Re: [PATCH v4 2/2] iommu/arm-smmu-v3: avoid redundant CMD_SYNCs if possible

[John Garry <john.garry@huawei.com>]

On 19/08/2018 08:51, Zhen Lei wrote:
> More than two CMD_SYNCs maybe adjacent in the command queue, and the first
> one has done what others want to do. Drop the redundant CMD_SYNCs can
> improve IO performance especially under the pressure scene.
>
> I did the statistics in my test environment, the number of CMD_SYNCs can
> be reduced about 1/3. See below:
> CMD_SYNCs reduced:	19542181
> CMD_SYNCs total:	58098548	(include reduced)
> CMDs total:		116197099	(TLBI:SYNC about 1:1)
>
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
>  drivers/iommu/arm-smmu-v3.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index ac6d6df..f3a56e1 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -567,6 +567,7 @@ struct arm_smmu_device {
>  	int				gerr_irq;
>  	int				combined_irq;
>  	u32				sync_nr;
> +	u8				prev_cmd_opcode;
>
>  	unsigned long			ias; /* IPA */
>  	unsigned long			oas; /* PA */
> @@ -786,6 +787,11 @@ void arm_smmu_cmdq_build_sync_msi_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>  	cmd[1]  = ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
>  }
>
> +static inline u8 arm_smmu_cmd_opcode_get(u64 *cmd)
> +{
> +	return cmd[0] & CMDQ_0_OP;
> +}
> +
>  /* High-level queue accessors */
>  static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>  {
> @@ -906,6 +912,8 @@ static void arm_smmu_cmdq_insert_cmd(struct arm_smmu_device *smmu, u64 *cmd)
>  	struct arm_smmu_queue *q = &smmu->cmdq.q;
>  	bool wfe = !!(smmu->features & ARM_SMMU_FEAT_SEV);
>
> +	smmu->prev_cmd_opcode = arm_smmu_cmd_opcode_get(cmd);
> +
>  	while (queue_insert_raw(q, cmd) == -ENOSPC) {
>  		if (queue_poll_cons(q, false, wfe))
>  			dev_err_ratelimited(smmu->dev, "CMDQ timeout\n");
> @@ -958,9 +966,17 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
>  	};
>
>  	spin_lock_irqsave(&smmu->cmdq.lock, flags);
> -	ent.sync.msidata = ++smmu->sync_nr;
> -	arm_smmu_cmdq_build_sync_msi_cmd(cmd, &ent);
> -	arm_smmu_cmdq_insert_cmd(smmu, cmd);
> +	if (smmu->prev_cmd_opcode == CMDQ_OP_CMD_SYNC) {
> +		/*
> +		 * Previous command is CMD_SYNC also, there is no need to add
> +		 * one more. Just poll it.
> +		 */
> +		ent.sync.msidata = smmu->sync_nr;
> +	} else {
> +		ent.sync.msidata = ++smmu->sync_nr;
> +		arm_smmu_cmdq_build_sync_msi_cmd(cmd, &ent);
> +		arm_smmu_cmdq_insert_cmd(smmu, cmd);
> +	}
>  	spin_unlock_irqrestore(&smmu->cmdq.lock, flags);

I find something like this adds support for combining CMD_SYNC commands 
for regular polling mode:

@@ -569,6 +569,7 @@ struct arm_smmu_device {
         int                             combined_irq;
         u32                             sync_nr;
         u8                              prev_cmd_opcode;
+       int                             prev_cmd_sync_res;

         unsigned long                   ias; /* IPA */
         unsigned long                   oas; /* PA */
@@ -985,17 +986,33 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct 
arm_smmu_device *smmu)

  static int __arm_smmu_cmdq_issue_sync(struct arm_smmu_device *smmu)
  {
-       u64 cmd[CMDQ_ENT_DWORDS];
+       static u64 cmd[CMDQ_ENT_DWORDS] = {
+               _FIELD_PREP(CMDQ_0_OP, CMDQ_OP_CMD_SYNC) |
+               _FIELD_PREP(CMDQ_SYNC_0_CS, CMDQ_SYNC_0_CS_SEV) |
+               _FIELD_PREP(CMDQ_SYNC_0_MSH, ARM_SMMU_SH_ISH) |
+               _FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB)
+       };
         unsigned long flags;
         bool wfe = !!(smmu->features & ARM_SMMU_FEAT_SEV);
-       struct arm_smmu_cmdq_ent ent = { .opcode = CMDQ_OP_CMD_SYNC };
-       int ret;
+       int ret = 0;

-       arm_smmu_cmdq_build_cmd(cmd, &ent);

         spin_lock_irqsave(&smmu->cmdq.lock, flags);
-       arm_smmu_cmdq_insert_cmd(smmu, cmd);
-       ret = queue_poll_cons(&smmu->cmdq.q, true, wfe);
+       if (smmu->prev_cmd_opcode != CMDQ_OP_CMD_SYNC ||
+               smmu->prev_cmd_sync_res != 0) {
+               arm_smmu_cmdq_insert_cmd(smmu, cmd);
+               smmu->prev_cmd_sync_res = ret =
+                       queue_poll_cons(&smmu->cmdq.q, true, wfe);
+       }

I tested iperf on a 1G network link and was seeing 6-10% CMD_SYNC 
commands combined. I would really need to test this on a faster 
connection to see any throughout difference.

 From the above figures, I think leizhen was seeing 25% combine rate, right?

As for this code, it could be neatened...

Cheers,
John

>
>  	return __arm_smmu_sync_poll_msi(smmu, ent.sync.msidata);
> --
> 1.8.3
>
>
>
> .
>