Message-ID: <20200616094616.ddwmtqczmd3qfcl6@butterfly.localdomain>
Date:   Tue, 16 Jun 2020 11:46:16 +0200
From:   Oleksandr Natalenko <oleksandr@...hat.com>
To:     Nitin Gupta <nigupta@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Nitin Gupta <ngupta@...ingupta.dev>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v7] mm: Proactive compaction

Hello.

Please see the notes inline.

On Mon, Jun 15, 2020 at 07:36:14AM -0700, Nitin Gupta wrote:
> For some applications, we need to allocate almost all memory as
> hugepages. However, on a running system, higher-order allocations can
> fail if the memory is fragmented. Linux kernel currently does on-demand
> compaction as we request more hugepages, but this style of compaction
> incurs very high latency. Experiments with one-time full memory
> compaction (followed by hugepage allocations) show that kernel is able
> to restore a highly fragmented memory state to a fairly compacted memory
> state within <1 sec for a 32G system. Such data suggests that a more
> proactive compaction can help us allocate a large fraction of memory as
> hugepages keeping allocation latencies low.
> 
> For a more proactive compaction, the approach taken here is to define a
> new sysctl called 'vm.compaction_proactiveness' which dictates bounds
> for external fragmentation which kcompactd tries to maintain.
> 
> The tunable takes a value in range [0, 100], with a default of 20.
> 
> Note that a previous version of this patch [1] was found to introduce
> too many tunables (per-order extfrag{low, high}), but this one reduces
> them to just one sysctl. Also, the new tunable is an opaque value
> instead of asking for specific bounds of "external fragmentation", which
> would have been difficult to estimate. The internal interpretation of
> this opaque value allows for future fine-tuning.
> 
> Currently, we use a simple translation from this tunable to [low, high]
> "fragmentation score" thresholds (low=100-proactiveness, high=low+10%).
> The score for a node is defined as weighted mean of per-zone external
> fragmentation. A zone's present_pages determines its weight.
> 
> To periodically check per-node score, we reuse per-node kcompactd
> threads, which are woken up every 500 milliseconds to check the same. If
> a node's score exceeds its high threshold (as derived from user-provided
> proactiveness value), proactive compaction is started until its score
> reaches its low threshold value. By default, proactiveness is set to 20,
> which implies threshold values of low=80 and high=90.
> 
> This patch is largely based on ideas from Michal Hocko [2]. See also the
> LWN article [3].
> 
> Performance data
> ================
> 
> System: x64_64, 1T RAM, 80 CPU threads.
> Kernel: 5.6.0-rc3 + this patch
> 
> echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/enabled
> echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/defrag
> 
> Before starting the driver, the system was fragmented from a userspace
> program that allocates all memory and then for each 2M aligned section,
> frees 3/4 of base pages using munmap. The workload is mainly anonymous
> userspace pages, which are easy to move around. I intentionally avoided
> unmovable pages in this test to see how much latency we incur when
> hugepage allocations hit direct compaction.
> 
> 1. Kernel hugepage allocation latencies
> 
> With the system in such a fragmented state, a kernel driver then
> allocates as many hugepages as possible and measures allocation
> latency:
> 
> (all latency values are in microseconds)
> 
> - With vanilla 5.6.0-rc3
> 
>   percentile latency
>   –––––––––– –––––––
> 	   5    7894
> 	  10    9496
> 	  25   12561
> 	  30   15295
> 	  40   18244
> 	  50   21229
> 	  60   27556
> 	  75   30147
> 	  80   31047
> 	  90   32859
> 	  95   33799
> 
> Total 2M hugepages allocated = 383859 (749G worth of hugepages out of
> 762G total free => 98% of free memory could be allocated as hugepages)
> 
> - With 5.6.0-rc3 + this patch, with proactiveness=20
> 
> sysctl -w vm.compaction_proactiveness=20
> 
>   percentile latency
>   –––––––––– –––––––
> 	   5       2
> 	  10       2
> 	  25       3
> 	  30       3
> 	  40       3
> 	  50       4
> 	  60       4
> 	  75       4
> 	  80       4
> 	  90       5
> 	  95     429
> 
> Total 2M hugepages allocated = 384105 (750G worth of hugepages out of
> 762G total free => 98% of free memory could be allocated as hugepages)
> 
> 2. JAVA heap allocation
> 
> In this test, we first fragment memory using the same method as for (1).
> 
> Then, we start a Java process with a heap size set to 700G and request
> the heap to be allocated with THP hugepages. We also set THP to madvise
> to allow hugepage backing of this heap.
> 
> /usr/bin/time
>  java -Xms700G -Xmx700G -XX:+UseTransparentHugePages -XX:+AlwaysPreTouch
> 
> The above command allocates 700G of Java heap using hugepages.
> 
> - With vanilla 5.6.0-rc3
> 
> 17.39user 1666.48system 27:37.89elapsed
> 
> - With 5.6.0-rc3 + this patch, with proactiveness=20
> 
> 8.35user 194.58system 3:19.62elapsed
> 
> Elapsed time remains around 3:15, as proactiveness is further increased.
> 
> Note that proactive compaction happens throughout the runtime of these
> workloads. The situation of one-time compaction, sufficient to supply
> hugepages for following allocation stream, can probably happen for more
> extreme proactiveness values, like 80 or 90.
> 
> In the above Java workload, proactiveness is set to 20. The test starts
> with a node's score of 80 or higher, depending on the delay between the
> fragmentation step and starting the benchmark, which gives more-or-less
> time for the initial round of compaction. As t	he benchmark consumes
> hugepages, node's score quickly rises above the high threshold (90) and
> proactive compaction starts again, which brings down the score to the
> low threshold level (80).  Repeat.
> 
> bpftrace also confirms proactive compaction running 20+ times during the
> runtime of this Java benchmark. kcompactd threads consume 100% of one of
> the CPUs while it tries to bring a node's score within thresholds.
> 
> Backoff behavior
> ================
> 
> Above workloads produce a memory state which is easy to compact.
> However, if memory is filled with unmovable pages, proactive compaction
> should essentially back off. To test this aspect:
> 
> - Created a kernel driver that allocates almost all memory as hugepages
>   followed by freeing first 3/4 of each hugepage.
> - Set proactiveness=40
> - Note that proactive_compact_node() is deferred maximum number of times
>   with HPAGE_FRAG_CHECK_INTERVAL_MSEC of wait between each check
>   (=> ~30 seconds between retries).
> 
> [1] https://patchwork.kernel.org/patch/11098289/
> [2] https://lore.kernel.org/linux-mm/20161230131412.GI13301@dhcp22.suse.cz/
> [3] https://lwn.net/Articles/817905/
> 
> Signed-off-by: Nitin Gupta <nigupta@...dia.com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Reviewed-by: Khalid Aziz <khalid.aziz@...cle.com>
> To: Andrew Morton <akpm@...ux-foundation.org>
> CC: Vlastimil Babka <vbabka@...e.cz>
> CC: Khalid Aziz <khalid.aziz@...cle.com>
> CC: Michal Hocko <mhocko@...e.com>
> CC: Mel Gorman <mgorman@...hsingularity.net>
> CC: Matthew Wilcox <willy@...radead.org>
> CC: Mike Kravetz <mike.kravetz@...cle.com>
> CC: Joonsoo Kim <iamjoonsoo.kim@....com>
> CC: David Rientjes <rientjes@...gle.com>
> CC: Nitin Gupta <ngupta@...ingupta.dev>
> CC: Oleksandr Natalenko <oleksandr@...hat.com>
> CC: linux-kernel <linux-kernel@...r.kernel.org>
> CC: linux-mm <linux-mm@...ck.org>
> CC: Linux API <linux-api@...r.kernel.org>
> 
> ---
> Changelog v7 vs v6:
>  - Fix compile error while THP is disabled (Oleksandr)

Thank you for taking this.

> 
> Changelog v6 vs v5:
>  - Fallback to HUGETLB_PAGE_ORDER if HPAGE_PMD_ORDER is not defined, and
>    some cleanups (Vlastimil)
>  - Cap min threshold to avoid excess compaction load in case user sets
>    extreme values like 100 for `vm.compaction_proactiveness` sysctl (Khalid)
>  - Add some more explanation about the effect of tunable on compaction
>    behavior in user guide (Khalid)
> 
> Changelog v5 vs v4:
>  - Change tunable from sysfs to sysctl (Vlastimil)
>  - Replace HUGETLB_PAGE_ORDER with HPAGE_PMD_ORDER (Vlastimil)
>  - Minor cleanups (remove redundant initializations, ...)
> 
> Changelog v4 vs v3:
>  - Document various functions.
>  - Added admin-guide for the new tunable `proactiveness`.
>  - Rename proactive_compaction_score to fragmentation_score for clarity.
> 
> Changelog v3 vs v2:
>  - Make proactiveness a global tunable and not per-node. Also upadated
> the
>    patch description to reflect the same (Vlastimil Babka).
>  - Don't start proactive compaction if kswapd is running (Vlastimil
> Babka).
>  - Clarified in the description that compaction runs in parallel with
>    the workload, instead of a one-time compaction followed by a stream
> of
>    hugepage allocations.
> 
> Changelog v2 vs v1:
>  - Introduce per-node and per-zone "proactive compaction score". This
>    score is compared against watermarks which are set according to
>    user provided proactiveness value.
>  - Separate code-paths for proactive compaction from targeted compaction
>    i.e. where pgdat->kcompactd_max_order is non-zero.
>  - Renamed hpage_compaction_effort -> proactiveness. In future we may
>    use more than extfrag wrt hugepage size to determine proactive
>    compaction score.
> ---
>  Documentation/admin-guide/sysctl/vm.rst |  15 ++
>  include/linux/compaction.h              |   2 +
>  kernel/sysctl.c                         |   9 ++
>  mm/compaction.c                         | 183 +++++++++++++++++++++++-
>  mm/internal.h                           |   1 +
>  mm/vmstat.c                             |  18 +++
>  6 files changed, 223 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index 0329a4d3fa9e..360914b4f346 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -119,6 +119,21 @@ all zones are compacted such that free memory is available in contiguous
>  blocks where possible. This can be important for example in the allocation of
>  huge pages although processes will also directly compact memory as required.
>  
> +compaction_proactiveness
> +========================
> +
> +This tunable takes a value in the range [0, 100] with a default value of
> +20. This tunable determines how aggressively compaction is done in the
> +background. Setting it to 0 disables proactive compaction.
> +
> +Note that compaction has a non-trivial system-wide impact as pages
> +belonging to different processes are moved around, which could also lead
> +to latency spikes in unsuspecting applications. The kernel employs
> +various heuristics to avoid wasting CPU cycles if it detects that
> +proactive compaction is not being effective.
> +
> +Be careful when setting it to extreme values like 100, as that may
> +cause excessive background compaction activity.
>  
>  compact_unevictable_allowed
>  ===========================
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index 4b898cdbdf05..ccd28978b296 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -85,11 +85,13 @@ static inline unsigned long compact_gap(unsigned int order)
>  
>  #ifdef CONFIG_COMPACTION
>  extern int sysctl_compact_memory;
> +extern int sysctl_compaction_proactiveness;
>  extern int sysctl_compaction_handler(struct ctl_table *table, int write,
>  			void __user *buffer, size_t *length, loff_t *ppos);

Based on __user notation here, I guess the patch is based on v5.7, not
on something newer, right?

>  extern int sysctl_extfrag_threshold;
>  extern int sysctl_compact_unevictable_allowed;
>  
> +extern int extfrag_for_order(struct zone *zone, unsigned int order);
>  extern int fragmentation_index(struct zone *zone, unsigned int order);
>  extern enum compact_result try_to_compact_pages(gfp_t gfp_mask,
>  		unsigned int order, unsigned int alloc_flags,
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 8a176d8727a3..51c90906efbc 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1458,6 +1458,15 @@ static struct ctl_table vm_table[] = {
>  		.mode		= 0200,
>  		.proc_handler	= sysctl_compaction_handler,
>  	},
> +	{
> +		.procname	= "compaction_proactiveness",
> +		.data		= &sysctl_compaction_proactiveness,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= SYSCTL_ZERO,
> +		.extra2		= &one_hundred,
> +	},

Again, as a highlight, in v5.8 the table was shuffled around, so you may want
to rebase the patch on top of something newer in order for people to not
get conflicts when doing `git am`.

>  	{
>  		.procname	= "extfrag_threshold",
>  		.data		= &sysctl_extfrag_threshold,
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 46f0fcc93081..99579a1fa582 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -50,6 +50,24 @@ static inline void count_compact_events(enum vm_event_item item, long delta)
>  #define pageblock_start_pfn(pfn)	block_start_pfn(pfn, pageblock_order)
>  #define pageblock_end_pfn(pfn)		block_end_pfn(pfn, pageblock_order)
>  
> +/*
> + * Fragmentation score check interval for proactive compaction purposes.
> + */
> +static const int HPAGE_FRAG_CHECK_INTERVAL_MSEC = 500;
> +
> +/*
> + * Page order with-respect-to which proactive compaction
> + * calculates external fragmentation, which is used as
> + * the "fragmentation score" of a node/zone.
> + */
> +#if defined CONFIG_TRANSPARENT_HUGEPAGE
> +#define COMPACTION_HPAGE_ORDER	HPAGE_PMD_ORDER
> +#elif defined HUGETLB_PAGE_ORDER
> +#define COMPACTION_HPAGE_ORDER	HUGETLB_PAGE_ORDER
> +#else
> +#define COMPACTION_HPAGE_ORDER	(PMD_SHIFT - PAGE_SHIFT)
> +#endif
> +
>  static unsigned long release_freepages(struct list_head *freelist)
>  {
>  	struct page *page, *next;
> @@ -1855,6 +1873,76 @@ static inline bool is_via_compact_memory(int order)
>  	return order == -1;
>  }
>  
> +static bool kswapd_is_running(pg_data_t *pgdat)
> +{
> +	return pgdat->kswapd && (pgdat->kswapd->state == TASK_RUNNING);
> +}
> +
> +/*
> + * A zone's fragmentation score is the external fragmentation wrt to the
> + * COMPACTION_HPAGE_ORDER scaled by the zone's size. It returns a value
> + * in the range [0, 100].
> + *
> + * The scaling factor ensures that proactive compaction focuses on larger
> + * zones like ZONE_NORMAL, rather than smaller, specialized zones like
> + * ZONE_DMA32. For smaller zones, the score value remains close to zero,
> + * and thus never exceeds the high threshold for proactive compaction.
> + */
> +static int fragmentation_score_zone(struct zone *zone)
> +{
> +	unsigned long score;
> +
> +	score = zone->present_pages *
> +			extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
> +	return div64_ul(score, zone->zone_pgdat->node_present_pages + 1);
> +}
> +
> +/*
> + * The per-node proactive (background) compaction process is started by its
> + * corresponding kcompactd thread when the node's fragmentation score
> + * exceeds the high threshold. The compaction process remains active till
> + * the node's score falls below the low threshold, or one of the back-off
> + * conditions is met.
> + */
> +static int fragmentation_score_node(pg_data_t *pgdat)
> +{
> +	unsigned long score = 0;
> +	int zoneid;
> +
> +	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
> +		struct zone *zone;
> +
> +		zone = &pgdat->node_zones[zoneid];
> +		score += fragmentation_score_zone(zone);
> +	}
> +
> +	return score;
> +}
> +
> +static int fragmentation_score_wmark(pg_data_t *pgdat, bool low)
> +{
> +	int wmark_low;
> +
> +	/*
> +	 * Cap the low watermak to avoid excessive compaction
> +	 * activity in case a user sets the proactivess tunable
> +	 * close to 100 (maximum).
> +	 */
> +	wmark_low = max(100 - sysctl_compaction_proactiveness, 5);
> +	return low ? wmark_low : min(wmark_low + 10, 100);
> +}
> +
> +static bool should_proactive_compact_node(pg_data_t *pgdat)
> +{
> +	int wmark_high;
> +
> +	if (!sysctl_compaction_proactiveness || kswapd_is_running(pgdat))
> +		return false;
> +
> +	wmark_high = fragmentation_score_wmark(pgdat, false);
> +	return fragmentation_score_node(pgdat) > wmark_high;
> +}
> +
>  static enum compact_result __compact_finished(struct compact_control *cc)
>  {
>  	unsigned int order;
> @@ -1881,6 +1969,25 @@ static enum compact_result __compact_finished(struct compact_control *cc)
>  			return COMPACT_PARTIAL_SKIPPED;
>  	}
>  
> +	if (cc->proactive_compaction) {
> +		int score, wmark_low;
> +		pg_data_t *pgdat;
> +
> +		pgdat = cc->zone->zone_pgdat;
> +		if (kswapd_is_running(pgdat))
> +			return COMPACT_PARTIAL_SKIPPED;
> +
> +		score = fragmentation_score_zone(cc->zone);
> +		wmark_low = fragmentation_score_wmark(pgdat, true);
> +
> +		if (score > wmark_low)
> +			ret = COMPACT_CONTINUE;
> +		else
> +			ret = COMPACT_SUCCESS;
> +
> +		goto out;
> +	}
> +
>  	if (is_via_compact_memory(cc->order))
>  		return COMPACT_CONTINUE;
>  
> @@ -1939,6 +2046,7 @@ static enum compact_result __compact_finished(struct compact_control *cc)
>  		}
>  	}
>  
> +out:
>  	if (cc->contended || fatal_signal_pending(current))
>  		ret = COMPACT_CONTENDED;
>  
> @@ -2412,6 +2520,41 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
>  	return rc;
>  }
>  
> +/*
> + * Compact all zones within a node till each zone's fragmentation score
> + * reaches within proactive compaction thresholds (as determined by the
> + * proactiveness tunable).
> + *
> + * It is possible that the function returns before reaching score targets
> + * due to various back-off conditions, such as, contention on per-node or
> + * per-zone locks.
> + */
> +static void proactive_compact_node(pg_data_t *pgdat)
> +{
> +	int zoneid;
> +	struct zone *zone;
> +	struct compact_control cc = {
> +		.order = -1,
> +		.mode = MIGRATE_SYNC_LIGHT,
> +		.ignore_skip_hint = true,
> +		.whole_zone = true,
> +		.gfp_mask = GFP_KERNEL,
> +		.proactive_compaction = true,
> +	};
> +
> +	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
> +		zone = &pgdat->node_zones[zoneid];
> +		if (!populated_zone(zone))
> +			continue;
> +
> +		cc.zone = zone;
> +
> +		compact_zone(&cc, NULL);
> +
> +		VM_BUG_ON(!list_empty(&cc.freepages));
> +		VM_BUG_ON(!list_empty(&cc.migratepages));

Can this actually happen here? I'd expect some comment in the code
regarding being overcautious here. IIUC, you follow what
kcompactd_do_work() does, but even there it is not explained.

> +	}
> +}
>  
>  /* Compact all zones within a node */
>  static void compact_node(int nid)
> @@ -2458,6 +2601,13 @@ static void compact_nodes(void)
>  /* The written value is actually unused, all memory is compacted */
>  int sysctl_compact_memory;
>  
> +/*
> + * Tunable for proactive compaction. It determines how
> + * aggressively the kernel should compact memory in the
> + * background. It takes values in the range [0, 100].
> + */
> +int __read_mostly sysctl_compaction_proactiveness = 20;

Excuse me if I missed previous discussion and this question was already
addressed, but given possible latency spikes as described in the commit
message, shall this value be amended to conserve current kernel
behaviour (IOW, sysctl_compaction_proactiveness = 0)?

> +
>  /*
>   * This is the entry point for compacting all nodes via
>   * /proc/sys/vm/compact_memory
> @@ -2637,6 +2787,7 @@ static int kcompactd(void *p)
>  {
>  	pg_data_t *pgdat = (pg_data_t*)p;
>  	struct task_struct *tsk = current;
> +	unsigned int proactive_defer = 0;
>  
>  	const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
>  
> @@ -2652,12 +2803,34 @@ static int kcompactd(void *p)
>  		unsigned long pflags;
>  
>  		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
> -		wait_event_freezable(pgdat->kcompactd_wait,
> -				kcompactd_work_requested(pgdat));
> +		if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
> +			kcompactd_work_requested(pgdat),
> +			msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {
> +
> +			psi_memstall_enter(&pflags);
> +			kcompactd_do_work(pgdat);
> +			psi_memstall_leave(&pflags);

I wonder if wrapping kcompactd_do_work() into
psi_memstall_{enter,leave} is a too big hammer that may cause mem PSI to
be bigger that it really is, but this question seems to be out of scope
of current patch, so feel free to ignore it.

> +			continue;
> +		}
>  
> -		psi_memstall_enter(&pflags);
> -		kcompactd_do_work(pgdat);
> -		psi_memstall_leave(&pflags);
> +		/* kcompactd wait timeout */
> +		if (should_proactive_compact_node(pgdat)) {
> +			unsigned int prev_score, score;
> +
> +			if (proactive_defer) {
> +				proactive_defer--;
> +				continue;
> +			}
> +			prev_score = fragmentation_score_node(pgdat);
> +			proactive_compact_node(pgdat);
> +			score = fragmentation_score_node(pgdat);
> +			/*
> +			 * Defer proactive compaction if the fragmentation
> +			 * score did not go down i.e. no progress made.
> +			 */
> +			proactive_defer = score < prev_score ?
> +					0 : 1 << COMPACT_MAX_DEFER_SHIFT;
> +		}
>  	}
>  
>  	return 0;
> diff --git a/mm/internal.h b/mm/internal.h
> index b5634e78f01d..9671bccd97d5 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -228,6 +228,7 @@ struct compact_control {
>  	bool no_set_skip_hint;		/* Don't mark blocks for skipping */
>  	bool ignore_block_suitable;	/* Scan blocks considered unsuitable */
>  	bool direct_compaction;		/* False from kcompactd or /proc/... */
> +	bool proactive_compaction;	/* kcompactd proactive compaction */
>  	bool whole_zone;		/* Whole zone should/has been scanned */
>  	bool contended;			/* Signal lock or sched contention */
>  	bool rescan;			/* Rescanning the same pageblock */
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 96d21a792b57..cc88f7533b8d 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1074,6 +1074,24 @@ static int __fragmentation_index(unsigned int order, struct contig_page_info *in
>  	return 1000 - div_u64( (1000+(div_u64(info->free_pages * 1000ULL, requested))), info->free_blocks_total);
>  }
>  
> +/*
> + * Calculates external fragmentation within a zone wrt the given order.
> + * It is defined as the percentage of pages found in blocks of size
> + * less than 1 << order. It returns values in range [0, 100].
> + */
> +int extfrag_for_order(struct zone *zone, unsigned int order)
> +{
> +	struct contig_page_info info;
> +
> +	fill_contig_page_info(zone, order, &info);
> +	if (info.free_pages == 0)
> +		return 0;
> +
> +	return div_u64((info.free_pages -
> +			(info.free_blocks_suitable << order)) * 100,
> +			info.free_pages);
> +}
> +
>  /* Same as __fragmentation index but allocs contig_page_info on stack */
>  int fragmentation_index(struct zone *zone, unsigned int order)
>  {
> -- 
> 2.27.0
> 

Modulo the minor nits above and given I run this submission for quite
some time on various machines:

Reviewed-by: Oleksandr Natalenko <oleksandr@...hat.com>
Tested-by: Oleksandr Natalenko <oleksandr@...hat.com>

Thank you.

-- 
  Best regards,
    Oleksandr Natalenko (post-factum)
    Principal Software Maintenance Engineer

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