[PATCH 4/4] percpu_ida: Do not steal all remote CPU tags at once

[Alexander Gordeev]

When stealing from a remote CPU all available tags are moved
from the remote CPU's cache to the stealing CPU's one. Since
the best CPU to steal from is not selected the victim might
actively performing IO and (as result of losing all local
tags) a further cycle of cache rebouncing and/or stealing
could be provoked.

This update is an attempt to soften the described scenario
and limit the number of tags to be stolen at once to the
value of percpu_batch_size.

Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxx>
Cc: Kent Overstreet <kmo@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: "Nicholas A. Bellinger" <nab@xxxxxxxxxxxxxxx>
---
 lib/percpu_ida.c |   18 ++++++++++--------
 1 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
index fad029c..b4c4cc7 100644
--- a/lib/percpu_ida.c
+++ b/lib/percpu_ida.c
@@ -84,20 +84,22 @@ static inline void steal_tags(struct percpu_ida *pool,
 		pool->cpu_last_stolen = cpu;
 		remote = per_cpu_ptr(pool->tag_cpu, cpu);
 
-		cpumask_clear_cpu(cpu, &pool->cpus_have_tags);
-
-		if (remote == tags)
+		if (remote == tags) {
+			cpumask_clear_cpu(cpu, &pool->cpus_have_tags);
 			continue;
+		}
 
 		spin_lock(&remote->lock);
 
 		if (remote->nr_free) {
-			memcpy(tags->freelist,
-			       remote->freelist,
-			       sizeof(unsigned) * remote->nr_free);
+			const struct percpu_ida *p = pool;
+
+			move_tags(tags->freelist, &tags->nr_free,
+				  remote->freelist, &remote->nr_free,
+				  min(remote->nr_free, p->percpu_batch_size));
 
-			tags->nr_free = remote->nr_free;
-			remote->nr_free = 0;
+			if (!remote->nr_free)
+				cpumask_clear_cpu(cpu, &pool->cpus_have_tags);
 		}
 
 		spin_unlock(&remote->lock);
-- 
1.7.7.6

-- 
Regards,
Alexander Gordeev
agordeev@xxxxxxxxxx
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