Re: [PATCH v8 6/6] x86/tsc: use tsc early

[Dou Liyang]

Hi Pavel,

At 11/15/2017 05:46 AM, Pavel Tatashin wrote:
> Hi Dou,
>
> Great comments, my replies below:
>
> > >  static inline unsigned long long paravirt_sched_clock(void)
> > >  {
> > > -       return PVOP_CALL0(unsigned long long, pv_time_ops.sched_clock);
> > > +       return PVOP_CALL0(unsigned long long,
> > > pv_time_ops.active_sched_clock);
> > >  }
> >
> > Should in the 5th patch
>
> Actually, it has to be in patch 6, because otherwise patch 5 without
> patch 6 would cause native_sched_clock() to be used even when a
> platform specific clock is set, thus may cause performance regressions
> where it is not optimal to use tsc for clock.
Indeed.

>> +	tsc_early_disable();
>> +	__sched_clock_offset = sched_clock_early() - sched_clock();
>> +	pr_info("sched clock early is finished, offset[%lld.%09lds]\n", t, r);
>> +}

s/sched clock early/early sched clock/

>> +#endif /* CONFIG_X86_TSC */
> > Add definitions for the situation of X86_TSC = no :
> >
> > #else /* CONFIG_X86_TSC */
> > static inline void tsc_early_init(unsigned int khz) { }
> > static inline void tsc_early_fini(void) { }
>
> Excellent point, I totally forgot about  X86_TSC = no, however, a
> better fix is to simply remove #ifdef CONFIG_X86_TSC from my
> functions. Apparently, even with X86_TSC=no we can use TSC unless

Agree with removing #ifdef CONFIG_X86_TSC

> notsc kernel parameter is passed. This will be in the next patchset.
>
> > According to tsc_early_delay_calibrate(), if (!boot_cpu_has(X86_FEATURE_TSC
> > || !tsc_khz ), tsc_early_init(tsc_khz)
> > will never be called, so here is  redundant.
> >
> > >                 return;
> > >         }
> > >
> > > @@ -1302,6 +1385,7 @@ void __init tsc_init(void)
> > >         if (!tsc_khz) {
> > >                 mark_tsc_unstable("could not calculate TSC khz");
> > >                 setup_clear_cpu_cap(X86_FEATURE_TSC_DEADLINE_TIMER);
> > > +               tsc_early_fini();
> >
> >
> > ditto
>
> Right, in both case we still want to call tsc_early_fini(). Because,
> it calls tsc_early_disable() even when tsc_early_init() was never
> called.  tsc_early_disable()  either sets static branch
> __tsc_early_static to false or changes active_sched_clock to be
> platform specific, depending on CONFIG_PARAVIRT.

Yes, the function names confused me, the actually purpose of
tsc_early_fini() is switching schedule clock to the final one. right?

How about replacing *return;* with *goto final_sched_clock;*

Thanks,
	dou.
> > BTW, seems you forgot to cc Peter Zijlstra <peterz@xxxxxxxxxxxxx> in both V7
> > and V8 patchsets.
>
> Thank you for noticing this! I will include Peter when I send out
> patchset version 9.
>
> Thank you,
> Pavel