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

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Message-ID: <20200716101940.GA7036@willie-the-truck>
Date:   Thu, 16 Jul 2020 11:19:41 +0100
From:   Will Deacon <will@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     robin.murphy@....com, joro@...tes.org, trivial@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com, maz@...nel.org
Subject: Re: [PATCH 0/4] iommu/arm-smmu-v3: Improve cmdq lock efficiency

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?

Will

--->8

diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index f578677a5c41..705d99ec8219 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, cas_failed = 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 +1388,35 @@ 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 (cas_failed)
+			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)
-			break;
+		cas_failed = old != llq.val;
+
+		if (cas_failed)
+			spin_lock(&cmdq->slock);
+	} while (cas_failed);
 
-		llq.val = old;
-	} while (1);
 	owner = !(llq.prod & CMDQ_PROD_OWNED_FLAG);
 	head.prod &= ~CMDQ_PROD_OWNED_FLAG;
 	llq.prod &= ~CMDQ_PROD_OWNED_FLAG;
@@ -3192,6 +3201,7 @@ 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) {