Messages in this thread | | | Date | Mon, 21 Sep 2020 14:43:25 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency |
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On Fri, Aug 21, 2020 at 09:54:20PM +0800, John Garry wrote: > As mentioned in [0], the CPU may consume many cycles processing > arm_smmu_cmdq_issue_cmdlist(). One issue we find is the cmpxchg() loop to > get space on the queue takes a lot of time once we start getting many > CPUs contending - from experiment, for 64 CPUs contending the cmdq, > success rate is ~ 1 in 12, which is poor, but not totally awful. > > This series removes that cmpxchg() and replaces with an atomic_add, > same as how the actual cmdq deals with maintaining the prod pointer.
I'm still not a fan of this. Could you try to adapt the hacks I sent before, please? I know they weren't quite right (I have no hardware to test on), but the basic idea is to fall back to a spinlock if the cmpxchg() fails. The queueing in the spinlock implementation should avoid the contention.
Thanks,
Will
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