Re: [PATCH v5 1/4] mm: add per-order mTHP anon_fault_alloc and anon_fault_fallback counters

[Barry Song <21cnbao@gmail.com>]

On Fri, Apr 12, 2024 at 10:38 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 12/04/2024 11:29, Barry Song wrote:
> > On Fri, Apr 12, 2024 at 10:25 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>
> >> On 12/04/2024 11:17, Barry Song wrote:
> >>> On Fri, Apr 12, 2024 at 9:56 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>>>
> >>>> On 12/04/2024 10:43, Barry Song wrote:
> >>>>> On Fri, Apr 12, 2024 at 9:27 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>>>>>
> >>>>>> Hi Barry,
> >>>>>>
> >>>>>> 2 remaining comments - otherwise looks good. (same comments I just made in the
> >>>>>> v4 conversation).
> >>>>>>
> >>>>>> On 12/04/2024 08:37, Barry Song wrote:
> >>>>>>> From: Barry Song <v-songbaohua@oppo.com>
> >>>>>>>
> >>>>>>> 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 establishes the framework for per-order mTHP
> >>>>>>> counters. It begins by introducing the anon_fault_alloc and
> >>>>>>> anon_fault_fallback counters. Additionally, to maintain consistency
> >>>>>>> with thp_fault_fallback_charge in /proc/vmstat, this patch also tracks
> >>>>>>> anon_fault_fallback_charge when mem_cgroup_charge fails for mTHP.
> >>>>>>> Incorporating additional counters should now be straightforward as well.
> >>>>>>>
> >>>>>>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> >>>>>>> Cc: Chris Li <chrisl@kernel.org>
> >>>>>>> Cc: David Hildenbrand <david@redhat.com>
> >>>>>>> Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> >>>>>>> Cc: Kairui Song <kasong@tencent.com>
> >>>>>>> Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> >>>>>>> Cc: Peter Xu <peterx@redhat.com>
> >>>>>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
> >>>>>>> Cc: Suren Baghdasaryan <surenb@google.com>
> >>>>>>> Cc: Yosry Ahmed <yosryahmed@google.com>
> >>>>>>> Cc: Yu Zhao <yuzhao@google.com>
> >>>>>>> ---
> >>>>>>>  include/linux/huge_mm.h | 51 ++++++++++++++++++++++++++++++++++
> >>>>>>>  mm/huge_memory.c        | 61 +++++++++++++++++++++++++++++++++++++++++
> >>>>>>>  mm/memory.c             |  3 ++
> >>>>>>>  mm/page_alloc.c         |  4 +++
> >>>>>>>  4 files changed, 119 insertions(+)
> >>>>>>>
> >>>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> >>>>>>> index e896ca4760f6..c5beb54b97cb 100644
> >>>>>>> --- a/include/linux/huge_mm.h
> >>>>>>> +++ b/include/linux/huge_mm.h
> >>>>>>> @@ -264,6 +264,57 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
> >>>>>>>                                         enforce_sysfs, orders);
> >>>>>>>  }
> >>>>>>>
> >>>>>>> +enum mthp_stat_item {
> >>>>>>> +     MTHP_STAT_ANON_FAULT_ALLOC,
> >>>>>>> +     MTHP_STAT_ANON_FAULT_FALLBACK,
> >>>>>>> +     MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
> >>>>>>> +     __MTHP_STAT_COUNT
> >>>>>>> +};
> >>>>>>> +
> >>>>>>> +struct mthp_stat {
> >>>>>>> +     unsigned long stats[0][__MTHP_STAT_COUNT];
> >>>>>>> +};
> >>>>>>> +
> >>>>>>> +extern struct mthp_stat __percpu *mthp_stats;
> >>>>>>> +
> >>>>>>> +static inline void count_mthp_stat(int order, enum mthp_stat_item item)
> >>>>>>> +{
> >>>>>>> +     if (order <= 0 || order > PMD_ORDER || !mthp_stats)
> >>>>>>> +             return;
> >>>>>>> +
> >>>>>>> +     this_cpu_inc(mthp_stats->stats[order][item]);
> >>>>>>> +}
> >>>>>>> +
> >>>>>>> +static inline void count_mthp_stats(int order, enum mthp_stat_item item, long delta)
> >>>>>>> +{
> >>>>>>> +     if (order <= 0 || order > PMD_ORDER || !mthp_stats)
> >>>>>>> +             return;
> >>>>>>> +
> >>>>>>> +     this_cpu_add(mthp_stats->stats[order][item], delta);
> >>>>>>> +}
> >>>>>>> +
> >>>>>>> +/*
> >>>>>>> + * Fold the foreign cpu mthp stats into our own.
> >>>>>>> + *
> >>>>>>> + * This is adding to the stats on one processor
> >>>>>>> + * but keeps the global counts constant.
> >>>>>>> + */
> >>>>>>> +static inline void mthp_stats_fold_cpu(int cpu)
> >>>>>>> +{
> >>>>>>> +     struct mthp_stat *fold_stat;
> >>>>>>> +     int i, j;
> >>>>>>> +
> >>>>>>> +     if (!mthp_stats)
> >>>>>>> +             return;
> >>>>>>> +     fold_stat = per_cpu_ptr(mthp_stats, cpu);
> >>>>>>> +     for (i = 1; i <= PMD_ORDER; i++) {
> >>>>>>> +             for (j = 0; j < __MTHP_STAT_COUNT; j++) {
> >>>>>>> +                     count_mthp_stats(i, j, fold_stat->stats[i][j]);
> >>>>>>> +                     fold_stat->stats[i][j] = 0;
> >>>>>>> +             }
> >>>>>>> +     }
> >>>>>>> +}
> >>>>>>
> >>>>>> This is a pretty horrible hack; I'm pretty sure just summing for all *possible*
> >>>>>> cpus should work.
> >>>>>>
> >>>>>>> +
> >>>>>>>  #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 dc30139590e6..21c4ac74b484 100644
> >>>>>>> --- a/mm/huge_memory.c
> >>>>>>> +++ b/mm/huge_memory.c
> >>>>>>> @@ -526,6 +526,50 @@ static const struct kobj_type thpsize_ktype = {
> >>>>>>>       .sysfs_ops = &kobj_sysfs_ops,
> >>>>>>>  };
> >>>>>>>
> >>>>>>> +struct mthp_stat __percpu *mthp_stats;
> >>>>>>> +
> >>>>>>> +static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)
> >>>>>>> +{
> >>>>>>> +     unsigned long sum = 0;
> >>>>>>> +     int cpu;
> >>>>>>> +
> >>>>>>> +     cpus_read_lock();
> >>>>>>> +     for_each_online_cpu(cpu) {
> >>>>>>> +             struct mthp_stat *this = per_cpu_ptr(mthp_stats, cpu);
> >>>>>>> +
> >>>>>>> +             sum += this->stats[order][item];
> >>>>>>> +     }
> >>>>>>> +     cpus_read_unlock();
> >>>>>>> +
> >>>>>>> +     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)
> >>>>>>> +
> >>>>>>> +DEFINE_MTHP_STAT_ATTR(anon_fault_alloc, MTHP_STAT_ANON_FAULT_ALLOC);
> >>>>>>> +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);
> >>>>>>> +
> >>>>>>> +static struct attribute *stats_attrs[] = {
> >>>>>>> +     &anon_fault_alloc_attr.attr,
> >>>>>>> +     &anon_fault_fallback_attr.attr,
> >>>>>>> +     &anon_fault_fallback_charge_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 +593,12 @@ 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;
> >>>>>>>  }
> >>>>>>> @@ -691,6 +741,11 @@ static int __init hugepage_init(void)
> >>>>>>>        */
> >>>>>>>       MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER < 2);
> >>>>>>>
> >>>>>>> +     mthp_stats = __alloc_percpu((PMD_ORDER + 1) * sizeof(mthp_stats->stats[0]),
> >>>>>>> +                     sizeof(unsigned long));
> >>>>>>
> >>>>>> Personally I think it would be cleaner to allocate statically using
> >>>>>> ilog2(MAX_PTRS_PER_PTE) instead of PMD_ORDER.
> >>>>>
> >>>>> Hi Ryan,
> >>>>>
> >>>>> I don't understand why MAX_PTRS_PER_PTE is the correct size. For ARM64,
> >>>>>
> >>>>> #define PMD_ORDER       (PMD_SHIFT - PAGE_SHIFT)
> >>>>>
> >>>>> #define MAX_PTRS_PER_PTE PTRS_PER_PTE
> >>>>>
> >>>>> #define PTRS_PER_PTE            (1 << (PAGE_SHIFT - 3))
> >>>>>
> >>>>> while PAGE_SIZE is 16KiB or 64KiB, PTRS_PER_PTE can be a huge number?
> >>>>>
> >>>>>
> >>>>> Am I missing something?
> >>>>
> >>>> PTRS_PER_PTE is the number of PTE entries in a PTE table. On arm64 its as follows:
> >>>>
> >>>> PAGE_SIZE       PAGE_SHIFT      PTRS_PER_PTE
> >>>> 4K              12              512
> >>>> 16K             14              2048
> >>>> 64K             16              8192
> >>>>
> >>>> So (PTRS_PER_PTE * PAGE_SIZE) = PMD_SIZE
> >>>>
> >>>> PMD_ORDER is ilog2(PMD_SIZE / PAGE_SIZE) = ilog2(PTRS_PER_PTE)
> >>>>
> >>>> MAX_PTRS_PER_PTE is just the maximum value that PTRS_PER_PTE will ever have,
> >>>> (and its equal to PTRS_PER_PTE except for powerpc).
> >>>>
> >>>> Pretty sure the math is correct?
> >>>
> >>> I am not convinced the math is correct :-)
> >>>
> >>> while page size is 64KiB, the page table is as below,
> >>> PMD_ORDER = L2 index bits = [41:29] = 13 != ilog2(8192)
> >>
> >> 1 << 13 = 8192
> >>
> >> Right? So:
> >>
> >> ilog2(8192) = 13
> >>
> >> What's wrong with that?
> >>
> >> I even checked in Python to make sure I'm not going mad:
> >>
> >>>>> import math
> >>>>> math.log2(8192)
> >> 13.0
> >
> > You're correct. My mind fixated on the '16' in the line '64K 16 8192'.
> > I mistakenly thought ilog2(8192) equals 16. Apologies for the confusion.
>
> No worries! We got there in the end :)
>
> Of course my suggestion relies on being able to get a compile-time constant from
> ilog2(MAX_PTRS_PER_PTE). I think that should work, right?

I guess so, ilog2 can detect compile-time const, otherwise, it will find the
last (most-significant) bit set.

I've implemented the following change, and the build all passed.
Currently conducting testing.

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index c5beb54b97cb..d4fdb2641070 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -272,47 +272,17 @@ enum mthp_stat_item {
 };

 struct mthp_stat {
-       unsigned long stats[0][__MTHP_STAT_COUNT];
+       unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
 };

-extern struct mthp_stat __percpu *mthp_stats;
+DECLARE_PER_CPU(struct mthp_stat, mthp_stats);

 static inline void count_mthp_stat(int order, enum mthp_stat_item item)
 {
-       if (order <= 0 || order > PMD_ORDER || !mthp_stats)
+       if (order <= 0 || order > PMD_ORDER)
                return;

-       this_cpu_inc(mthp_stats->stats[order][item]);
-}
-
-static inline void count_mthp_stats(int order, enum mthp_stat_item
item, long delta)
-{
-       if (order <= 0 || order > PMD_ORDER || !mthp_stats)
-               return;
-
-       this_cpu_add(mthp_stats->stats[order][item], delta);
-}
-
-/*
- * Fold the foreign cpu mthp stats into our own.
- *
- * This is adding to the stats on one processor
- * but keeps the global counts constant.
- */
-static inline void mthp_stats_fold_cpu(int cpu)
-{
-       struct mthp_stat *fold_stat;
-       int i, j;
-
-       if (!mthp_stats)
-               return;
-       fold_stat = per_cpu_ptr(mthp_stats, cpu);
-       for (i = 1; i <= PMD_ORDER; i++) {
-               for (j = 0; j < __MTHP_STAT_COUNT; j++) {
-                       count_mthp_stats(i, j, fold_stat->stats[i][j]);
-                       fold_stat->stats[i][j] = 0;
-               }
-       }
+       this_cpu_inc(mthp_stats.stats[order][item]);
 }

 #define transparent_hugepage_use_zero_page()                           \
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 21c4ac74b484..e88961ffc398 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -526,20 +526,18 @@ static const struct kobj_type thpsize_ktype = {
        .sysfs_ops = &kobj_sysfs_ops,
 };

-struct mthp_stat __percpu *mthp_stats;
+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;

-       cpus_read_lock();
-       for_each_online_cpu(cpu) {
-               struct mthp_stat *this = per_cpu_ptr(mthp_stats, cpu);
+       for_each_possible_cpu(cpu) {
+               struct mthp_stat *this = &per_cpu(mthp_stats, cpu);

                sum += this->stats[order][item];
        }
-       cpus_read_unlock();

        return sum;
 }
@@ -741,11 +739,6 @@ static int __init hugepage_init(void)
         */
        MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER < 2);

-       mthp_stats = __alloc_percpu((PMD_ORDER + 1) *
sizeof(mthp_stats->stats[0]),
-                       sizeof(unsigned long));
-       if (!mthp_stats)
-               return -ENOMEM;
-
        err = hugepage_init_sysfs(&hugepage_kobj);
        if (err)
                goto err_sysfs;
@@ -780,8 +773,6 @@ static int __init hugepage_init(void)
 err_slab:
        hugepage_exit_sysfs(hugepage_kobj);
 err_sysfs:
-       free_percpu(mthp_stats);
-       mthp_stats = NULL;
        return err;
 }
 subsys_initcall(hugepage_init);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3135b5ca2457..b51becf03d1e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5840,10 +5840,6 @@ static int page_alloc_cpu_dead(unsigned int cpu)
         */
        vm_events_fold_cpu(cpu);

-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
-       mthp_stats_fold_cpu(cpu);
-#endif
-
        /*
         * Zero the differential counters of the dead processor
         * so that the vm statistics are consistent.


Thanks
Barry