On 02/04/2020 13:10, John Garry wrote:
On 18/03/2020 20:53, Will Deacon wrote:
As for arm_smmu_cmdq_issue_cmdlist(), I do note that during the
testing our
batch size is 1, so we're not seeing the real benefit of the batching. I
can't help but think that we could improve this code to try to
combine CMD
SYNCs for small batches.
Anyway, let me know your thoughts or any questions. I'll have a look
if a
get a chance for other possible bottlenecks.
Did you ever get any more information on this? I don't have any SMMUv3
hardware any more, so I can't really dig into this myself.
Hi Will,
JFYI, I added some debug in arm_smmu_cmdq_issue_cmdlist() to get some
idea of what is going on. Perf annotate did not tell much.
I tested NVMe performance with and without Marc's patchset to spread
LPIs for managed interrupts.
Average duration of arm_smmu_cmdq_issue_cmdlist() mainline [all results
are approximations]:
owner: 6ms
non-owner: 4ms
mainline + LPI spreading patchset:
owner: 25ms
non-owner: 22ms
For this, a list would be a itlb+cmd_sync.
Please note that the LPI spreading patchset is still giving circa 25%
NVMe throughput increase. What happens there would be that we get many
more cpus involved, which creates more inter-cpu contention. But the
performance increase comes from just alleviating pressure on those
overloaded cpus.
I also notice that with the LPI spreading patchset, on average a cpu is
an "owner" in arm_smmu_cmdq_issue_cmdlist() 1 in 8, as opposed to 1 in 3
for mainline. This means that we're just creating longer chains of lists
to be published.
But I found that for a non-owner, average msi cmd_sync polling time is
12ms with the LPI spreading patchset. As such, it seems to be really
taking approx (12*2/8-1=) ~3ms to consume a single list. This seems
consistent with my finding that an owner polls consumption for 3ms also.
Without the LPI speading patchset, polling time is approx 2 and 3ms for
both owner and non-owner, respectively.
As an experiment, I did try to hack the code to use a spinlock again for
protecting the command queue, instead of current solution - and always
saw a performance drop there. To be expected. But maybe we can try to
not use a spinlock, but still serialise production+consumption to
alleviate the long polling periods.
Let me know your thoughts.
Cheers,
John
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