linux-kernel - Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0 controller

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Date:   Wed, 28 Dec 2022 10:19:58 +0800
From:   Xinghui Li <korantwork@...il.com>
To:     Jonathan Derrick <jonathan.derrick@...ux.dev>
Cc:     Keith Busch <kbusch@...nel.org>, nirmal.patel@...ux.intel.com,
        lpieralisi@...nel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xinghui Li <korantli@...cent.com>
Subject: Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0 controller

Jonathan Derrick <jonathan.derrick@...ux.dev> 于2022年12月28日周三 06:32写道：
>
> The bypass mode should help in the cases where drives irqs (eg nproc) exceed
> VMD I/O irqs. VMD I/O irqs for 28c0 should be min(63, nproc). You have
> very few cpus for a Skylake system with that many drives, unless you mean you
> are explicitly restricting the 12 drives to only 6 cpus. Either way, bypass mode
> is effectively VMD-disabled, which points to other issues. Though I have also seen
> much smaller interrupt aggregation benefits.

Firstly，I am sorry for my words misleading you. We totally tested 12 drives.
And each drive run in 6 CPU cores with 8 jobs.

Secondly, I try to test the drives with VMD disabled,I found the results to
be largely consistent with bypass mode. I suppose the bypass mode just
"bypass" the VMD controller.

The last one，we found in bypass mode the CPU idle is 91%. But in remapping mode
the CPU idle is 78%. And the bypass's context-switchs is much fewer
than the remapping
mode's. It seems the system is watiing for something in bypass mode.