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

From: John Garry
Date: Mon Sep 21 2020 - 10:01:05 EST

On 21/09/2020 14:43, Will Deacon wrote:
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.

OK, so if you're asking me to try this again, then I can do that, and see what it gives us.