On Fri, Aug 21, 2020 at 09:54:20PM +0800, John Garry wrote:
As mentioned in , the CPU may consume many cycles processing> I'm still not a fan of this.
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.
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.