Messages in this thread | | | Subject | Re: [PATCH v3 1/1] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd() | From | John Garry <> | Date | Wed, 8 Dec 2021 18:17:45 +0000 |
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>> Did you notice any performance change with this change? > > Hi John: > Thanks for the tip. I wrote a test case today, and I found that the > performance did not go up but down.
I very quickly tested on a DMA mapping benchmark very similar to the kernel DMA benchmark module - I got mixed results. For fewer CPUs (<8), a small improvement, like 0.7%. For more CPUs, a dis-improvement - that's surprising, I did expect just no change as any improvement would get dwarfed from the slower unmap rates for more CPUs. I can check this more tomorrow.
> It's so weird. So I decided not to > change it, because it's also poorly readable. So I plan to make only > the following modifications: > @@ -237,7 +237,7 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent) > static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent) > { > memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT); > - cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode); > + cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode); > > switch (ent->opcode) { > case CMDQ_OP_TLBI_EL2_ALL: > > This prevents the compiler from generating the following two inefficient > instructions: > 394: f9400002 ldr x2, [x0] //x2 = cmd[0] > 398: aa020062 orr x2, x3, x2 //x3 = FIELD_PREP(CMDQ_0_OP, ent->opcode) > > Maybe it's not worth changing because I've only seen a 0.x nanosecond reduction > in performance. But one thing is, it only comes with benefits, no side effects. >
I just think that with the original code that cmd[] is on the stack and cached, so if we have write-back attribute (which I think we do) then there would not necessarily a write to external memory per write to cmd[].
So, apart from this approach, I think that if we can just reduce the instructions through other efficiencies in the function then that would be good.
Thanks, John
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