Messages in this thread | | | Subject | Re: [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency | From | John Garry <> | Date | Mon, 21 Sep 2020 14:58:10 +0100 |
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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
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