Message-ID: <b30dedf6393ed9b3ecd4118b97233cc4186b0e62.1661461643.git.alexlzhu@fb.com>
Date:   Thu, 25 Aug 2022 14:30:52 -0700
From:   <alexlzhu@...com>
To:     <linux-mm@...ck.org>
CC:     <willy@...radead.org>, <hannes@...xchg.org>,
        <akpm@...ux-foundation.org>, <riel@...riel.com>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>,
        Alexander Zhu <alexlzhu@...com>
Subject: [RFC 1/3] mm: add thp_utilization metrics to debugfs

From: Alexander Zhu <alexlzhu@...com>

This change introduces a tool that scans through all of physical
memory for anonymous THPs and groups them into buckets based
on utilization. It also includes an interface under
/sys/kernel/debug/thp_utilization.

Sample Output:

Utilized[0-50]: 1331 680884
Utilized[51-101]: 9 3983
Utilized[102-152]: 3 1187
Utilized[153-203]: 0 0
Utilized[204-255]: 2 539
Utilized[256-306]: 5 1135
Utilized[307-357]: 1 192
Utilized[358-408]: 0 0
Utilized[409-459]: 1 57
Utilized[460-512]: 400 13
Last Scan Time: 223.98
Last Scan Duration: 70.65

This indicates that there are 1331 THPs that have between 0 and 50
utilized (non zero) pages. In total there are 680884 zero pages in
this utilization bucket. THPs in the [0-50] bucket compose 76% of total
THPs, and are responsible for 99% of total zero pages across all
THPs. In other words, the least utilized THPs are responsible for almost
all of the memory waste when THP is always enabled. Similar results
have been observed across production workloads.

The last two lines indicate the timestamp and duration of the most recent
scan through all of physical memory. Here we see that the last scan
occurred 223.98 seconds after boot time and took 70.65 seconds.

Utilization of a THP is defined as the percentage of nonzero
pages in the THP. The worker thread will scan through all
of physical memory and obtain utilization of all anonymous
THPs. It will gather this information by periodically scanning
through all of physical memory for anonymous THPs, group them
into buckets based on utilization, and report utilization
information through debugfs under /sys/kernel/debug/thp_utilization.

Signed-off-by: Alexander Zhu <alexlzhu@...com>
---
 Documentation/admin-guide/mm/transhuge.rst |   9 +
 include/linux/huge_mm.h                    |   2 +
 mm/huge_memory.c                           | 198 +++++++++++++++++++++
 3 files changed, 209 insertions(+)

diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index c9c37f16eef8..d883ff9fddc7 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -297,6 +297,15 @@ To identify what applications are mapping file transparent huge pages, it
 is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields
 for each mapping.
 
+The utilization of transparent hugepages can be viewed by reading
+``/sys/kernel/debug/thp_utilization``. The utilization of a THP is defined
+as the ratio of non zero filled 4kb pages to the total number of pages in a
+THP. The buckets are labelled by the range of total utilized 4kb pages with
+one line per utilization bucket. Each line contains the total number of
+THPs in that bucket and the total number of zero filled 4kb pages summed
+over all THPs in that bucket. The last two lines show the timestamp and
+duration respectively of the most recent scan over all of physical memory.
+
 Note that reading the smaps file is expensive and reading it
 frequently will incur overhead.
 
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 768e5261fdae..c9086239deb7 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -179,6 +179,8 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
 unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
 		unsigned long len, unsigned long pgoff, unsigned long flags);
 
+int thp_number_utilized_pages(struct page *page);
+
 void prep_transhuge_page(struct page *page);
 void free_transhuge_page(struct page *page);
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8a7c1b344abe..8be1e320e70c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -45,6 +45,21 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/thp.h>
 
+/*
+ * The number of utilization buckets THPs will be grouped in
+ * under /sys/kernel/debug/thp_utilization.
+ */
+#define THP_UTIL_BUCKET_NR 10
+/*
+ * The maximum number of hugepages to scan through on each periodic
+ * run of the scanner that generates /sys/kernel/debug/thp_utilization.
+ * We scan through physical memory in chunks of size PMD_SIZE and
+ * record the timestamp and duration of each scan. In practice we have
+ * found that scanning THP_UTIL_SCAN_SIZE hugepages per second is sufficient
+ * for obtaining useful utilization metrics and does not have a noticeable
+ * impact on CPU.
+ */
+#define THP_UTIL_SCAN_SIZE 256
 /*
  * By default, transparent hugepage support is disabled in order to avoid
  * risking an increased memory footprint for applications that are not
@@ -70,6 +85,25 @@ static atomic_t huge_zero_refcount;
 struct page *huge_zero_page __read_mostly;
 unsigned long huge_zero_pfn __read_mostly = ~0UL;
 
+static void thp_utilization_workfn(struct work_struct *work);
+static DECLARE_DELAYED_WORK(thp_utilization_work, thp_utilization_workfn);
+
+struct thp_scan_info_bucket {
+	int nr_thps;
+	int nr_zero_pages;
+};
+
+struct thp_scan_info {
+	struct thp_scan_info_bucket buckets[THP_UTIL_BUCKET_NR];
+	struct zone *scan_zone;
+	struct timespec64 last_scan_duration;
+	struct timespec64 last_scan_time;
+	unsigned long pfn;
+};
+
+static struct thp_scan_info thp_scan_debugfs;
+static struct thp_scan_info thp_scan;
+
 bool hugepage_vma_check(struct vm_area_struct *vma,
 			unsigned long vm_flags,
 			bool smaps, bool in_pf)
@@ -486,6 +520,7 @@ static int __init hugepage_init(void)
 	if (err)
 		goto err_slab;
 
+	schedule_delayed_work(&thp_utilization_work, HZ);
 	err = register_shrinker(&huge_zero_page_shrinker, "thp-zero");
 	if (err)
 		goto err_hzp_shrinker;
@@ -600,6 +635,11 @@ static inline bool is_transparent_hugepage(struct page *page)
 	       page[1].compound_dtor == TRANSHUGE_PAGE_DTOR;
 }
 
+static inline bool is_anon_transparent_hugepage(struct page *page)
+{
+	return PageAnon(page) && is_transparent_hugepage(page);
+}
+
 static unsigned long __thp_get_unmapped_area(struct file *filp,
 		unsigned long addr, unsigned long len,
 		loff_t off, unsigned long flags, unsigned long size)
@@ -650,6 +690,38 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
 }
 EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
 
+int thp_number_utilized_pages(struct page *page)
+{
+	struct folio *folio;
+	unsigned long page_offset, value;
+	int thp_nr_utilized_pages = HPAGE_PMD_NR;
+	int step_size = sizeof(unsigned long);
+	bool is_all_zeroes;
+	void *kaddr;
+	int i;
+
+	if (!page || !is_anon_transparent_hugepage(page))
+		return -1;
+
+	folio = page_folio(page);
+	for (i = 0; i < folio_nr_pages(folio); i++) {
+		kaddr = kmap_local_folio(folio, i);
+		is_all_zeroes = true;
+		for (page_offset = 0; page_offset < PAGE_SIZE; page_offset += step_size) {
+			value = *(unsigned long *)(kaddr + page_offset);
+			if (value != 0) {
+				is_all_zeroes = false;
+				break;
+			}
+		}
+		if (is_all_zeroes)
+			thp_nr_utilized_pages--;
+
+		kunmap_local(kaddr);
+	}
+	return thp_nr_utilized_pages;
+}
+
 static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
 			struct page *page, gfp_t gfp)
 {
@@ -3135,6 +3207,42 @@ static int __init split_huge_pages_debugfs(void)
 	return 0;
 }
 late_initcall(split_huge_pages_debugfs);
+
+static int thp_utilization_show(struct seq_file *seqf, void *pos)
+{
+	int i;
+	int start;
+	int end;
+
+	for (i = 0; i < THP_UTIL_BUCKET_NR; i++) {
+		start = i * HPAGE_PMD_NR / THP_UTIL_BUCKET_NR;
+		end = (i + 1 == THP_UTIL_BUCKET_NR)
+			   ? HPAGE_PMD_NR
+			   : ((i + 1) * HPAGE_PMD_NR / THP_UTIL_BUCKET_NR - 1);
+		/* The last bucket will need to contain 100 */
+		seq_printf(seqf, "Utilized[%d-%d]: %d %d\n", start, end,
+			   thp_scan_debugfs.buckets[i].nr_thps,
+			   thp_scan_debugfs.buckets[i].nr_zero_pages);
+	}
+	seq_printf(seqf, "Last Scan Time: %lu.%02lu\n",
+		   (unsigned long)thp_scan_debugfs.last_scan_time.tv_sec,
+		   (thp_scan_debugfs.last_scan_time.tv_nsec / (NSEC_PER_SEC / 100)));
+
+	seq_printf(seqf, "Last Scan Duration: %lu.%02lu\n",
+		   (unsigned long)thp_scan_debugfs.last_scan_duration.tv_sec,
+		   (thp_scan_debugfs.last_scan_duration.tv_nsec / (NSEC_PER_SEC / 100)));
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(thp_utilization);
+
+static int __init thp_utilization_debugfs(void)
+{
+	debugfs_create_file("thp_utilization", 0200, NULL, NULL,
+			    &thp_utilization_fops);
+	return 0;
+}
+late_initcall(thp_utilization_debugfs);
 #endif
 
 #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
@@ -3220,3 +3328,93 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
 	trace_remove_migration_pmd(address, pmd_val(pmde));
 }
 #endif
+
+static void thp_scan_next_zone(void)
+{
+	struct timespec64 current_time;
+	int i;
+	bool update_debugfs;
+	/*
+	 * THP utilization worker thread has reached the end
+	 * of the memory zone. Proceed to the next zone.
+	 */
+	thp_scan.scan_zone = next_zone(thp_scan.scan_zone);
+	update_debugfs = !thp_scan.scan_zone;
+	thp_scan.scan_zone = update_debugfs ? (first_online_pgdat())->node_zones
+			: thp_scan.scan_zone;
+	thp_scan.pfn = (thp_scan.scan_zone->zone_start_pfn + HPAGE_PMD_NR - 1)
+			& ~(HPAGE_PMD_SIZE - 1);
+	if (!update_debugfs)
+		return;
+	/*
+	 * If the worker has scanned through all of physical
+	 * memory. Then update information displayed in /sys/kernel/debug/thp_utilization
+	 */
+	ktime_get_ts64(&current_time);
+	thp_scan_debugfs.last_scan_duration = timespec64_sub(current_time,
+							     thp_scan_debugfs.last_scan_time);
+	thp_scan_debugfs.last_scan_time = current_time;
+
+	for (i = 0; i < THP_UTIL_BUCKET_NR; i++) {
+		thp_scan_debugfs.buckets[i].nr_thps = thp_scan.buckets[i].nr_thps;
+		thp_scan_debugfs.buckets[i].nr_zero_pages = thp_scan.buckets[i].nr_zero_pages;
+		thp_scan.buckets[i].nr_thps = 0;
+		thp_scan.buckets[i].nr_zero_pages = 0;
+	}
+}
+
+static void thp_util_scan(unsigned long pfn_end)
+{
+	struct page *page = NULL;
+	int bucket, num_utilized_pages, current_pfn;
+	int i;
+	/*
+	 * Scan through each memory zone in chunks of up to THP_UTIL_SCAN_SIZE
+	 * hugepages every second looking for anonymous THPs.
+	 */
+	for (i = 0; i < THP_UTIL_SCAN_SIZE; i++) {
+		current_pfn = thp_scan.pfn;
+		thp_scan.pfn += HPAGE_PMD_NR;
+		if (current_pfn >= pfn_end)
+			return;
+
+		if (!pfn_valid(current_pfn))
+			continue;
+
+		page = pfn_to_page(current_pfn);
+		num_utilized_pages = thp_number_utilized_pages(page);
+		 /* Not a THP; skip it. */
+		if (num_utilized_pages < 0)
+			continue;
+		/* Group THPs into utilization buckets */
+		bucket = num_utilized_pages * THP_UTIL_BUCKET_NR / HPAGE_PMD_NR;
+		bucket = min(bucket, THP_UTIL_BUCKET_NR - 1);
+		thp_scan.buckets[bucket].nr_thps++;
+		thp_scan.buckets[bucket].nr_zero_pages += (HPAGE_PMD_NR - num_utilized_pages);
+	}
+}
+
+static void thp_utilization_workfn(struct work_struct *work)
+{
+	unsigned long pfn_end;
+
+	if (!thp_scan.scan_zone)
+		thp_scan.scan_zone = (first_online_pgdat())->node_zones;
+	/*
+	 * Worker function that scans through all of physical memory
+	 * for anonymous THPs.
+	 */
+	pfn_end = (thp_scan.scan_zone->zone_start_pfn +
+			thp_scan.scan_zone->spanned_pages + HPAGE_PMD_NR - 1)
+			& ~(HPAGE_PMD_SIZE - 1);
+	/* If we have reached the end of the zone or end of physical memory
+	 * move on to the next zone. Otherwise, scan the next PFNs in the
+	 * current zone.
+	 */
+	if (!populated_zone(thp_scan.scan_zone) || thp_scan.pfn >= pfn_end)
+		thp_scan_next_zone();
+	else
+		thp_util_scan(pfn_end);
+
+	schedule_delayed_work(&thp_utilization_work, HZ);
+}
-- 
2.30.2

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