From: Will Deacon <will@kernel.org>
To: Nicolin Chen <nicolinc@nvidia.com>
Cc: robin.murphy@arm.com, jgg@nvidia.com, joro@8bytes.org,
jean-philippe@linaro.org, apopple@nvidia.com,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, iommu@lists.linux.dev
Subject: Re: [PATCH v2] iommu/arm-smmu-v3: Add a user-configurable tlb_invalidate_threshold
Date: Fri, 18 Aug 2023 17:11:19 +0100 [thread overview]
Message-ID: <20230818161119.GA16216@willie-the-truck> (raw)
In-Reply-To: <ZN5oojF6vKOKB/eI@Asurada-Nvidia>
On Thu, Aug 17, 2023 at 11:36:18AM -0700, Nicolin Chen wrote:
> On Wed, Aug 16, 2023 at 01:43:50PM -0700, Nicolin Chen wrote:
>
> > When receiving an __arm_smmu_tlb_inv_range() call with a large size, there
> > could be a long latency at this function call: one part is coming from a
> > large software overhead in the routine of building commands, and the other
> > part is coming from CMDQ hardware consuming the large number of commands.
> > This latency could be significantly large on an SMMU that does not support
> > range invalidation commands, i.e. no ARM_SMMU_FEAT_RANGE_INV.
> >
> > One way to optimize this is to replace a large number of VA invalidation
> > commands with one single per-asid invalidation command, when the requested
> > size reaches a threshold. This threshold can be configurable depending on
> > the SMMU implementaion.
>
> I'm rethinking about this size-based threshold, since what really
> affects the latency is the number of the invalidation commands in
> the request. So having an npages-based threshold might be optimal,
> though the idea and implementation would be similar.
On the CPU side, we just have:
#define MAX_TLBI_OPS PTRS_PER_PTE
in asm/tlbflush.h
Can we start off with something similar for the SMMU? I'm not massively
keen on exposing this as a knob to userspace, because I don't think most
people will have a clue about how to tune it.
Will
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next prev parent reply other threads:[~2023-08-18 16:11 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-16 20:43 [PATCH v2] iommu/arm-smmu-v3: Add a user-configurable tlb_invalidate_threshold Nicolin Chen
2023-08-17 18:36 ` Nicolin Chen
2023-08-18 16:11 ` Will Deacon [this message]
2023-08-18 17:18 ` Nicolin Chen
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