Commit 0d87308c authored Aug 28, 2019 by Eric Dumazet Committed by Joerg Roedel Aug 30, 2019

iommu/iova: Avoid false sharing on fq_timer_on



In commit 14bd9a60 ("iommu/iova: Separate atomic variables
to improve performance") Jinyu Qi identified that the atomic_cmpxchg()
in queue_iova() was causing a performance loss and moved critical fields
so that the false sharing would not impact them.

However, avoiding the false sharing in the first place seems easy.
We should attempt the atomic_cmpxchg() no more than 100 times
per second. Adding an atomic_read() will keep the cache
line mostly shared.

This false sharing came with commit 9a005a80
("iommu/iova: Add flush timer").

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 9a005a80 ('iommu/iova: Add flush timer')
Cc: Jinyu Qi <jinyuqi@huawei.com>
Cc: Joerg Roedel <jroedel@suse.de>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

parent c8fb436b

drivers/iommu/iova.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -577,7 +577,9 @@ void queue_iova(struct iova_domain *iovad,

		spin_unlock_irqrestore(&fq->lock, flags);

		if (atomic_cmpxchg(&iovad->fq_timer_on, 0, 1) == 0)
		/* Avoid false sharing as much as possible. */
		if (!atomic_read(&iovad->fq_timer_on) &&
		!atomic_cmpxchg(&iovad->fq_timer_on, 0, 1))
		mod_timer(&iovad->fq_timer,
		jiffies + msecs_to_jiffies(IOVA_FQ_TIMEOUT));
		}

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