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

[Marc Zyngier <maz@kernel.org>]

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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