Re: [patch] x86: reduce srat verbosity in the kernel log

Ingo Molnar
2009-11-13T10:58:34Z

* David Rientjes <rientjes@google.com> wrote:

> On Fri, 13 Nov 2009, Ingo Molnar wrote:
>
> > There's two problems outlined in this discussion:
> >
> > A) too verbose bootup that is annoying with 64 CPUs and a show-stopper
> > with 4096 CPUs.
> >
> > B) the ad-hoc nature of our topology enumeration. Some of it is in
> > /sys, some of it is in printk logs. None really works well and
> > there's no structure in it.
> >
> > The simplest solution for (A) is what i suggested a few mails ago: dont
> > print the information by default, but allow (for trouble-shooting)
> > purposes for it to be printed when a boot option is passed.
> >
>
> Sigh, and even if that were done with a subsequent patch, you would
> still want to reduce the debugging output from 1272 lines to 40, just
> like my patch does without losing any information. It's insane to
> emit 1272 lines even if they are emitted only for a certain kernel
> parameter. I'm sure we agree on that.

For _debug_ output, we want it pretty simple. 1272 lines is nothing if
it's only done for debugging/trouble-shooting.

Furthermore, if this _only_ gets used in the debug case, we want it
simple for the pure reason that it's uncommon code. It might work now,
but it might regress later without anyone noticing it - up to the point
when someone might need it when it breaks in the worst possible moment.

We've been through this many times before and that's the core principle
of all debug printout code: keep it simple.

> > Problem (B), topology info enumeration of a successful bootup is a
> > different matter. It should be exposed to user-space via proper /sys
> > abstractions, not via ad-hoc printks. There's ongoing work in that
> > area, from Andreas Hermann, with patches posted. hpa expressed the
> > view there that topology structure should be expressed via a nice
> > vfs abstraction - i share that opinion.
>
> Ingo, what do you want?

My goal is to accept good patches and to reject patches that are bad or
not good enough.

> Your first criticism was that it should be limited only to a kernel
> parameter but now it seems like you're insisting that the printk's get
> removed completely and its exported via userspace. Then what is the
> kernel parameter that you suggested for?

Please read my mail. There's three usecases:

- default bootup. We want no messages in the log.

- troubleshooting bootup with a boot flag specified. (an existing
example is apic=verbose) We want simple messages in that case and
obvious logic. (verbosity is not an overriding issue - we are
troubleshooting)

- Successful bootup and we want to retrieve topology information for
the system. Using the boot logs for it is not the right channel -
/sys is.

Your patch is not a 'good enough' solution to either of these usecases,
because:

- In the default case it prints 40 lines more than it should.

- In the troubleshooting case it provides the information but it is
not a simple mechanism. (anything with state and a bitmap in it is
out pretty much - stick to simple printks, those are in the least
danger of regressing down the road. It's also less bloated in terms
of code. )

- For extracing topology information kernel log messages is not
something that tools can rely on very well.

Thanks,

Ingo