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

Will Deacon
2020-07-16T10:28:25Z

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

--->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)
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) {