> >  next:
> >               order = next_order(&orders, order);
> >       }
>

Thanks
Barry