From 48edcf2bc2f54a6dfd4920d1ee6d159d87492631 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 9 Oct 2019 10:29:16 +0800 Subject: [PATCH] iommu/iova: Avoid false sharing on fq_timer_on [ 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 Fixes: 9a005a800ae8 ('iommu/iova: Add flush timer') Cc: Jinyu Qi Cc: Joerg Roedel Acked-by: Robin Murphy Signed-off-by: Joerg Roedel Signed-off-by: Sasha Levin Signed-off-by: Yang Yingliang --- 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 137b8cee06fc..168ca95daaed 100644 --- a/drivers/iommu/iova.c +++ b/drivers/iommu/iova.c @@ -588,7 +588,9 @@ 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)); } -- GitLab