Re: [PATCH 5/4] iommu/arm-smmu: Poll for TLB sync completion more effectively

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Thu, Mar 23, 2017 at 05:59:40PM +0000, Robin Murphy wrote:
> On relatively slow development platforms and software models, the
> inefficiency of our TLB sync loop tends not to show up - for instance on
> a Juno r1 board I typically see the TLBI has completed of its own accord
> by the time we get to the sync, such that the latter finishes instantly.
> 
> However, on larger systems doing real I/O, it's less realistic for the
> TLBs to go idle immediately, and at that point falling into the 1MHz
> polling loop turns out to throw away performance drastically. Let's
> strike a balance by polling more than once between pauses, such that we
> have much more chance of catching normal operations completing before
> committing to the fixed delay, but also backing off exponentially, since
> if a sync really hasn't completed within one or two "reasonable time"
> periods, it becomes increasingly unlikely that it ever will.
> 
> Signed-off-by: Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
> ---
>  drivers/iommu/arm-smmu.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)

Thanks, I like this patch.

> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> index f7411109670f..aa17f3d937a0 100644
> --- a/drivers/iommu/arm-smmu.c
> +++ b/drivers/iommu/arm-smmu.c
> @@ -162,6 +162,7 @@
>  #define ARM_SMMU_GR0_sTLBGSTATUS	0x74
>  #define sTLBGSTATUS_GSACTIVE		(1 << 0)
>  #define TLB_LOOP_TIMEOUT		1000000	/* 1s! */
> +#define TLB_SPIN_COUNT			10
>  
>  /* Stream mapping registers */
>  #define ARM_SMMU_GR0_SMR(n)		(0x800 + ((n) << 2))
> @@ -574,18 +575,17 @@ static void __arm_smmu_free_bitmap(unsigned long *map, int idx)
>  static void __arm_smmu_tlb_sync(struct arm_smmu_device *smmu,
>  				void __iomem *sync, void __iomem *status)
>  {
> -	int count = 0;
> +	unsigned int spin_count, delay;
>  
>  	writel_relaxed(0, sync);
> -	while (readl_relaxed(status) & sTLBGSTATUS_GSACTIVE) {
> -		cpu_relax();
> -		if (++count == TLB_LOOP_TIMEOUT) {
> -			dev_err_ratelimited(smmu->dev,
> -			"TLB sync timed out -- SMMU may be deadlocked\n");
> -			return;
> -		}
> -		udelay(1);
> +	for (delay = 1; delay < TLB_LOOP_TIMEOUT; delay *= 2) {
> +		for (spin_count = TLB_SPIN_COUNT; spin_count > 0; spin_count--)
> +			if (!(readl_relaxed(status) & sTLBGSTATUS_GSACTIVE))
> +				return;

Can you keep the cpu_relax in the inner loop please?

> +		udelay(delay);
>  	}
> +	dev_err_ratelimited(smmu->dev,
> +			    "TLB sync timed out -- SMMU may be deadlocked\n");

Whilst we can have WFE-based spinning with SMMUv3, I suppose we should
do something similar in queue_poll_cons... Fancy taking a look?

Will