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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