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:Ah, scratch that, I don't drop the lock if we fail the cas with it held.
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? >>
Let me hack it some more (I have no hardware so I can only build-test this).
Right, second attempt...
I can try it, but if performance if not as good, then please check mine further (patch 4/4 specifically) - performance is really good, IMHO.
Thanks,
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 {
ÂÂÂÂÂ atomic_long_tÂÂÂÂÂÂÂÂÂÂÂ *valid_map;
ÂÂÂÂÂ atomic_tÂÂÂÂÂÂÂÂÂÂÂ owner_prod;
ÂÂÂÂÂ atomic_tÂÂÂÂÂÂÂÂÂÂÂ lock;
+ÂÂÂ spinlock_tÂÂÂÂÂÂÂÂÂÂÂ slock;
 };
 struct arm_smmu_cmdq_batch {
@@ -1378,7 +1379,7 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu,
ÂÂÂÂÂ u64 cmd_sync[CMDQ_ENT_DWORDS];
ÂÂÂÂÂ u32 prod;
ÂÂÂÂÂ unsigned long flags;
-ÂÂÂ bool owner;
+ÂÂÂ bool owner, locked = false;
ÂÂÂÂÂ struct arm_smmu_cmdq *cmdq = &smmu->cmdq;
ÂÂÂÂÂ struct arm_smmu_ll_queue llq = {
ÂÂÂÂÂÂÂÂÂ .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,
ÂÂÂÂÂ /* 1. Allocate some space in the queue */
ÂÂÂÂÂ local_irq_save(flags);
-ÂÂÂ llq.val = READ_ONCE(cmdq->q.llq.val);
ÂÂÂÂÂ do {
ÂÂÂÂÂÂÂÂÂ u64 old;
+ÂÂÂÂÂÂÂ llq.val = READ_ONCE(cmdq->q.llq.val);
-ÂÂÂÂÂÂÂ while (!queue_has_space(&llq, n + sync)) {
+ÂÂÂÂÂÂÂ if (queue_has_space(&llq, n + sync))
+ÂÂÂÂÂÂÂÂÂÂÂ goto try_cas;
+
+ÂÂÂÂÂÂÂ if (locked)
+ÂÂÂÂÂÂÂÂÂÂÂ spin_unlock(&cmdq->slock);
+
+ÂÂÂÂÂÂÂ do {
ÂÂÂÂÂÂÂÂÂÂÂÂÂ local_irq_restore(flags);
ÂÂÂÂÂÂÂÂÂÂÂÂÂ if (arm_smmu_cmdq_poll_until_not_full(smmu, &llq))
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ dev_err_ratelimited(smmu->dev, "CMDQ timeout\n");
ÂÂÂÂÂÂÂÂÂÂÂÂÂ local_irq_save(flags);
-ÂÂÂÂÂÂÂ }
+ÂÂÂÂÂÂÂ } while (!queue_has_space(&llq, n + sync));
+try_cas:
ÂÂÂÂÂÂÂÂÂ head.cons = llq.cons;
ÂÂÂÂÂÂÂÂÂ head.prod = queue_inc_prod_n(&llq, n + sync) |
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ CMDQ_PROD_OWNED_FLAG;
ÂÂÂÂÂÂÂÂÂ old = cmpxchg_relaxed(&cmdq->q.llq.val, llq.val, head.val);
-ÂÂÂÂÂÂÂ if (old == llq.val)
+ÂÂÂÂÂÂÂ if (old != llq.val)
ÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
-ÂÂÂÂÂÂÂ llq.val = old;
+ÂÂÂÂÂÂÂ if (!locked) {
+ÂÂÂÂÂÂÂÂÂÂÂ spin_lock(&cmdq->slock);
+ÂÂÂÂÂÂÂÂÂÂÂ locked = true;
+ÂÂÂÂÂÂÂ }
ÂÂÂÂÂ } while (1);
+
ÂÂÂÂÂ owner = !(llq.prod & CMDQ_PROD_OWNED_FLAG);
ÂÂÂÂÂ head.prod &= ~CMDQ_PROD_OWNED_FLAG;
ÂÂÂÂÂ llq.prod &= ~CMDQ_PROD_OWNED_FLAG;
@@ -3192,6 +3204,7 @@ static int arm_smmu_cmdq_init(struct arm_smmu_device *smmu)
ÂÂÂÂÂ atomic_set(&cmdq->owner_prod, 0);
ÂÂÂÂÂ atomic_set(&cmdq->lock, 0);
+ÂÂÂ spin_lock_init(&cmdq->slock);
ÂÂÂÂÂ bitmap = (atomic_long_t *)bitmap_zalloc(nents, GFP_KERNEL);
ÂÂÂÂÂ if (!bitmap) {
.