Re: [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist()
From: Robin Murphy <robin.murphy@arm.com>
Date: 2021-08-05 11:24:42
Also in:
linux-iommu, lkml
On 2021-06-21 17:36, John Garry wrote:
Members of struct "llq" will be zero-inited, apart from member max_n_shift. But we write llq.val straight after the init, so it was pointless to zero init those other members. As such, separately init member max_n_shift only. In addition, struct "head" is initialised to "llq" only so that member max_n_shift is set. But that member is never referenced for "head", so remove any init there. Removing these initializations is seen as a small performance optimisation, as this code is (very) hot path.
I looked at this and immediately thought "surely the compiler can see that all the prod/cons/val fields are written anyway and elide the initialisation?", so I dumped the before and after disassembly, and... oh. You should probably clarify that it's zero-initialising all the cacheline padding which is both pointless and painful. With that, Reviewed-by: Robin Murphy <robin.murphy@arm.com> However, having looked this closely I'm now tangentially wondering why max_n_shift isn't inside the padded union? It's read at the same time as both prod and cons by queue_has_space(), and never updated, so there doesn't appear to be any benefit to it being in a separate cacheline all by itself, and llq is already twice as big as it needs to be. Sorting that would also be a good opportunity to store the value of interest in its appropriate form so we're not needlessly recalculating 1 << shift every flippin' time... Robin.
quoted hunk ↗ jump to hunk
Signed-off-by: John Garry <redacted>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 54b2f27b81d4..8a8ad49bb7fd 100644 --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c@@ -727,11 +727,11 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu, unsigned long flags; bool owner; struct arm_smmu_cmdq *cmdq = &smmu->cmdq; - struct arm_smmu_ll_queue llq = { - .max_n_shift = cmdq->q.llq.max_n_shift, - }, head = llq; + struct arm_smmu_ll_queue llq, head; int ret = 0; + llq.max_n_shift = cmdq->q.llq.max_n_shift; + /* 1. Allocate some space in the queue */ local_irq_save(flags); llq.val = READ_ONCE(cmdq->q.llq.val);
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