iommu/iova: Avoid false sharing on fq_timer_on

In commit 14bd9a6 ("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 9a005a8
("iommu/iova: Add flush timer").

Signed-off-by: Eric Dumazet <[email protected]>
Fixes: 9a005a8 ('iommu/iova: Add flush timer')
Cc: Jinyu Qi <[email protected]>
Cc: Joerg Roedel <[email protected]>
Acked-by: Robin Murphy <[email protected]>
Signed-off-by: Joerg Roedel <[email protected]>

Author: edumazet

drivers/iommu/iova.c

@@ -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));
 }