Re: [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency

From: Will Deacon
Date: Mon Sep 21 2020 - 09:43:35 EST

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.