linux-kernel - Re: [PATCH 0/5] x86, MSI, AHCI: Support multiple MSIs

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Message-ID: <502D1D9F.9020203@pobox.com>
Date:	Thu, 16 Aug 2012 12:19:43 -0400
From:	Jeff Garzik <jgarzik@...ox.com>
To:	Alexander Gordeev <agordeev@...hat.com>
CC:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Matthew Wilcox <willy@...ux.intel.com>, x86@...nel.org,
	linux-pci@...r.kernel.org, linux-ide@...r.kernel.org
Subject: Re: [PATCH 0/5] x86, MSI, AHCI: Support multiple MSIs

On 08/16/2012 10:45 AM, Alexander Gordeev wrote:
> Currently multiple MSI mode is limited to a single vector per device (at
> least on x86 and PPC). This series breathes life into pci_enable_msi_block()
> and makes it possible to set interrupt affinity for multiple IRQs, similarly
> to MSI-X. Yet, only for x86 and only when IOMMUs are present.
>
> Although IRQ and PCI subsystems are modified, the current behaviour left
> intact. The drivers could just start using multiple MSIs just by following
> the existing documentation.
>
> The AHCI device driver makes use of the new mode and a new function -
> pci_enable_msi_block_auto() - patches 4,5.
>
> The series is adapted to Ingo's -tip repository, x86/apic branch.
> Patches 4,5 could be applied independently of patches 1-3.
>
> Attached patches:
>    1/5 x86, MSI: Support multiple MSIs in presense of IRQ remapping
>    2/5 x86, MSI: Allocate as many multiple IRQs as requested
>    3/5 x86, MSI: Minor readability fixes
>    4/5 PCI, MSI: Enable multiple MSIs with pci_enable_msi_block_auto()
>    5/5 AHCI: Support multiple MSIs

Numbers?  I would like to see measurements that show this is a benefit. 
  I'm especially wondering about lock contention for the case of 
multiple, fast devices (e.g. RAM over SATA, or PCIe flash).

Because I see the following obvious problems immediately:

1) AHCI takes a host-wide lock during interrupt processing, which quite 
reduces the value of "interrupts generated by different
ports could be serviced on different CPUs"

2) We do not put AHCI-specific code in libata-core.  Try libahci.c or 
ahci.c.

Regards,

	Jeff




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