mm, sched/numa: Remove rate-limiting of automatic NUMA balancing migration

gormanm · Ingo Molnar · commit efaffc5e40ae · 2018-10-02T11:31:14.000+02:00
Rate limiting of page migrations due to automatic NUMA balancing was introduced to mitigate the worst-case scenario of migrating at high frequency due to false sharing or slowly ping-ponging between nodes. Since then, a lot of effort was spent on correctly identifying these pages and avoiding unnecessary migrations and the safety net may no longer be required. Jirka Hladky reported a regression in 4.17 due to a scheduler patch that avoids spreading STREAM tasks wide prematurely. However, once the task was properly placed, it delayed migrating the memory due to rate limiting. Increasing the limit fixed the problem for him. Currently, the limit is hard-coded and does not account for the real capabilities of the hardware. Even if an estimate was attempted, it would not properly account for the number of memory controllers and it could not account for the amount of bandwidth used for normal accesses. Rather than fudging, this patch simply eliminates the rate limiting. However, Jirka reports that a STREAM configuration using multiple processes achieved similar performance to 4.16. In local tests, this patch improved performance of STREAM relative to the baseline but it is somewhat machine-dependent. Most workloads show little or not performance difference implying that there is not a heavily reliance on the throttling mechanism and it is safe to remove. STREAM on 2-socket machine 4.19.0-rc5 4.19.0-rc5 numab-v1r1 noratelimit-v1r1 MB/sec copy 43298.52 ( 0.00%) 44673.38 ( 3.18%) MB/sec scale 30115.06 ( 0.00%) 31293.06 ( 3.91%) MB/sec add 32825.12 ( 0.00%) 34883.62 ( 6.27%) MB/sec triad 32549.52 ( 0.00%) 34906.60 ( 7.24% Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Reviewed-by: Rik van Riel <riel@surriel.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Jirka Hladky <jhladky@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Linux-MM <linux-mm@kvack.org> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20181001100525.29789-2-mgorman@techsingularity.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
@@ -671,12 +671,6 @@ typedef struct pglist_data {
 #ifdef CONFIG_NUMA_BALANCING
 	/* Lock serializing the migrate rate limiting window */
 	spinlock_t numabalancing_migrate_lock;
-
-	/* Rate limiting time interval */
-	unsigned long numabalancing_migrate_next_window;
-
-	/* Number of pages migrated during the rate limiting time interval */
-	unsigned long numabalancing_migrate_nr_pages;
 #endif
 	/*
 	 * This is a per-node reserve of pages that are not available
diff --git a/include/trace/events/migrate.h b/include/trace/events/migrate.h
@@ -70,33 +70,6 @@ TRACE_EVENT(mm_migrate_pages,
 		__print_symbolic(__entry->mode, MIGRATE_MODE),
 		__print_symbolic(__entry->reason, MIGRATE_REASON))
 );
-
-TRACE_EVENT(mm_numa_migrate_ratelimit,
-
-	TP_PROTO(struct task_struct *p, int dst_nid, unsigned long nr_pages),
-
-	TP_ARGS(p, dst_nid, nr_pages),
-
-	TP_STRUCT__entry(
-		__array(	char,		comm,	TASK_COMM_LEN)
-		__field(	pid_t,		pid)
-		__field(	int,		dst_nid)
-		__field(	unsigned long,	nr_pages)
-	),
-
-	TP_fast_assign(
-		memcpy(__entry->comm, p->comm, TASK_COMM_LEN);
-		__entry->pid		= p->pid;
-		__entry->dst_nid	= dst_nid;
-		__entry->nr_pages	= nr_pages;
-	),
-
-	TP_printk("comm=%s pid=%d dst_nid=%d nr_pages=%lu",
-		__entry->comm,
-		__entry->pid,
-		__entry->dst_nid,
-		__entry->nr_pages)
-);
 #endif /* _TRACE_MIGRATE_H */
 
 /* This part must be outside protection */
diff --git a/mm/migrate.c b/mm/migrate.c
@@ -1855,54 +1855,6 @@ static struct page *alloc_misplaced_dst_page(struct page *page,
 	return newpage;
 }
 
-/*
- * page migration rate limiting control.
- * Do not migrate more than @pages_to_migrate in a @migrate_interval_millisecs
- * window of time. Default here says do not migrate more than 1280M per second.
- */
-static unsigned int migrate_interval_millisecs __read_mostly = 100;
-static unsigned int ratelimit_pages __read_mostly = 128 << (20 - PAGE_SHIFT);
-
-/* Returns true if the node is migrate rate-limited after the update */
-static bool numamigrate_update_ratelimit(pg_data_t *pgdat,
-					unsigned long nr_pages)
-{
-	unsigned long next_window, interval;
-
-	next_window = READ_ONCE(pgdat->numabalancing_migrate_next_window);
-	interval = msecs_to_jiffies(migrate_interval_millisecs);
-
-	/*
-	 * Rate-limit the amount of data that is being migrated to a node.
-	 * Optimal placement is no good if the memory bus is saturated and
-	 * all the time is being spent migrating!
-	 */
-	if (time_after(jiffies, next_window) &&
-			spin_trylock(&pgdat->numabalancing_migrate_lock)) {
-		pgdat->numabalancing_migrate_nr_pages = 0;
-		do {
-			next_window += interval;
-		} while (unlikely(time_after(jiffies, next_window)));
-
-		WRITE_ONCE(pgdat->numabalancing_migrate_next_window, next_window);
-		spin_unlock(&pgdat->numabalancing_migrate_lock);
-	}
-	if (pgdat->numabalancing_migrate_nr_pages > ratelimit_pages) {
-		trace_mm_numa_migrate_ratelimit(current, pgdat->node_id,
-								nr_pages);
-		return true;
-	}
-
-	/*
-	 * This is an unlocked non-atomic update so errors are possible.
-	 * The consequences are failing to migrate when we potentiall should
-	 * have which is not severe enough to warrant locking. If it is ever
-	 * a problem, it can be converted to a per-cpu counter.
-	 */
-	pgdat->numabalancing_migrate_nr_pages += nr_pages;
-	return false;
-}
-
 static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
 {
 	int page_lru;
@@ -1975,14 +1927,6 @@ int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
 	if (page_is_file_cache(page) && PageDirty(page))
 		goto out;
 
-	/*
-	 * Rate-limit the amount of data that is being migrated to a node.
-	 * Optimal placement is no good if the memory bus is saturated and
-	 * all the time is being spent migrating!
-	 */
-	if (numamigrate_update_ratelimit(pgdat, 1))
-		goto out;
-
 	isolated = numamigrate_isolate_page(pgdat, page);
 	if (!isolated)
 		goto out;
@@ -2029,14 +1973,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 	unsigned long mmun_start = address & HPAGE_PMD_MASK;
 	unsigned long mmun_end = mmun_start + HPAGE_PMD_SIZE;
 
-	/*
-	 * Rate-limit the amount of data that is being migrated to a node.
-	 * Optimal placement is no good if the memory bus is saturated and
-	 * all the time is being spent migrating!
-	 */
-	if (numamigrate_update_ratelimit(pgdat, HPAGE_PMD_NR))
-		goto out_dropref;
-
 	new_page = alloc_pages_node(node,
 		(GFP_TRANSHUGE_LIGHT | __GFP_THISNODE),
 		HPAGE_PMD_ORDER);
@@ -2133,7 +2069,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 
 out_fail:
 	count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
-out_dropref:
 	ptl = pmd_lock(mm, pmd);
 	if (pmd_same(*pmd, entry)) {
 		entry = pmd_modify(entry, vma->vm_page_prot);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
@@ -6197,8 +6197,6 @@ static unsigned long __init calc_memmap_size(unsigned long spanned_pages,
 static void pgdat_init_numabalancing(struct pglist_data *pgdat)
 {
 	spin_lock_init(&pgdat->numabalancing_migrate_lock);
-	pgdat->numabalancing_migrate_nr_pages = 0;
-	pgdat->numabalancing_migrate_next_window = jiffies;
 }
 #else
 static void pgdat_init_numabalancing(struct pglist_data *pgdat) {}

Original file line number	Diff line number	Diff line change
`@@ -6197,8 +6197,6 @@ static unsigned long __init calc_memmap_size(unsigned long spanned_pages,`
`6197`	`6197`	`static void pgdat_init_numabalancing(struct pglist_data *pgdat)`
`6198`	`6198`	`{`
`6199`	`6199`	`spin_lock_init(&pgdat->numabalancing_migrate_lock);`
`6200`		`- pgdat->numabalancing_migrate_nr_pages = 0;`
`6201`		`- pgdat->numabalancing_migrate_next_window = jiffies;`
`6202`	`6200`	`}`
`6203`	`6201`	`#else`
`6204`	`6202`	`static void pgdat_init_numabalancing(struct pglist_data *pgdat) {}`