Messages in this thread | | | Subject | Re: [PATCH] iommu/iova: avoid false sharing on fq_timer_on | From | Robin Murphy <> | Date | Fri, 30 Aug 2019 13:27:25 +0100 |
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On 30/08/2019 11:49, Joerg Roedel wrote: > Looks good to me, but adding Robin for his opinion.
Sounds reasonable to me too - that should also be true for the majority of Arm systems that we know of. Will suggested that atomic_try_cmpxchg() might be relevant, but AFAICS that's backwards compared to what we want to do here, which I guess is more of an "atomic_unlikely_cmpxchg".
Acked-by: Robin Murphy <robin.murphy@arm.com>
Cheers, Robin.
> 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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