Re: arm-smmu-v3 high cpu usage for NVMe

[Marc Zyngier]

Hi John,

On 2020-03-20 16:20, John Garry wrote:


I've run a bunch of netperf instances on multiple cores and 
collecting
SMMU usage (on TaiShan 2280). I'm getting the following ratio pretty
consistently.

- 6.07% arm_smmu_iotlb_sync
     - 5.74% arm_smmu_tlb_inv_range
          5.09% arm_smmu_cmdq_issue_cmdlist
          0.28% __pi_memset
          0.08% __pi_memcpy
          0.08% arm_smmu_atc_inv_domain.constprop.37
          0.07% arm_smmu_cmdq_build_cmd
          0.01% arm_smmu_cmdq_batch_add
       0.31% __pi_memset

So arm_smmu_atc_inv_domain() takes about 1.4% of 
arm_smmu_iotlb_sync(),
when ATS is not used. According to the annotations, the load from 
the
atomic_read(), that checks whether the domain uses ATS, is 77% of 
the
samples in arm_smmu_atc_inv_domain() (265 of 345 samples), so I'm 
not sure
there is much room for optimization there.

Well I did originally suggest using RCU protection to scan the list 
of
devices, instead of reading an atomic and checking for non-zero 
value. But
that would be an optimsation for ATS also, and there was no ATS 
devices at
the time (to verify performance).

Heh, I have yet to get my hands on one. Currently I can't evaluate ATS
performance, but I agree that using RCU to scan the list should get 
better
results when using ATS.

When ATS isn't in use however, I suspect reading nr_ats_masters should 
be
more efficient than taking the RCU lock + reading an "ats_devices" 
list
(since the smmu_domain->devices list also serves context descriptor
invalidation, even when ATS isn't in use). I'll run some tests 
however, to
see if I can micro-optimize this case, but I don't expect noticeable
improvements.

ok, cheers. I, too, would not expect a significant improvement there.

JFYI, I've been playing for "perf annotate" today and it's giving
strange results for my NVMe testing. So "report" looks somewhat sane,
if not a worryingly high % for arm_smmu_cmdq_issue_cmdlist():


    55.39%  irq/342-nvme0q1  [kernel.kallsyms]  [k] 
arm_smmu_cmdq_issue_cmdlist
     9.74%  irq/342-nvme0q1  [kernel.kallsyms]  [k] 
_raw_spin_unlock_irqrestore
     2.02%  irq/342-nvme0q1  [kernel.kallsyms]  [k] nvme_irq
     1.86%  irq/342-nvme0q1  [kernel.kallsyms]  [k] fput_many
     1.73%  irq/342-nvme0q1  [kernel.kallsyms]  [k]
arm_smmu_atc_inv_domain.constprop.42
     1.67%  irq/342-nvme0q1  [kernel.kallsyms]  [k] __arm_lpae_unmap
     1.49%  irq/342-nvme0q1  [kernel.kallsyms]  [k] aio_complete_rw

But "annotate" consistently tells me that a specific instruction
consumes ~99% of the load for the enqueue function:

         :                      /* 5. If we are inserting a CMD_SYNC,
we must wait for it to complete */
         :                      if (sync) {
    0.00 :   ffff80001071c948:       ldr     w0, [x29, #108]
         :                      int ret = 0;
    0.00 :   ffff80001071c94c:       mov     w24, #0x0      // #0
         :                      if (sync) {
    0.00 :   ffff80001071c950:       cbnz    w0, ffff80001071c990
<arm_smmu_cmdq_issue_cmdlist+0x420>
         :                      arch_local_irq_restore():
    0.00 :   ffff80001071c954:       msr     daif, x21
         :                      arm_smmu_cmdq_issue_cmdlist():
         :                      }
         :                      }
         :
         :                      local_irq_restore(flags);
         :                      return ret;
         :                      }
   99.51 :   ffff80001071c958:       adrp    x0, ffff800011909000
<page_wait_table+0x14c0>

This is likely the side effect of the re-enabling of interrupts (msr 
daif, x21)
on the previous instruction which causes the perf interrupt to fire 
right after.

Time to enable pseudo-NMIs in the PMUv3 driver...

         M.
--
Jazz is not dead. It just smells funny...
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