[gentoo-amd64] Re: Identifying CPUs in the kernel

[Duncan]

Peter Humphrey <prh@××××××××××.uk> posted
200706230916.07711.prh@××××××××××.uk, excerpted below, on  Sat, 23 Jun
2007 09:16:07 +0100:

> Here's what top showed then. Look at the /nice/ values on lines 3 and 4,
> and compare those with the %CPU and Processor fields of processes 5279
> and 5280 (sorry about the line wraps). This has me deeply puzzled:

Fixed the line wraps and removed a bit of extraneous information. =8^)
 
> top - 09:04:59 up 23 min, 5 users, load average: 3.60, 4.79, 3.91
> Tasks: 124 total, 2 running, 122 sleeping, 0 stopped, 0 zombie

> Cpu0: 0.3%us, 0.3%sy,  0.0%ni, 99.3%id, [zeroes]
> Cpu1: 0.0%us, 0.3%sy, 99.7%ni,  0.0%id, [zeroes]

> PID USER  PR NI S %CPU %MEM  TIME+   P COMMAND
> 5279 prh  34 19 S   50  1.0  6:53.97 1 setiathome-5.12
> 5280 prh  34 19 S   50  1.0  6:54.08 0 setiathome-5.12


> I don't think this is a scheduling problem; it goes deeper, so that the
> kernel doesn't have a consistent picture of which processor is which.

Critical question here, is that in SMP Irix or SMP Solaris mode?  (See
the top manpage if you don't know what I mean.)  Asked another way, is
that displaying percent of total CPU time (both CPUs) or percent of
total divided by number of CPUs (so percent of one CPU)?

If it's Irix mode (percent total CPU time), then it's reporting full
usage of both CPUs, one on each.  The CPU0 line would then be the one
screwed up, since it's reporting idle when it clearly has to be in use.

If it's Solaris mode (percent of a single CPU's time, so total of all
percentages should be 200% if you have two CPUs), then the CPUs
lines would seem to be correct, both processes would appear to be
running on CPU1, maxing it out, and the P column of the 5280 line
would have to be screwed up.  (That's assuming you let the figures
stabilize after the last schedtool call you made.)

In either case, I'm not sure where your bug is, but you are correct,
the problem appears to be way deeper than scheduling.  I'd guess it's
ultimately a kernel bug, possibly due to a hardware bug, possibly not,
but you might wish to file it on top initially, just to see if they've
seen similar and can tell you what's going on.  Unless you want to
double-check patching status yourself, you might as well file the bug
with Gentoo first, in case it's a Gentoo bug.  They'll probably end
up closing it "upstream", but at least then when you file it upstream,
you can say you've cleared it with Gentoo first. 

As for top, note that there's a trick you can use with it.  You'll
likely want to trim the memory columns etc as I did for your bug
report, but you may not want to mess up your regular config to do
so.  Not a problem! =8^)  Create a symlink to top called something
else (say topbug).  Then run it using the symlink, and you can change
and save your setttings, and it'll save them in a different rc file
(topbugrc using my example).  That way, you can run it with the bug
report settings when you want to, without messing up your regular
config.

Of course, don't forget to mention in your bug report whether you were
in Solaris or Irix SMP mode, because as I explained, it /does/ make a
difference.

Let me know how this goes, post the bug number when you file it or
whatever, as I'd like to follow it too.  You definitely have a
strange one here, and I'd /love/ to see what the real experts have
to say about it!  You are absolutely correct, it doesn't seem to
make any sense at all!

Good luck.  That's one /strange/ problem you have going there!
No /wonder/ you were expressing frustration earlier!

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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