linux-kernel - Re: [PATCH] msi: skip calling pci_find_capability from msi_set_mask

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Message-Id: <200805191311.46162.jbarnes@virtuousgeek.org>
Date:	Mon, 19 May 2008 13:11:45 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Matthew Wilcox <matthew@....cx>, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH] msi: skip calling pci_find_capability from msi_set_mask_bits

On Sunday, May 18, 2008 9:48 pm Hidetoshi Seto wrote:
> Jesse Barnes wrote:
> > Since we know MSI is a problem, let's just go with fixing that for now. 
> > If we find that other caps are also causing problems we can revisit
> > caching all of them; the patch is simple enough.
>
> Humm...
> I suppose it can be more simple. How about this patch?
>
> > everytime handle_edge_irq is called it needs to mask and unmask MSI, and
> > that leads to a series of very expensive calls to pci_find_capability
>
> The position of MSI capability is already cached in the msi_desc when
> we enter the msi_set_mask_bits().  Use it instead.
>
> Signed-off-by: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>

Yeah, this looks really nice.  It should also fix Arnaldo's latency problem, 
and really looks like a bug fix for the MSI code more than anything.

Arnaldo, can you take a look & test and ack/nack?

Thanks,
Jesse
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