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

[John Garry]

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.

John