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

[John Garry]

On 21/09/2020 14:58, John Garry wrote:
> 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.

JFYI, to prove that this is not a problem which affects only our HW, I 
managed to test an arm64 platform from another vendor. Generally I see 
the same issue, and this patchset actually helps that platform even more.

		CPUs	Before	After	% Increase
Huawei D06	8	282K	302K	7%
Other			379K	420K	11%

Huawei D06	16	115K	193K	68K
Other			102K	291K	185K

Huawei D06	32	36K	80K	122%
Other			41K	156K	280%

Huawei D06	64	11K	30K	172%
Other			6K	47K	683%

I tested with something like [1], so unit is map+unmaps per cpu per 
second - higher is better.

My D06 is memory poor, so would expect higher results otherwise (with 
more memory). Indeed, my D05 has memory on all nodes and performs better.

Anyway, I see that the implementation here is not perfect, and I could 
not get suggested approach to improve performance significantly. So back 
to the drawing board...

Thanks,
John

[1] 
https://lore.kernel.org/linux-iommu/20201102080646.2180-1-song.bao.hua@xxxxxxxxxxxxx/