Messages in this thread | | | Date | Fri, 30 Aug 2019 12:49:25 +0200 | From | Joerg Roedel <> | Subject | Re: [PATCH] iommu/iova: avoid false sharing on fq_timer_on |
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Looks good to me, but adding Robin for his opinion.
On Wed, Aug 28, 2019 at 06:13:38AM -0700, Eric Dumazet wrote: > 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
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