Messages in this thread Patch in this message | | | From | John Garry <> | Subject | Re: [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency | Date | Wed, 23 Sep 2020 15:47:17 +0100 |
| |
On 21/09/2020 14:58, John Garry wrote: > >> Could you try to adapt the hacks I sent before, >> please? I know they weren't quite right (I have no hardware to test >> on
Could the ARM Rev C FVP be used to at least functionally test? Can't seem to access myself, even though it's gratis...
), but >> the basic idea is to fall back to a spinlock if the cmpxchg() fails. The >> queueing in the spinlock implementation should avoid the contention. >
So I modified that suggested change to get it functioning, and it looks like this:
diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c index 7196207be7ea..f907b7c233a2 100644 --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c +++ b/drivers/iommu/arm/arm-smmu-v3/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, @@ -1387,26 +1388,42 @@ 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;
- while (!queue_has_space(&llq, n + sync)) { + llq.val = READ_ONCE(cmdq->q.llq.val); + + if (queue_has_space(&llq, n + sync)) + goto try_cas; + + if (locked) { + spin_unlock(&cmdq->slock); + locked = 0; // added + } + + 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) { // was if (old != llq.val) + if (locked) // break; + spin_unlock(&cmdq->slock);// 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; @@ -3192,6 +3209,7 @@ 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) { -- 2.26.2 I annotated my mods with comments. Maybe those mods would not be as you intend.
So I'm not sure that we solve the problem of a new CPU coming along and trying the cmpxchg immediately, while another CPU has the slock and will try the cmpxchg also.
Anyway, the results are a bit mixed depending on the CPU count, but generally positive compared to mainline:
CPUs 2 4 8 16 32 64 96 v5.9-rc1 453K 409K 295K 157K 33.6K 9.5K 5.2K Will's change 459K 414K 281K 131K 44K 15.5K 8.6K $subject change 481K 406K 305K 190K 81K 30K 18.7K
(Unit is DMA map+unmap per CPU per second, using test harness. Higher is better.)
Please let me know of any way to progress.
Thanks, John
| |