powertop "pread cpu0 0xe8 : Bad file descriptor" fixed by CONFIG_MSR

powertop "pread cpu0 0xe8 : Bad file descriptor" fixed by CONFIG_MSR

[Andy Isaacson]

I hadn't seen this issue discussed on the list, and I don't know of a
powertop mailing list, so here's what stymied my use of powertop until I
straced it.

With CONFIG_MSR=n, powertop "1.97-2" (from Debian unstable) on 3.0.1
(and presumably many other recent kernels) exits at startup with:

pread cpu0 0xe8 : Bad file descriptor

strace shows:

open("/dev/cpu/0/msr", O_RDONLY) = -1 ENOENT (No such file or directory)
pread(4294967295, 0x7fff4c8347c8, 8, 232) = -1 EBADF (Bad file descriptor)

Setting CONFIG_MSR=m and loading msr.ko fixes the issue, allowing
powertop to work again (tested against 3.0.1).  I'm pretty sure this is
a regression from long ago -- I have had CONFIG_X86_MSR=n in almost all
configs since 2.6.34-07097-gf4b87de, and powertop worked in most of
those kernels.  But I don't have references for what versions of
powertop and what kernels I was running at various points.

-andy

Re: powertop "pread cpu0 0xe8 : Bad file descriptor" fixed by CONFIG_MSR

[Américo Wang]

On Sun, Aug 7, 2011 at 6:52 AM, Andy Isaacson <adi@hexapodia.org> wrote:
> I hadn't seen this issue discussed on the list, and I don't know of a
> powertop mailing list, so here's what stymied my use of powertop until I
> straced it.
>
> With CONFIG_MSR=n, powertop "1.97-2" (from Debian unstable) on 3.0.1
> (and presumably many other recent kernels) exits at startup with:
>
> pread cpu0 0xe8 : Bad file descriptor
>
> strace shows:
>
> open("/dev/cpu/0/msr", O_RDONLY) = -1 ENOENT (No such file or directory)
> pread(4294967295, 0x7fff4c8347c8, 8, 232) = -1 EBADF (Bad file descriptor)
>

Seems powertop forgot to check the return value of that open().
This is a bug of powertop, not kernel.