Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit_lock if possible

Ingo Molnar
2011-03-25T19:22:32Z

* Andi Kleen <andi@firstfloor.org> wrote:

> > Also seriously complicated by the kexec case where the previous kernel
> > didn't clean up PMU state. There is simply no sane way to detect if its
>
> That's a good point, but we can easily stop the PMU before kexec.

Wrong - there's lots of existing Linux versions out there that will kexec with
PMU state active. We cannot change them retroactively.

> > actually used and by whoem.
>
> You check if the counter is enabled. If it's already enabled it's used by
> someone else.

Wrong - or it can be enabled if it was left enabled accidentally. We treat PMU
state like we treat other CPU state.

> > The whole PMU 'sharing' concept championed by Andi is utter crap.
>
> Why? It's the same thing as having some less counters.

Wrong again - 25% or 50% of the events stolen with no user choice is a pretty
big deal.

Consider the example in this thread: cachemiss profiling done via perf, which
needs two events. If one of the generic counters is 'stolen' by a BIOS and
Linux accepts this silently then it means the loss of real functionality.

In other words, '25% of a specific hardware functionality stolen by the BIOS'
(or more) is absolutely not acceptable.

> [...] You need to already support that for architectural perfmon with
> variable counters anyways or for sharing with oprofile.

Wrong, that's different - multiplexing the PMU is obviously done within the OS.
It's evidently user configurable - users can use oprofile or perf - and the
user can still fully utilise the PMU to the extent he wishes to - it's his
hardware.

It is not possible for the kernel to stop the BIOS from using the PMU though,
so it's not 'sharing' no matter what 'protocol' is used, it's 'stealing'.

> > As for simply using it despite the BIOS corrupting it, that might not
> > always work, the BIOS might simply over-write your state because it
> > one-sidedly declares to own the MSRs (observed behaviour).
>
> Yes, that doesn't work. If someone else is active you have to step back.
>
> > Its all a big clusterfuck and really the best way (IMO) is what we have
> > now to put pressure on and force the BIOS vendors to play nice.
>
> It just results in users like Eric being screwed. I doubt that any
> BIOS writer ever heard about it. Congratulations for a great plan.

I'd encourage you to think through this topic instead of making derisive
comments about others ...

Thanks,

Ingo