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Date:   Wed, 28 Aug 2019 06:13:38 -0700
Message-Id: <20190828131338.89832-1-edumazet@google.com>
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Subject: [PATCH] iommu/iova: avoid false sharing on fq_timer_on
From:   Eric Dumazet <edumazet@google.com>
To:     Joerg Roedel <jroedel@suse.de>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        iommu@lists.linux-foundation.org,
        Eric Dumazet <edumazet@google.com>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Jinyu Qi <jinyuqi@huawei.com>
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Sender: linux-kernel-owner@vger.kernel.org
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In commit 14bd9a607f90 ("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 9a005a800ae8
("iommu/iova: Add flush timer").

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jinyu Qi <jinyuqi@huawei.com>
Cc: Joerg Roedel <jroedel@suse.de>
---
 drivers/iommu/iova.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 3e1a8a6755723a927a7942a7429ab7e6c19a0027..41c605b0058f9615c2dbdd83f1de2404a9b1d255 100644
--- a/drivers/iommu/iova.c
+++ b/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));
 }
-- 
2.23.0.187.g17f5b7556c-goog