mm: convert mm's rss stats into percpu_counter

Currently mm_struct maintains rss_stats which are updated on page fault
and the unmapping codepaths. For page fault codepath the updates are
cached per thread with the batch of TASK_RSS_EVENTS_THRESH which is 64.
The reason for caching is performance for multithreaded applications
otherwise the rss_stats updates may become hotspot for such applications.

However this optimization comes with the cost of error margin in the rss
stats. The rss_stats for applications with large number of threads can be
very skewed. At worst the error margin is (nr_threads * 64) and we have a
lot of applications with 100s of threads, so the error margin can be very
high. Internally we had to reduce TASK_RSS_EVENTS_THRESH to 32.

Recently we started seeing the unbounded errors for rss_stats for specific
applications which use TCP rx0cp. It seems like vm_insert_pages()
codepath does not sync rss_stats at all.

This patch converts the rss_stats into percpu_counter to convert the error
margin from (nr_threads * 64) to approximately (nr_cpus ^ 2). However
this conversion enable us to get the accurate stats for situations where
accuracy is more important than the cpu cost.

This patch does not make such tradeoffs - we can just use
percpu_counter_add_local() for the updates and percpu_counter_sum() (or
percpu_counter_sync() + percpu_counter_read) for the readers. At the
moment the readers are either procfs interface, oom_killer and memory
reclaim which I think are not performance critical and should be ok with
slow read. However I think we can make that change in a separate patch.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Shakeel Butt <[e-mail]>
Cc: Marek Szyprowski <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/mm.h

···
  */
 static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
 {
-	long val = atomic_long_read(&mm->rss_stat.count[member]);
-
-#ifdef SPLIT_RSS_COUNTING
-	/*
-	 * counter is updated in asynchronous manner and may go to minus.
-	 * But it's never be expected number for users.
-	 */
-	if (val < 0)
-		val = 0;
-#endif
-	return (unsigned long)val;
+	return percpu_counter_read_positive(&mm->rss_stat[member]);
 }
 
-void mm_trace_rss_stat(struct mm_struct *mm, int member, long count);
+void mm_trace_rss_stat(struct mm_struct *mm, int member);
 
 static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
 {
-	long count = atomic_long_add_return(value, &mm->rss_stat.count[member]);
+	percpu_counter_add(&mm->rss_stat[member], value);
 
-	mm_trace_rss_stat(mm, member, count);
+	mm_trace_rss_stat(mm, member);
 }
 
 static inline void inc_mm_counter(struct mm_struct *mm, int member)
 {
-	long count = atomic_long_inc_return(&mm->rss_stat.count[member]);
+	percpu_counter_inc(&mm->rss_stat[member]);
 
-	mm_trace_rss_stat(mm, member, count);
+	mm_trace_rss_stat(mm, member);
 }
 
 static inline void dec_mm_counter(struct mm_struct *mm, int member)
 {
-	long count = atomic_long_dec_return(&mm->rss_stat.count[member]);
+	percpu_counter_dec(&mm->rss_stat[member]);
 
-	mm_trace_rss_stat(mm, member, count);
+	mm_trace_rss_stat(mm, member);
 }
 
 /* Optimized variant when page is already known not to be PageAnon */

include/linux/mm_types.h

···
 #include <linux/page-flags-layout.h>
 #include <linux/workqueue.h>
 #include <linux/seqlock.h>
+#include <linux/percpu_counter.h>
 
 #include <asm/mmu.h>
 
···
 
 		unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */
 
-		/*
-		 * Special counters, in some configurations protected by the
-		 * page_table_lock, in other configurations by being atomic.
-		 */
-		struct mm_rss_stat rss_stat;
+		struct percpu_counter rss_stat[NR_MM_COUNTERS];
 
 		struct linux_binfmt *binfmt;
 

include/linux/mm_types_task.h

···
 	NR_MM_COUNTERS
 };
 
-#if USE_SPLIT_PTE_PTLOCKS && defined(CONFIG_MMU)
-#define SPLIT_RSS_COUNTING
-/* per-thread cached information, */
-struct task_rss_stat {
-	int events;	/* for synchronization threshold */
-	int count[NR_MM_COUNTERS];
-};
-#endif /* USE_SPLIT_PTE_PTLOCKS */
-
-struct mm_rss_stat {
-	atomic_long_t count[NR_MM_COUNTERS];
-};
-
 struct page_frag {
 	struct page *page;
 #if (BITS_PER_LONG > 32) || (PAGE_SIZE >= 65536)

include/linux/percpu_counter.h

···
 #include <linux/threads.h>
 #include <linux/percpu.h>
 #include <linux/types.h>
-#include <linux/gfp.h>
 
 /* percpu_counter batch for local add or sub */
 #define PERCPU_COUNTER_LOCAL_BATCH	INT_MAX

include/linux/sched.h

···
 	struct mm_struct		*mm;
 	struct mm_struct		*active_mm;
 
-#ifdef SPLIT_RSS_COUNTING
-	struct task_rss_stat		rss_stat;
-#endif
 	int				exit_state;
 	int				exit_code;
 	int				exit_signal;

include/trace/events/kmem.h

···
 TRACE_EVENT(rss_stat,
 
 	TP_PROTO(struct mm_struct *mm,
-		int member,
-		long count),
+		int member),
 
-	TP_ARGS(mm, member, count),
+	TP_ARGS(mm, member),
 
 	TP_STRUCT__entry(
 		__field(unsigned int, mm_id)
···
 		__entry->mm_id = mm_ptr_to_hash(mm);
 		__entry->curr = !!(current->mm == mm);
 		__entry->member = member;
-		__entry->size = (count << PAGE_SHIFT);
+		__entry->size = (percpu_counter_sum_positive(&mm->rss_stat[member])
+							    << PAGE_SHIFT);
 	),
 
 	TP_printk("mm_id=%u curr=%d type=%s size=%ldB",

kernel/fork.c

···
 			 "Please make sure 'struct resident_page_types[]' is updated as well");
 
 	for (i = 0; i < NR_MM_COUNTERS; i++) {
-		long x = atomic_long_read(&mm->rss_stat.count[i]);
+		long x = percpu_counter_sum(&mm->rss_stat[i]);
 
 		if (unlikely(x))
 			pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
···
  */
 void __mmdrop(struct mm_struct *mm)
 {
+	int i;
+
 	BUG_ON(mm == &init_mm);
 	WARN_ON_ONCE(mm == current->mm);
 	WARN_ON_ONCE(mm == current->active_mm);
···
 	check_mm(mm);
 	put_user_ns(mm->user_ns);
 	mm_pasid_drop(mm);
+
+	for (i = 0; i < NR_MM_COUNTERS; i++)
+		percpu_counter_destroy(&mm->rss_stat[i]);
 	free_mm(mm);
 }
 EXPORT_SYMBOL_GPL(__mmdrop);
···
 static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	struct user_namespace *user_ns)
 {
+	int i;
+
 	mt_init_flags(&mm->mm_mt, MM_MT_FLAGS);
 	mt_set_external_lock(&mm->mm_mt, &mm->mmap_lock);
 	atomic_set(&mm->mm_users, 1);
···
 	if (init_new_context(p, mm))
 		goto fail_nocontext;
 
+	for (i = 0; i < NR_MM_COUNTERS; i++)
+		if (percpu_counter_init(&mm->rss_stat[i], 0, GFP_KERNEL_ACCOUNT))
+			goto fail_pcpu;
+
 	mm->user_ns = get_user_ns(user_ns);
 	lru_gen_init_mm(mm);
 	return mm;
 
+fail_pcpu:
+	while (i > 0)
+		percpu_counter_destroy(&mm->rss_stat[--i]);
 fail_nocontext:
 	mm_free_pgd(mm);
 fail_nopgd:

mm/memory.c

···
 }
 early_initcall(init_zero_pfn);
 
-void mm_trace_rss_stat(struct mm_struct *mm, int member, long count)
+void mm_trace_rss_stat(struct mm_struct *mm, int member)
 {
-	trace_rss_stat(mm, member, count);
+	trace_rss_stat(mm, member);
 }
-
-#if defined(SPLIT_RSS_COUNTING)
-
-void sync_mm_rss(struct mm_struct *mm)
-{
-	int i;
-
-	for (i = 0; i < NR_MM_COUNTERS; i++) {
-		if (current->rss_stat.count[i]) {
-			add_mm_counter(mm, i, current->rss_stat.count[i]);
-			current->rss_stat.count[i] = 0;
-		}
-	}
-	current->rss_stat.events = 0;
-}
-
-static void add_mm_counter_fast(struct mm_struct *mm, int member, int val)
-{
-	struct task_struct *task = current;
-
-	if (likely(task->mm == mm))
-		task->rss_stat.count[member] += val;
-	else
-		add_mm_counter(mm, member, val);
-}
-#define inc_mm_counter_fast(mm, member) add_mm_counter_fast(mm, member, 1)
-#define dec_mm_counter_fast(mm, member) add_mm_counter_fast(mm, member, -1)
-
-/* sync counter once per 64 page faults */
-#define TASK_RSS_EVENTS_THRESH	(64)
-static void check_sync_rss_stat(struct task_struct *task)
-{
-	if (unlikely(task != current))
-		return;
-	if (unlikely(task->rss_stat.events++ > TASK_RSS_EVENTS_THRESH))
-		sync_mm_rss(task->mm);
-}
-#else /* SPLIT_RSS_COUNTING */
-
-#define inc_mm_counter_fast(mm, member) inc_mm_counter(mm, member)
-#define dec_mm_counter_fast(mm, member) dec_mm_counter(mm, member)
-
-static void check_sync_rss_stat(struct task_struct *task)
-{
-}
-
-#endif /* SPLIT_RSS_COUNTING */
 
 /*
  * Note: this doesn't free the actual pages themselves. That
···
 		return -EBUSY;
 	/* Ok, finally just insert the thing.. */
 	get_page(page);
-	inc_mm_counter_fast(vma->vm_mm, mm_counter_file(page));
+	inc_mm_counter(vma->vm_mm, mm_counter_file(page));
 	page_add_file_rmap(page, vma, false);
 	set_pte_at(vma->vm_mm, addr, pte, mk_pte(page, prot));
 	return 0;
···
 	if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
 		if (old_page) {
 			if (!PageAnon(old_page)) {
-				dec_mm_counter_fast(mm,
-						mm_counter_file(old_page));
-				inc_mm_counter_fast(mm, MM_ANONPAGES);
+				dec_mm_counter(mm, mm_counter_file(old_page));
+				inc_mm_counter(mm, MM_ANONPAGES);
 			}
 		} else {
-			inc_mm_counter_fast(mm, MM_ANONPAGES);
+			inc_mm_counter(mm, MM_ANONPAGES);
 		}
 		flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
 		entry = mk_pte(new_page, vma->vm_page_prot);
···
 	if (should_try_to_free_swap(folio, vma, vmf->flags))
 		folio_free_swap(folio);
 
-	inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
-	dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
+	inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
+	dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
 	pte = mk_pte(page, vma->vm_page_prot);
 
 	/*
···
 		return handle_userfault(vmf, VM_UFFD_MISSING);
 	}
 
-	inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
+	inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
 	page_add_new_anon_rmap(page, vma, vmf->address);
 	lru_cache_add_inactive_or_unevictable(page, vma);
 setpte:
···
 		entry = pte_mkuffd_wp(pte_wrprotect(entry));
 	/* copy-on-write page */
 	if (write && !(vma->vm_flags & VM_SHARED)) {
-		inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
+		inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
 		page_add_new_anon_rmap(page, vma, addr);
 		lru_cache_add_inactive_or_unevictable(page, vma);
 	} else {
-		inc_mm_counter_fast(vma->vm_mm, mm_counter_file(page));
+		inc_mm_counter(vma->vm_mm, mm_counter_file(page));
 		page_add_file_rmap(page, vma, false);
 	}
 	set_pte_at(vma->vm_mm, addr, vmf->pte, entry);
···
 
 	count_vm_event(PGFAULT);
 	count_memcg_event_mm(vma->vm_mm, PGFAULT);
-
-	/* do counter updates before entering really critical section. */
-	check_sync_rss_stat(current);
 
 	if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
 					    flags & FAULT_FLAG_INSTRUCTION,