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>
0.00 : ffff80001071c95c: add x21, x0, #0x908
0.02 : ffff80001071c960: ldr x2, [x29, #488]
0.14 : ffff80001071c964: ldr x1, [x21]
0.00 : ffff80001071c968: eor x1, x2, x1
0.00 : ffff80001071c96c: mov w0, w24
But there may be a hint that we're getting consuming a lot of time in
polling the CMD_SYNC consumption.
The files are available here:
https://raw.githubusercontent.com/hisilicon/kernel-dev/private-topic-nvme-5.6-profiling/ann.txt,
report
Or maybe I'm just not using the tool properly ...
Cheers,
John
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