linux-kernel - Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi

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Date:	Tue, 1 Oct 2013 17:19:55 +1000
From:	Michael Ellerman <michael@...erman.id.au>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Joerg Roedel <joro@...tes.org>,
	Jan Beulich <JBeulich@...e.com>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface

On Thu, Sep 26, 2013 at 04:39:02PM +0200, Alexander Gordeev wrote:
> On Thu, Sep 26, 2013 at 09:11:47AM -0400, Tejun Heo wrote:
> > > Because otherwise we will re-introduce a problem described by Michael:
> > > "We have a small number of MSIs available, limited by hardware &
> > > firmware, if we don't impose a quota then the first device that probes
> > > will get most/all of the MSIs and other devices miss out."
> > 
> > Still not following.  Why wouldn't just letting the drivers request
> > the optimal number they want and falling back to single interrupt mode
> > work?  ie. why can't we just have an all or nothing interface?
> 
> I can imagine a scenario where the first device probes in, requests its
> optimal number, acquires that number and exhausts MSIs in pSeries firmware.
> The next few devices possibly end up with single MSI, since no MSIs left
> to satisfy their optimal numbers. If one of those single-MSI'ed devices
> happened to be a high-performance HBA hitting a degraded performance that
> alone would force (IBM) to introduce the quotas.

Yes that's exactly the scenario, and I didn't imagine it, our test
people actually hit it and yelled at me.

I don't remember exactly which adapters it was, I might be able to find
the details if I looked hard, a quick search through my mail archive
didn't find it - it might have come in via irc / bugzilla etc.

cheers
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