Re: [PATCH] arch/x86/kernel/tsc.c : set X86_FEATURE_ART for TSC on CPUs like i7-4910MQ : bug #194609

[Jason Vas Dias]

Thank You for enlightening me -

I was just having a hard time believing that Intel would ship a chip
that features a monotonic, fixed frequency timestamp counter
without specifying in either documentation or on-chip or in ACPI what
precisely that hard-wired frequency is, but I now know that to
be the case for the unfortunate i7-4910MQ - I mean, how can the CPU
assert CPUID:80000007[8] ( InvariantTSC ) which it does, which is
difficult to reconcile with the statement in the SDM :
  17.16.4  Invariant Time-Keeping
    The invariant TSC is based on the invariant timekeeping hardware
    (called Always Running Timer or ART), that runs at the core crystal clock
    frequency. The ratio defined by CPUID leaf 15H expresses the frequency
    relationship between the ART hardware and TSC. If CPUID.15H:EBX[31:0] != 0
    and CPUID.80000007H:EDX[InvariantTSC] = 1, the following linearity
    relationship holds between TSC and the ART hardware:
    TSC_Value = (ART_Value * CPUID.15H:EBX[31:0] )
                         / CPUID.15H:EAX[31:0] + K
    Where 'K' is an offset that can be adjusted by a privileged agent*2.
     When ART hardware is reset, both invariant TSC and K are also reset.

So I'm just trying to figure out what CPUID.15H:EBX[31:0]  and
CPUID.15H:EAX[31:0]  are for my hardware.  I assumed (incorrectly)
that
the "Nominal TSC Frequency" formulae in the manul must apply to all
CPUs with InvariantTSC .

Do I understand correctly , that since I do have InvariantTSC ,  the
TSC_Value is in fact calculated according to the above formula, but with
a "hidden" ART Value,  & Core Crystal Clock frequency & its ratio to
TSC frequency ?
It was obvious this nominal TSC Frequency had nothing to do with the
actual TSC frequency used by Linux, which is 'tsc_khz' .
I guess wishful thinking led me to believe CPUID:15h was actually
supported somehow , because I thought InvariantTSC meant it had ART
hardware .

I do strongly suggest that Linux exports its calibrated TSC Khz
somewhere to user
space .

I think the best long-term solution would be to allow programs to
somehow read the TSC without invoking
clock_gettime(CLOCK_MONOTONIC_RAW,&ts), &
having to enter the kernel, which incurs an overhead of > 120ns on my system .


Couldn't linux export its 'tsc_khz' and / or 'clocksource->mult' and
'clocksource->shift' values to /sysfs somehow ?

For instance , only  if the 'current_clocksource' is 'tsc', then these
values could be exported as:
/sys/devices/system/clocksource/clocksource0/shift
/sys/devices/system/clocksource/clocksource0/mult
/sys/devices/system/clocksource/clocksource0/freq

So user-space programs could  know that the value returned by
    clock_gettime(CLOCK_MONOTONIC_RAW)
  would be
    {    .tv_sec =  ( ( rdtsc() * mult ) >> shift ) >> 32,
      , .tv_nsec = ( ( rdtsc() * mult ) >> shift ) >> &~0U
    }
  and that represents ticks of period (1.0 / ( freq * 1000 )) S.

That would save user-space programs from having to know 'tsc_khz' by
parsing the 'Refined TSC' frequency from log files or by examining the
running kernel with objdump to obtain this value & figure out 'mult' &
'shift' themselves.

And why not a
  /sys/devices/system/clocksource/clocksource0/value
file that actually prints this ( ( rdtsc() * mult ) >> shift )
expression as a long integer?
And perhaps a
  /sys/devices/pnp0/XX\:YY/rtc/rtc0/nanoseconds
file that actually prints out the number of real-time nano-seconds since the
contents of the existing
  /sys/devices/pnp0/XX\:YY/rtc/rtc0/{time,since_epoch}
files using the current TSC value?
To read the rtc0/{date,time} files is already faster than entering the
kernel to call
clock_gettime(CLOCK_REALTIME, &ts) & convert to integer for scripts.

I will work on developing a patch to this effect if no-one else is.

Also, am I right in assuming that the maximum granularity of the real-time clock
on my system is 1/64th of a second ? :
 $ cat /sys/devices/pnp0/00\:02/rtc/rtc0/max_user_freq
 64
This is the maximum granularity that can be stored in CMOS , not
returned by TSC? Couldn't we have something similar that gave an
accurate idea of TSC frequency and the precise formula applied to TSC
value to get clock_gettime
(CLOCK_MONOTONIC_RAW) value ?

Regards,
Jason


This code does produce good timestamps with a latency of @20ns
that correlate well with clock_gettIme(CLOCK_MONOTONIC_RAW,&ts)
values, but it depends on a global variable that  is initialized to
the 'tsc_khz' value
computed by running kernel parsed from objdump /proc/kcore output :

static inline __attribute__((always_inline))
U64_t
IA64_tsc_now()
{ if(!(    _ia64_invariant_tsc_enabled
      ||(( _cpu0id_fd == -1) && IA64_invariant_tsc_is_enabled(NULL,NULL))
      )
    )
  { fprintf(stderr, __FILE__":%d:(%s): must be called with invariant
TSC enabled.\n");
    return 0;
  }
  U32_t tsc_hi, tsc_lo;
  register UL_t tsc;
  asm volatile
  ( "rdtscp\n\t"
    "mov %%edx, %0\n\t"
    "mov %%eax, %1\n\t"
    "mov %%ecx, %2\n\t"
  : "=m" (tsc_hi) ,
    "=m" (tsc_lo) ,
    "=m" (_ia64_tsc_user_cpu) :
  : "%eax","%ecx","%edx"
  );
  tsc=(((UL_t)tsc_hi) << 32)|((UL_t)tsc_lo);
  return tsc;
}

__thread
U64_t _ia64_first_tsc = 0xffffffffffffffffUL;

static inline __attribute__((always_inline))
U64_t IA64_tsc_ticks_since_start()
{ if(_ia64_first_tsc == 0xffffffffffffffffUL)
  { _ia64_first_tsc = IA64_tsc_now();
    return 0;
  }
  return (IA64_tsc_now() - _ia64_first_tsc) ;
}

static inline __attribute__((always_inline))
void
ia64_tsc_calc_mult_shift
( register U32_t *mult,
  register U32_t *shift
)
{ /* paraphrases Linux clocksource.c's clocks_calc_mult_shift() function:
   * calculates second + nanosecond mult + shift in same way linux does.
   * we want to be compatible with what linux returns in struct
timespec ts after call to
   * clock_gettime(CLOCK_MONOTONIC_RAW, &ts).
   */
  const U32_t scale=1000U;
  register U32_t from= IA64_tsc_khz();
  register U32_t to  = NSEC_PER_SEC / scale;
  register U64_t sec = ( ~0UL / from ) / scale;
  sec = (sec > 600) ? 600 : ((sec > 0) ? sec : 1);
  register U64_t maxsec = sec * scale;
  UL_t tmp;
  U32_t sft, sftacc=32;
  /*
   * Calculate the shift factor which is limiting the conversion
   * range:
   */
  tmp = (maxsec * from) >> 32;
  while (tmp)
  { tmp >>=1;
    sftacc--;
  }
  /*
   * Find the conversion shift/mult pair which has the best
   * accuracy and fits the maxsec conversion range:
   */
  for (sft = 32; sft > 0; sft--)
  { tmp = ((UL_t) to) << sft;
    tmp += from / 2;
    tmp = tmp / from;
    if ((tmp >> sftacc) == 0)
      break;
  }
  *mult = tmp;
  *shift = sft;
}

__thread
U32_t _ia64_tsc_mult = ~0U, _ia64_tsc_shift=~0U;

static inline __attribute__((always_inline))
U64_t IA64_s_ns_since_start()
{ if( ( _ia64_tsc_mult == ~0U ) || ( _ia64_tsc_shift == ~0U ) )
    ia64_tsc_calc_mult_shift( &_ia64_tsc_mult, &_ia64_tsc_shift);
  register U64_t cycles = IA64_tsc_ticks_since_start();
  register U64_t ns = ((cycles *((UL_t)_ia64_tsc_mult))>>_ia64_tsc_shift);
  return( (((ns / NSEC_PER_SEC)&0xffffffffUL) << 32) | ((ns %
NSEC_PER_SEC)&0x3fffffffUL) );
  /* Yes, we are purposefully ignoring durations of more than 4.2
billion seconds here! */
}


I think Linux should export the 'tsc_khz', 'mult' and 'shift' values somehow,
then user-space libraries could have more confidence in using 'rdtsc'
or 'rdtscp'
if Linux's current_clocksource is 'tsc'.

Regards,
Jason



On 20/02/2017, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Sun, 19 Feb 2017, Jason Vas Dias wrote:
>
>> CPUID:15H is available in user-space, returning the integers : ( 7,
>> 832, 832 ) in EAX:EBX:ECX , yet boot_cpu_data.cpuid_level is 13 , so
>> in detect_art() in tsc.c,
>
> By some definition of available. You can feed CPUID random leaf numbers and
> it will return something, usually the value of the last valid CPUID leaf,
> which is 13 on your CPU. A similar CPU model has
>
> 0x0000000d 0x00: eax=0x00000007 ebx=0x00000340 ecx=0x00000340
> edx=0x00000000
>
> i.e. 7, 832, 832, 0
>
> Looks familiar, right?
>
> You can verify that with 'cpuid -1 -r' on your machine.
>
>> Linux does not think ART is enabled, and does not set the synthesized
>> CPUID +
>> ((3*32)+10) bit, so a program looking at /dev/cpu/0/cpuid would not
>> see this bit set .
>
> Rightfully so. This is a Haswell Core model.
>
>> if an e1000 NIC card had been installed, PTP would not be available.
>
> PTP is independent of the ART kernel feature . ART just provides enhanced
> PTP features. You are confusing things here.
>
> The ART feature as the kernel sees it is a hardware extension which feeds
> the ART clock to peripherals for timestamping and time correlation
> purposes. The ratio between ART and TSC is described by CPUID leaf 0x15 so
> the kernel can make use of that correlation, e.g. for enhanced PTP
> accuracy.
>
> It's correct, that the NONSTOP_TSC feature depends on the availability of
> ART, but that has nothing to do with the feature bit, which solely
> describes the ratio between TSC and the ART frequency which is exposed to
> peripherals. That frequency is not necessarily the real ART frequency.
>
>> Also, if the MSR TSC_ADJUST has not yet been written, as it seems to be
>> nowhere else in Linux,  the code will always think X86_FEATURE_ART is 0
>> because the CPU will always get a fault reading the MSR since it has
>> never been written.
>
> Huch? If an access to the TSC ADJUST MSR faults, then something is really
> wrong. And writing it unconditionally to 0 is not going to happen. 4.10 has
> new code which utilizes the TSC_ADJUST MSR.
>
>> It would be nice for user-space programs that want to use the TSC with
>> rdtsc / rdtscp instructions, such as the demo program attached to the
>> bug report,
>> could have confidence that Linux is actually generating the results of
>> clock_gettime(CLOCK_MONOTONIC_RAW, &timespec)
>> in a predictable way from the TSC by looking at the
>>  /dev/cpu/0/cpuid[bit(((3*32)+10)] value before enabling user-space
>> use of TSC values, so that they can correlate TSC values with linux
>> clock_gettime() values.
>
> What has ART to do with correct CLOCK_MONOTONIC_RAW values?
>
> Nothing at all, really.
>
> The kernel makes use of the proper information values already.
>
> The TSC frequency is determined from:
>
>     1) CPUID(0x16) if available
>     2) MSRs if available
>     3) By calibration against a known clock
>
> If the kernel uses TSC as clocksource then the CLOCK_MONOTONIC_* values are
> correct whether that machine has ART exposed to peripherals or not.
>
>> has tsc: 1 constant: 1
>> 832 / 7 = 118 : 832 - 9.888914286E+04hz : OK:1
>
> And that voodoo math tells us what? That you found a way to correlate
> CPUID(0xd) to the TSC frequency on that machine.
>
> Now I'm curious how you do that on this other machine which returns for
> cpuid(15): 1, 1, 1
>
> You can't because all of this is completely wrong.
>
> Thanks,
>
> 	tglx
>