Messages in this thread | | | Subject | Re: [PATCH] x86, smpboot: Disable frequency invariance when it's unsupported | From | Like Xu <> | Date | Thu, 16 Apr 2020 15:01:49 +0800 |
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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@linux.intel.com> >> --- >> 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 are 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@suse.cz/
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 >
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