Re: [PATCH v5] mm: Proactive compaction

[Khalid Aziz]

This looks good to me. I like the idea overall of controlling
aggressiveness of compaction with a single tunable for the whole
system. I wonder how an end user could arrive at what a reasonable
value would be for this based upon their workload. More comments below.

On Mon, 2020-05-18 at 11:14 -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 tunable called 'proactiveness' which dictates bounds for
> external
> fragmentation wrt HUGETLB_PAGE_ORDER order which kcompactd tries to
> maintain.
> 
> The tunable is exposed through sysctl:
>   /proc/sys/vm/compaction_proactiveness
> 
> It takes value in range [0, 100], with a default of 20.

Looking at the code, setting this to 100 would mean system would
continuously strive to drive level of fragmentation down to 0 which can
not be reasonable and would bog the system down. A cap lower than 100
might be a good idea to keep kcompactd from dragging system down.

> 
> 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 (proactiveness). 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 wrt HUGETLB_PAGE_ORDER order. 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 posted here:
> https://lore.kernel.org/linux-mm/20161230131412.GI13301@xxxxxxxxxxxxxx/
> 
> 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
> 
> echo 0 | sudo tee /sys/kernel/mm/compaction/node-*/proactiveness
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This is not needed here since there will be no
/proc/sys/vm/compaction_proactiveness without this patch on vanilla
kernel.

> 
>   percentile latency
>   ââââââââââ âââââââ
> 	   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
> 
> echo 20 | sudo tee /sys/kernel/mm/compaction/node-*/proactiveness

Should be "echo 20 | sudo tee /proc/sys/vm/compaction_proactiveness"

> 
>   percentile latency
>   ââââââââââ âââââââ
> 	   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 the 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/
> 
> Signed-off-by: Nitin Gupta <nigupta@xxxxxxxxxx>
> To: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> To: Michal Hocko <mhocko@xxxxxxxx>
> To: Vlastimil Babka <vbabka@xxxxxxx>
> CC: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> CC: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> CC: David Rientjes <rientjes@xxxxxxxxxx>
> CC: Nitin Gupta <ngupta@xxxxxxxxxxxxxx>
> CC: linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>
> CC: linux-mm <linux-mm@xxxxxxxxx>
> CC: Linux API <linux-api@xxxxxxxxxxxxxxx>
> 
> ---
> Changelog v5 vs v4:
>  - Change tunable from sysfs to sysctl (Vlastimil)
>  - HUGETLB_PAGE_ORDER -> 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 |  13 ++
>  include/linux/compaction.h              |   2 +
>  kernel/sysctl.c                         |   9 ++
>  mm/compaction.c                         | 165
> +++++++++++++++++++++++-
>  mm/internal.h                           |   1 +
>  mm/vmstat.c                             |  17 +++
>  6 files changed, 202 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst
> b/Documentation/admin-guide/sysctl/vm.rst
> index 0329a4d3fa9e..e5d88cabe980 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -119,6 +119,19 @@ 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.
> +

Value of 100 would cause kcompactd to try to bring fragmentation down
to 0. If hugepages are being consumed and released continuously by the
workload, it is possible that kcompactd keeps making progress (and
hence passes the test "proactive_defer = score < prev_score ?")
continuously but can not reach a fragmentation score of 0 and hence
gets stuck in compact_zone() for a long time. Page migration for
compaction is not inexpensive. Maybe either cap the value to something
less than 100 or set a floor for wmark_low above 0.

Some more guidance regarding the value for this tunable might be
helpful here, something along the lines of what does a value of 100
mean in terms of how kcompactd will behave. It can then give end user a
better idea of what they are getting at what cost. You touch upon the
cost above. Just add some more details so an end user can get a better
idea of size of the cost for higher values of this tunable.

--
Khalid

>  
>  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);
>  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,
> +	},
>  	{
>  		.procname	= "extfrag_threshold",
>  		.data		= &sysctl_extfrag_threshold,
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 46f0fcc93081..bf7f57a475ce 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -50,6 +50,11 @@ 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;
> +
>  static unsigned long release_freepages(struct list_head *freelist)
>  {
>  	struct page *page, *next;
> @@ -1855,6 +1860,71 @@ 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
> + * HUGETLB_PAGE_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, HPAGE_PMD_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;
> +
> +	wmark_low = 100 - sysctl_compaction_proactiveness;
> +	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 +1951,25 @@ 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 +2028,7 @@ static enum compact_result
> __compact_finished(struct compact_control *cc)
>  		}
>  	}
>  
> +out:
>  	if (cc->contended || fatal_signal_pending(current))
>  		ret = COMPACT_CONTENDED;
>  
> @@ -2412,6 +2502,41 @@ 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));
> +	}
> +}
>  
>  /* Compact all zones within a node */
>  static void compact_node(int nid)
> @@ -2458,6 +2583,13 @@ 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 sysctl_compaction_proactiveness = 20;
> +
>  /*
>   * This is the entry point for compacting all nodes via
>   * /proc/sys/vm/compact_memory
> @@ -2637,6 +2769,7 @@ 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 +2785,34 @@ 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);
> +			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..d7ab7dbdc3a5 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1074,6 +1074,23 @@ 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 (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)
>  {