Messages in this thread | | | Date | Wed, 8 Dec 2021 18:40:31 +0000 | Subject | Re: [PATCH v3 1/1] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd() | From | Robin Murphy <> |
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On 2021-12-08 18:17, John Garry wrote: >>> 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.
Not sure if it's still true, but FWIW last time the best result actually came from doing the ridiculously counter-intuitive:
https://lore.kernel.org/linux-iommu/141de3c3278e280712d16d9ac9ab305c3b80a810.1534344167.git.robin.murphy@arm.com/
Robin.
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