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Date:   Thu, 29 Apr 2021 18:25:17 -0700
From:   Roman Gushchin <guro@fb.com>
To:     kernel test robot <oliver.sang@intel.com>
CC:     Dennis Zhou <dennis@kernel.org>,
        Pratik Sampat <psampat@linux.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, <lkp@lists.01.org>,
        <lkp@intel.com>, <ying.huang@intel.com>, <feng.tang@intel.com>,
        <zhengjun.xing@intel.com>
Subject: Re: [percpu]  ace7e70901:  aim9.sync_disk_rw.ops_per_sec -2.3%
 regression
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On Tue, Apr 27, 2021 at 03:34:48PM +0800, kernel test robot wrote:
> 
> 
> Greeting,
> 
> FYI, we noticed a -2.3% regression of aim9.sync_disk_rw.ops_per_sec due to commit:

Wow, that's very surprising, given that there are no pcpu allocations on any hot
paths there.

I tried hard to reproduce it, and I think I see something, however the data is
very noisy. I'm not sure I can confidently attribute the regression to
ace7e70901 ("percpu: use reclaim threshold instead of running for every page")
rather than
f183324133 ("percpu: implement partial chunk depopulation").

Anyway, in my setup the following patch seems to fix the regression.
Is it possible to test it?

Thank you!

Roman

--

From 6ee182110126cf93cf43389923bcf49ba12cb9a0 Mon Sep 17 00:00:00 2001
From: Roman Gushchin <guro@fb.com>
Date: Thu, 29 Apr 2021 18:01:40 -0700
Subject: [PATCH] percpu: optimize locking in pcpu_balance_workfn()

pcpu_balance_workfn() unconditionally calls pcpu_balance_free(),
pcpu_reclaim_populated(), pcpu_balance_populated() and
pcpu_balance_free() again.

Each call to pcpu_balance_free() and pcpu_reclaim_populated() will
cause at least one acquisition of the pcpu_lock. So even if the
balancing was scheduled because of a failed atomic allocation,
pcpu_lock will be acquired at least 4 times. This obviously
increases the contention on the pcpu_lock.

To optimize the scheme let's grab the pcpu_lock on the upper level
(in pcpu_balance_workfn()) and keep it generally locked for the whole
duration of the scheduled work, but release conditionally to perform
any slow operations like chunk (de)population and creation of new chunks.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 mm/percpu.c | 41 +++++++++++++++++++++++++++++------------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 245d89f6f0a9..f6bc8157cb3e 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -2005,6 +2005,9 @@ void __percpu *__alloc_reserved_percpu(size_t size, size_t align)
  * If empty_only is %false, reclaim all fully free chunks regardless of the
  * number of populated pages.  Otherwise, only reclaim chunks that have no
  * populated pages.
+ *
+ * CONTEXT:
+ * pcpu_lock (can be dropped temporarily)
  */
 static void pcpu_balance_free(enum pcpu_chunk_type type, bool empty_only)
 {
@@ -2013,12 +2016,12 @@ static void pcpu_balance_free(enum pcpu_chunk_type type, bool empty_only)
 	struct list_head *free_head = &pcpu_slot[pcpu_free_slot];
 	struct pcpu_chunk *chunk, *next;
 
+	lockdep_assert_held(&pcpu_lock);
+
 	/*
 	 * There's no reason to keep around multiple unused chunks and VM
 	 * areas can be scarce.  Destroy all free chunks except for one.
 	 */
-	spin_lock_irq(&pcpu_lock);
-
 	list_for_each_entry_safe(chunk, next, free_head, list) {
 		WARN_ON(chunk->immutable);
 
@@ -2030,8 +2033,10 @@ static void pcpu_balance_free(enum pcpu_chunk_type type, bool empty_only)
 			list_move(&chunk->list, &to_free);
 	}
 
-	spin_unlock_irq(&pcpu_lock);
+	if (list_empty(&to_free))
+		return;
 
+	spin_unlock_irq(&pcpu_lock);
 	list_for_each_entry_safe(chunk, next, &to_free, list) {
 		unsigned int rs, re;
 
@@ -2045,6 +2050,7 @@ static void pcpu_balance_free(enum pcpu_chunk_type type, bool empty_only)
 		pcpu_destroy_chunk(chunk);
 		cond_resched();
 	}
+	spin_lock_irq(&pcpu_lock);
 }
 
 /**
@@ -2056,6 +2062,9 @@ static void pcpu_balance_free(enum pcpu_chunk_type type, bool empty_only)
  * OOM killer to be triggered.  We should avoid doing so until an actual
  * allocation causes the failure as it is possible that requests can be
  * serviced from already backed regions.
+ *
+ * CONTEXT:
+ * pcpu_lock (can be dropped temporarily)
  */
 static void pcpu_balance_populated(enum pcpu_chunk_type type)
 {
@@ -2065,6 +2074,8 @@ static void pcpu_balance_populated(enum pcpu_chunk_type type)
 	struct pcpu_chunk *chunk;
 	int slot, nr_to_pop, ret;
 
+	lockdep_assert_held(&pcpu_lock);
+
 	/*
 	 * Ensure there are certain number of free populated pages for
 	 * atomic allocs.  Fill up from the most packed so that atomic
@@ -2092,13 +2103,11 @@ static void pcpu_balance_populated(enum pcpu_chunk_type type)
 		if (!nr_to_pop)
 			break;
 
-		spin_lock_irq(&pcpu_lock);
 		list_for_each_entry(chunk, &pcpu_slot[slot], list) {
 			nr_unpop = chunk->nr_pages - chunk->nr_populated;
 			if (nr_unpop)
 				break;
 		}
-		spin_unlock_irq(&pcpu_lock);
 
 		if (!nr_unpop)
 			continue;
@@ -2108,12 +2117,13 @@ static void pcpu_balance_populated(enum pcpu_chunk_type type)
 					     chunk->nr_pages) {
 			int nr = min_t(int, re - rs, nr_to_pop);
 
+			spin_unlock_irq(&pcpu_lock);
 			ret = pcpu_populate_chunk(chunk, rs, rs + nr, gfp);
+			cond_resched();
+			spin_lock_irq(&pcpu_lock);
 			if (!ret) {
 				nr_to_pop -= nr;
-				spin_lock_irq(&pcpu_lock);
 				pcpu_chunk_populated(chunk, rs, rs + nr);
-				spin_unlock_irq(&pcpu_lock);
 			} else {
 				nr_to_pop = 0;
 			}
@@ -2125,11 +2135,12 @@ static void pcpu_balance_populated(enum pcpu_chunk_type type)
 
 	if (nr_to_pop) {
 		/* ran out of chunks to populate, create a new one and retry */
+		spin_unlock_irq(&pcpu_lock);
 		chunk = pcpu_create_chunk(type, gfp);
+		cond_resched();
+		spin_lock_irq(&pcpu_lock);
 		if (chunk) {
-			spin_lock_irq(&pcpu_lock);
 			pcpu_chunk_relocate(chunk, -1);
-			spin_unlock_irq(&pcpu_lock);
 			goto retry_pop;
 		}
 	}
@@ -2146,6 +2157,10 @@ static void pcpu_balance_populated(enum pcpu_chunk_type type)
  * populated pages threshold, reintegrate the chunk if it has empty free pages.
  * Each chunk is scanned in the reverse order to keep populated pages close to
  * the beginning of the chunk.
+ *
+ * CONTEXT:
+ * pcpu_lock (can be dropped temporarily)
+ *
  */
 static void pcpu_reclaim_populated(enum pcpu_chunk_type type)
 {
@@ -2155,7 +2170,7 @@ static void pcpu_reclaim_populated(enum pcpu_chunk_type type)
 	LIST_HEAD(to_depopulate);
 	int i, end;
 
-	spin_lock_irq(&pcpu_lock);
+	lockdep_assert_held(&pcpu_lock);
 
 	list_splice_init(&pcpu_slot[pcpu_to_depopulate_slot], &to_depopulate);
 
@@ -2231,8 +2246,6 @@ static void pcpu_reclaim_populated(enum pcpu_chunk_type type)
 				      &pcpu_slot[pcpu_to_depopulate_slot]);
 		pcpu_schedule_balance_work();
 	}
-
-	spin_unlock_irq(&pcpu_lock);
 }
 
 /**
@@ -2256,10 +2269,14 @@ static void pcpu_balance_workfn(struct work_struct *work)
 	 */
 	for (type = 0; type < PCPU_NR_CHUNK_TYPES; type++) {
 		mutex_lock(&pcpu_alloc_mutex);
+		spin_lock_irq(&pcpu_lock);
+
 		pcpu_balance_free(type, false);
 		pcpu_reclaim_populated(type);
 		pcpu_balance_populated(type);
 		pcpu_balance_free(type, true);
+
+		spin_unlock_irq(&pcpu_lock);
 		mutex_unlock(&pcpu_alloc_mutex);
 	}
 }
-- 
2.30.2