From: Eric Dumazet edumazet@google.com
[ Upstream commit 0d87308cca2c124f9bce02383f1d9632c9be89c4 ]
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 Fixes: 9a005a800ae8 ('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 Signed-off-by: Sasha Levin sashal@kernel.org --- 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 9f35b9a0d6d86..4edf65dbbcab5 100644 --- a/drivers/iommu/iova.c +++ b/drivers/iommu/iova.c @@ -607,7 +607,9 @@ 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));