Re: [PATCH 1/6] Rewrite MSI-HOWTO

Grant Grundler
2009-03-02T20:34:14Z

On Fri, Feb 27, 2009 at 05:14:43AM -0700, Matthew Wilcox wrote:
> On Thu, Feb 26, 2009 at 11:15:25PM -0700, Grant Grundler wrote:
> > ...
> > > +3. Why use MSIs?
> > > +
> > > +There are three reasons why using MSIs can give an advantage over
> > > +traditional pin-based interrupts.
> > ...
> > > +PCI devices can only support a single pin-based interrupt per function.
> >
> > Related to this is a 4th reason: distribute workload across CPUs
> > and enables construction of efficient, multi-queue devices.
> > Care to mention that?
>
> That's true for MSI-X, but not for MSIs in general. Workload is already
> distributed across CPUs with round-robin interrupts. I'm inclined to
> leave out this level of detail.

I'm Ok with omitting it.

AFAICT "round-robin" was a behavior of older kernels.
All the x86 platforms I've looked at direct the MSI to exactly
one CPU.

>
> > > +The MSI-X capability is much more flexible than the MSI capability.
> > > +It supports up to 2048 interrupts, each of which can be separately
> > > +assigned.
> >
> > Nothing describes "assignment" below or what is meant by "assigned".
> > My guess is you wanted to differentiate MSIX from MSI with:
> > ... and each MSIX can be directed at a different CPU.
>
> I think 'each of which can be controlled separately' might work better.
> For example, they're individually maskable which isn't (necessarily)
> true of plain MSI.

Sounds good to me.

...
> > The description for MSI is correct. But Linux will only allocate one
> > MSI as noted in an earlier section. This section implies more could
> > be allocated when using MSI and that won't happen.
> >
> > IIRC, for AHCI perf you were working on a patch to change that and
> > it should probably update this text at the same time when the
> > behavior changes.
>
> Did you see this is patch 1/6? ;-)

yes....after I hit send and continued reviewing the rest of the patches. ;)

> I removed the description of
> pci_enable_msi_block() from this patch, but missed updating this
> paragraph. By patch 6/6, this paragraph is true.

Yup - agreed.

> > ...
> > > +5.3. Disabling MSIs on a single device
> > > +
> > > +Some devices are known to have faulty MSI implementations. Usually this
> > > +is handled in the individual device driver but occasionally it's necessary
> > > +to handle this with a quirk. Some drivers have an option to disable MSIs;
> > > +this is deprecated.
> >
> > "this" is ambiguous. My guess is "quirks to disable MSI for a device is
> > deprecated" since recently some drivers have added module parameters to
> > disable MSI.
>
> Having an option to disable MSI is deprecated. That doesn't mean that
> individual driver authors aren't selfish and short-sighted.

Ok. Here's a suggestion on how to say that:
Driver options to disable MSI are deprecated and will be removed in the future.

But anything you like better is fine with me.

> > Should mention "fgrep MSI /proc/interrupts" to see if any devices have
> > MSI in use?
>
> Yes, you're right.
>
> > > +Then, lspci -t gives the list of bridges above a device. Reading
> > > +/sys/bus/pci/devices/*/msi_bus will tell you whether MSI are enabled (1)
> > > +or disabled (0). If 0 is found in any of the msi_bus files belonging
> > > +to bridges between the PCI root and the device, MSIs are disabled.
> > > +
> > > +It is also worth checking whether the device driver supports MSIs.
> >
> > Suggestions on how to check?
>
> 'eg has calls to pci_enable_msi(), pci_enable_msix() or
> pci_enable_msi_block()'?

Yeah, that should work.
Anyone reading this doc has obviously found a source tree. ;)

> > Conversely, one can easily check if the driver has MSI disabled by default
> > and MSI can be enabled. e.g. use "modinfo mvsas" to check driver parameters.
>
> I'm not going to give examples of bad practise.

*nod* I agree it would encourage use and should not be included.

cheers,
grant

> > Reviewed-by: Grant Grundler <grundler@parisc-linunx.org>
>
> Thanks.
>
> --
> Matthew Wilcox Intel Open Source Technology Centre
> "Bill, look, we understand that you're interested in selling us this
> operating system, but compare it to ours. We can't possibly take such
> a retrograde step."