Date:   Fri, 26 Aug 2022 20:11:31 -0400
From:   Zi Yan <ziy@...dia.com>
To:     alexlzhu@...com
Cc:     linux-mm@...ck.org, willy@...radead.org, hannes@...xchg.org,
        akpm@...ux-foundation.org, riel@...riel.com, kernel-team@...com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/3] mm: add thp_utilization metrics to debugfs

On 25 Aug 2022, at 17:30, alexlzhu@...com wrote:

> 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

How large is the memory? Just wonder the scanning speed.
Also, it might be better to explicitly add the time unit, second,
in the output.

>
> 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);

Is it possible to use cache-bypassing read to avoid cache
pollution? You are scanning for 256*2M at a time. Wouldn’t that
wipe out all the useful data in the cache?

> +			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

--
Best Regards,
Yan, Zi

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