From: Will Deacon <will@kernel.org>
To: John Garry <john.garry@huawei.com>
Cc: maz@kernel.org, linuxarm@huawei.com,
linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
robin.murphy@arm.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency
Date: Mon, 21 Sep 2020 14:43:25 +0100 [thread overview]
Message-ID: <20200921134324.GK2139@willie-the-truck> (raw)
In-Reply-To: <1598018062-175608-1-git-send-email-john.garry@huawei.com>
On Fri, Aug 21, 2020 at 09:54:20PM +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 a lot of time once we start getting many
> CPUs contending - from experiment, for 64 CPUs contending the cmdq,
> success rate is ~ 1 in 12, which is poor, but not totally awful.
>
> This series removes that cmpxchg() and replaces with an atomic_add,
> same as how the actual cmdq deals with maintaining the prod pointer.
I'm still not a fan of this. 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), 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.
Thanks,
Will
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next prev parent reply other threads:[~2020-09-21 13:43 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-21 13:54 [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency John Garry
2020-08-21 13:54 ` [PATCH v2 1/2] iommu/arm-smmu-v3: Calculate max commands per batch John Garry
2020-08-21 13:54 ` [PATCH v2 2/2] iommu/arm-smmu-v3: Remove cmpxchg() in arm_smmu_cmdq_issue_cmdlist() John Garry
2020-09-01 11:17 ` [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency Song Bao Hua (Barry Song)
2020-09-21 13:43 ` Will Deacon [this message]
2020-09-21 13:58 ` John Garry
2020-09-23 14:47 ` John Garry
2020-11-13 10:43 ` John Garry
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