[PATCH v4 2/2] timer: allow platform to override cpu TSC frequency

[Bruce Richardson]

On Wed, Oct 02, 2024 at 10:14:57PM +0300, Isaac Boukris wrote:
> On Wed, Oct 2, 2024 at 8:11 PM Bruce Richardson
> <bruce.richardson at intel.com> wrote:
> >
> > On Wed, Oct 02, 2024 at 07:56:36PM +0300, Isaac Boukris wrote:
> > > The CPU value is often not accurate, allow overriding it based
> > > on info from the host OS.
> > >
> > > On Linux X86, if the tsc_known_freq cpu flag is missing, it means
> > > the kernel doesn't trust it and calculates its own. We should do
> > > the same to avoid drift.
> > >
> > > On freebsd we have access to the kernel tsc_hz value, just use it.
> > >
> > > Signed-off-by: Isaac Boukris <iboukris at gmail.com>
> > > ---
> >
> > Looks good to me. Hope to test it out soon on my systems to see how it
> > goes.
> >
> > One comment inline below.
> >
> > /Bruce
> >
> > >  lib/eal/common/eal_common_timer.c |  3 +-
> > >  lib/eal/common/eal_private.h      |  2 +-
> > >  lib/eal/freebsd/eal_timer.c       |  8 +++--
> > >  lib/eal/linux/eal_timer.c         | 53 +++++++++++++++++++++++++++++--
> > >  lib/eal/windows/eal_timer.c       |  5 ++-
> > >  5 files changed, 62 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/lib/eal/common/eal_common_timer.c b/lib/eal/common/eal_common_timer.c
> > > index c5c4703f15..e00be0a5c8 100644
> > > --- a/lib/eal/common/eal_common_timer.c
> > > +++ b/lib/eal/common/eal_common_timer.c
> > > @@ -66,8 +66,7 @@ set_tsc_freq(void)
> > >       }
> > >
> > >       freq = get_tsc_freq_arch();
> > > -     if (!freq)
> > > -             freq = get_tsc_freq();
> > > +     freq = get_tsc_freq(freq);
> > >       if (!freq)
> > >               freq = estimate_tsc_freq();
> > >
> > > diff --git a/lib/eal/common/eal_private.h b/lib/eal/common/eal_private.h
> > > index af09620426..bb315dab04 100644
> > > --- a/lib/eal/common/eal_private.h
> > > +++ b/lib/eal/common/eal_private.h
> > > @@ -374,7 +374,7 @@ void set_tsc_freq(void);
> > >   *
> > >   * This function is private to the EAL.
> > >   */
> > > -uint64_t get_tsc_freq(void);
> > > +uint64_t get_tsc_freq(uint64_t arch_hz);
> > >
> > >  /**
> > >   * Get TSC frequency if the architecture supports.
> > > diff --git a/lib/eal/freebsd/eal_timer.c b/lib/eal/freebsd/eal_timer.c
> > > index 3dd70e24ba..19fc9a7228 100644
> > > --- a/lib/eal/freebsd/eal_timer.c
> > > +++ b/lib/eal/freebsd/eal_timer.c
> > > @@ -26,7 +26,7 @@
> > >  enum timer_source eal_timer_source = EAL_TIMER_TSC;
> > >
> > >  uint64_t
> > > -get_tsc_freq(void)
> > > +get_tsc_freq(uint64_t arch_hz)
> > >  {
> > >       size_t sz;
> > >       int tmp;
> > > @@ -50,9 +50,13 @@ get_tsc_freq(void)
> > >       sz = sizeof(tsc_hz);
> > >       if (sysctlbyname("machdep.tsc_freq", &tsc_hz, &sz, NULL, 0)) {
> > >               EAL_LOG(WARNING, "%s", strerror(errno));
> > > -             return 0;
> > > +             return arch_hz;
> > >       }
> > >
> > > +     if (arch_hz && arch_hz - tsc_hz > arch_hz / 100)
> >
> > Do we not need an "abs()" call on the "arch_hz - tsc_hz" in case tsc_hz is
> > the bigger value?
> 
> As they are unsigned it will overflow and result the absolute value, I
> actually did use abs() at first, and the compiler yelled at me:
> warning: taking the absolute value of unsigned type ‘uint64_t’ {aka
> ‘long unsigned int’} has no effect [-Wabsolute-value]

Yes, the compiler is right about them being unsigned. However, without
using signed arithmetic and using abs, tsc_hz being even 1Hz greater than
arch_hz will lead to the condition failing, so it is actually checking for
tsc_hz being within 1% only on the low side.

/Bruce