Re: [PATCH] x86, smpboot: Disable frequency invariance when it's unsupported

[Like Xu]

On 2020/4/16 14:08, Giovanni Gherdovich wrote:
> On Thu, 2020-04-16 at 10:12 +0800, Like Xu wrote:
> > On the Intel SNR processors such as "Intel Atom(R) C6562", the
> > turbo_freq for 4C turbo may be zero which causes a divide by zero
> > exception and blocks the boot process when arch_scale_freq_tick().
> >
> > When one of the preset base_freq or turbo_freq is meaningless,
> > we may disable frequency invariance.
> >
> > Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
> > Signed-off-by: Like Xu <like.xu@xxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kernel/smpboot.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index fe3ab9632f3b..741367ce4d14 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -1958,6 +1958,9 @@ static bool core_set_max_freq_ratio(u64 *base_freq, u64 *turbo_freq)
> >   	*base_freq = (*base_freq >> 8) & 0xFF;      /* max P state */
> >   	*turbo_freq = (*turbo_freq >> 24) & 0xFF;   /* 4C turbo    */
> >   
> > +	if (*turbo_freq == 0 || *base_freq == 0)
> > +		return false;
> > +
> >   	return true;
> >   }
>
> Hello Like Xu,
>
> thanks for reporting this and for the patch. My preferred solution for when
> the 4 cores turbo freq is detected as zero would be to look for the 1 core turbo
> frequency, as we're likely on a machine with less than 4 cores. Is that the
> case on your Atom C6562? I couldn't find it on ark.intel.com.

The Atom C6562 is "24 cores" based on 
https://www.intel.com/content/dam/www/public/us/en/documents/product-briefs/atom-p5900-product-brief.pdf

#define MSR_PLATFORM_INFO		0x000000ce

the value for this msr is 80820f9801600

#define MSR_TURBO_RATIO_LIMIT		0x000001ad

the value for this msr is 16

I know you didn't test your feature on this platform,
but combinations of other various values ââare also possible
(unless it's made clear in the specification).

> As per why I'd like to go with 1 core turbo instead of bailing out of freq
> invariance entirely, I've left a comment in the openSUSE bugzilla with some
> details: https://bugzilla.opensuse.org/show_bug.cgi?id=1166664#c35

> The relevant part is:
>
> :: The fix in this case is to take the 1 core turbo as normalizing factor. The
> :: other choice would be to use the base frequency (no turbo at all), but using
> :: base freq for normalization means that the ratio becomes current_freq / base_freq
> :: which is an over-estimation, which leads the frequency governor to think the
> :: CPU is more loaded than it really is and raise the frequency a bit too
> :: aggressively. This is tolerable in performance-oriented servers but
> :: inappropriate on small machines with 2 cores."
>
> Regarding base_freq being reported as zero, you're right, that can happen too
> and we've seen it in hypervisors.
>
> I've just sent fixes for these two problems here:
> https://lore.kernel.org/lkml/20200416054745.740-1-ggherdovich@xxxxxxx/

Hence the "less than 4 cores" comment is weird for C6562
but the use of "1C turbo" looks good to me.

Thanks,
Like Xu

> Thanks,
> Giovanni Gherdovich