Re: [PATCH 0/4] iommu/arm-smmu-v3: Improve cmdq lock efficiency

John Garry
2020-07-16T13:33:11Z

On 16/07/2020 11:28, Will Deacon wrote:
> On Thu, Jul 16, 2020 at 11:22:33AM +0100, Will Deacon wrote:
>> On Thu, Jul 16, 2020 at 11:19:41AM +0100, Will Deacon wrote:
>>> On Tue, Jun 23, 2020 at 01:28:36AM +0800, John Garry wrote:
>>>> As mentioned in [0], the CPU may consume many cycles processing
>>>> arm_smmu_cmdq_issue_cmdlist(). One issue we find is the cmpxchg() loop to
>>>> get space on the queue takes approx 25% of the cycles for this function.
>>>>
>>>> This series removes that cmpxchg().
>>>
>>> How about something much simpler like the diff below?
>>
>> Ah, scratch that, I don't drop the lock if we fail the cas with it held.
>> Let me hack it some more (I have no hardware so I can only build-test this).
>
> Right, second attempt...
>
> Will

Unfortunately that hangs my machine during boot:

[10.902893] 00:01: ttyS0 at MMIO 0x3f00003f8 (irq = 6, base_baud =
115200) is a 16550A
[10.912048] SuperH (H)SCI(F) driver initialized
[10.916811] msm_serial: driver initialized
[10.921371] arm-smmu-v3 arm-smmu-v3.0.auto: option mask 0x0
[10.926946] arm-smmu-v3 arm-smmu-v3.0.auto: ias 48-bit, oas 48-bit
(features 0x00000fef)
[10.935374] arm-smmu-v3 arm-smmu-v3.0.auto: allocated 65536 entries for cmdq
[10.942522] arm-smmu-v3 arm-smmu-v3.0.auto: allocated 32768 entries for evtq

>
> --->8
>
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index f578677a5c41..e6bcddd6ef69 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -560,6 +560,7 @@ struct arm_smmu_cmdq {
> atomic_long_t *valid_map;
> atomic_t owner_prod;
> atomic_t lock;
> + spinlock_t slock;
> };
>
> struct arm_smmu_cmdq_batch {
> @@ -1378,7 +1379,7 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
> u64 cmd_sync[CMDQ_ENT_DWORDS];
> u32 prod;
> unsigned long flags;
> - bool owner;
> + bool owner, locked = false;
> struct arm_smmu_cmdq *cmdq = &smmu->cmdq;
> struct arm_smmu_ll_queue llq = {
> .max_n_shift = cmdq->q.llq.max_n_shift,
> @@ -1387,27 +1388,38 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
>
> /* 1. Allocate some space in the queue */
> local_irq_save(flags);
> - llq.val = READ_ONCE(cmdq->q.llq.val);
> do {
> u64 old;
> + llq.val = READ_ONCE(cmdq->q.llq.val);
>
> - while (!queue_has_space(&llq, n + sync)) {
> + if (queue_has_space(&llq, n + sync))
> + goto try_cas;
> +
> + if (locked)
> + spin_unlock(&cmdq->slock);
> +
> + do {
> local_irq_restore(flags);
> if (arm_smmu_cmdq_poll_until_not_full(smmu, &llq))
> dev_err_ratelimited(smmu->dev, "CMDQ timeout\n");
> local_irq_save(flags);
> - }
> + } while (!queue_has_space(&llq, n + sync));
>
> +try_cas:
> head.cons = llq.cons;
> head.prod = queue_inc_prod_n(&llq, n + sync) |
> CMDQ_PROD_OWNED_FLAG;
>
> old = cmpxchg_relaxed(&cmdq->q.llq.val, llq.val, head.val);
> - if (old == llq.val)
> + if (old != llq.val)

Not sure why you changed this. And if I change it back, it seems that we
could drop out of the loop with cmdq->slock held, so need to drop the
lock also.

I tried that and it stops my machine hanging. Let me know that was the
intention, so I can test.

Thanks,
John

> break;
>
> - llq.val = old;
> + if (!locked) {
> + spin_lock(&cmdq->slock);
> + locked = true;
> + }
> } while (1);
> +
> owner = !(llq.prod & CMDQ_PROD_OWNED_FLAG);
> head.prod &= ~CMDQ_PROD_OWNED_FLAG;
> llq.prod &= ~CMDQ_PROD_OWNED_FLAG;
> @@ -3192,6 +3204,7 @@ static int arm_smmu_cmdq_init(struct arm_smmu_device *smmu)
>
> atomic_set(&cmdq->owner_prod, 0);
> atomic_set(&cmdq->lock, 0);
> + spin_lock_init(&cmdq->slock);
>
> bitmap = (atomic_long_t *)bitmap_zalloc(nents, GFP_KERNEL);
> if (!bitmap) {
> .
>