[PATCH+DATAPOINT] LinuxPPC IRQ Latency

[Iain Sandoe]

Hi List(s),

This is cross-posted to linuxppc-dev - apologies to any of you who sit on
both.

Here is a patch to install an option to profile the LinuxPPC kernel wrt IRQ
latency.

All the clever stuff is courtesy of Jun Sun see:
http://www.mvista.com/realtime/latency/  and...
http://www.mvista.com/realtime/rtsched_wp.html

All the mistakes are, undoubtedly, *mine* ;-)

It installs code to measure the amount of time for which interrupts are
blocked.  An x86 version is also available via the links IIRC.

You will need "perfview.c" to start/stop & examine the profiling.  I've
attached a (slightly) modified version of this - small changes in formatting
of output only.

The patch is against linuxcare rsync 2.2.17pre7 (two days ago).

====

I have tested the patch with/without the option and with/without modules on
a G3/300 (beige).

The worst I've seen is 6.3ms IRQ blocking during the transition between
single and init 5 (bmac.c - eth0 start-up).  Probably not relevant to
general usage.  The rest of the time IRQ latency <= 600 us - well that's two
midi bytes but not that dramtic.

What I've tried is a simple configuration:
G3 minitower - no extraneous options, no USB, SCSI Xpmac rev 9 (IIRC).
no particular structure to stressing the system - kernel compiles, pings,
xmms etc.

I will test G3 Lombard, G4 (when I get to grips with yaboot - the machine
only arrived yesterday) and I'm about to put a USB PCI card in the G3.

The patch is conservative about machines - it only appears for CONFIG_PMAC
or CONFIG_PPC_ALL.

I would expect that it will work for other machine types - and welcome
feedback on results with different configs.

happy testing,
Iain.

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