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

[Ryan Roberts]

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@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 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@xxxxxxxx>
> Cc: Chris Li <chrisl@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx>
> Cc: Kairui Song <kasong@xxxxxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> ---
>  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.

> +	if (!mthp_stats)
> +		return -ENOMEM;
> +
>  	err = hugepage_init_sysfs(&hugepage_kobj);
>  	if (err)
>  		goto err_sysfs;
> @@ -725,6 +780,8 @@ 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);
> @@ -880,6 +937,8 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>  		folio_put(folio);
>  		count_vm_event(THP_FAULT_FALLBACK);
>  		count_vm_event(THP_FAULT_FALLBACK_CHARGE);
> +		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
> +		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>  		return VM_FAULT_FALLBACK;
>  	}
>  	folio_throttle_swaprate(folio, gfp);
> @@ -929,6 +988,7 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>  		mm_inc_nr_ptes(vma->vm_mm);
>  		spin_unlock(vmf->ptl);
>  		count_vm_event(THP_FAULT_ALLOC);
> +		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
>  		count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>  	}
>  
> @@ -1050,6 +1110,7 @@ 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_FAULT_FALLBACK);
>  		return VM_FAULT_FALLBACK;
>  	}
>  	return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp);
> diff --git a/mm/memory.c b/mm/memory.c
> index 649a547fe8e3..06048af7cf9a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4368,6 +4368,7 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>  		folio = vma_alloc_folio(gfp, order, vma, addr, true);
>  		if (folio) {
>  			if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
> +				count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>  				folio_put(folio);
>  				goto next;
>  			}
> @@ -4376,6 +4377,7 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>  			return folio;
>  		}
>  next:
> +		count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>  		order = next_order(&orders, order);
>  	}
>  
> @@ -4485,6 +4487,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  
>  	folio_ref_add(folio, nr_pages - 1);
>  	add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> +	count_mthp_stat(folio_order(folio), MTHP_STAT_ANON_FAULT_ALLOC);
>  	folio_add_new_anon_rmap(folio, vma, addr);
>  	folio_add_lru_vma(folio, vma);
>  setpte:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b51becf03d1e..3135b5ca2457 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5840,6 +5840,10 @@ 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.