Re: [dpdk-dev] [PATCH v2 5/5] eal/timer: honor architecture specific rdtsc hz function

[Thomas Monjalon <thomas@monjalon.net>]

11/10/2017 20:57, Jerin Jacob:
> From: Thomas Monjalon <thomas@monjalon.net>
> > 22/09/2017 10:25, Gowrishankar:
> > > From: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > > 
> > > When calibrating the tsc frequency, first, probe the architecture specific
> > > rdtsc hz function. if not available, use the existing calibrate scheme
> > > to calibrate the tsc frequency.
> > > 
> > > Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > 
> > I agree on the idea.
> 
> OK
> 
> > The namespace of cycles related function in DPDK is a real mess.
> 
> Absolutely!!
> 
> > I think we can choose better names in this series as a first step
> > to tidy this mess.
> > I will explain below.
> > 
> > At first, we should avoid TSC and RDTSC which are Intel-only wording.
> > The generic word could be "cycles" (the word used in arch headers),
> > or "ticks".
> > We should also name the timer sources or their function in a generic way.
> > Examples: CPU cycles? fast counter? precise counter?
> > 
> > Sometimes we use "hz", sometimes "freq".
> > It would better to keep one of them.
> > 
> > > --- a/lib/librte_eal/common/eal_common_timer.c
> > > +++ b/lib/librte_eal/common/eal_common_timer.c
> > > @@ -80,8 +80,11 @@
> > >  void
> > >  set_tsc_freq(void)
> > >  {
> > > -	uint64_t freq = get_tsc_freq();
> > > +	uint64_t freq;
> > >  
> > > +	freq = rte_rdtsc_arch_hz();
> > 
> > This new function is arch-specific and exported as a new API.
> 
> I thought of avoid exporting it. But then if the function is in
> lib/librte_eal/common/include/arch/../rte_cycles.h it is anyway exposed to
> application. i.e whatever files in lib/librte_eal/common/include/arch/../
> anyway exposed to application.

Ah yes, you are right!

> See last comment.
> 
> > 
> > > +	if (!freq)
> > > +		freq = get_tsc_freq();
> > 
> > The function get_tsc_freq is guessing the freq with OS-specific method.
> > 
> > >  	if (!freq)
> > >  		freq = estimate_tsc_freq();
> > 
> > The function estimate_tsc_freq is doing an estimation based on sleep().
> > 
> > At the end, the most accurate frequency is saved in eal_tsc_resolution_hz
> > and can be retrieved with rte_get_tsc_hz().
> > I don't understand why rte_rdtsc_arch_hz() is also exported to the apps.
> > 
> > TSC and HPET timer sources are wrapped in rte_get_timer_hz() in the
> > Similarly we can get the current timer with rte_get_timer_cycles().
> > In the case of TSC, it calls rte_get_tsc_cycles() which is an alias
> > of rte_rdtsc().
> > Some code is still using directly rte_rdtsc().
> > There is also rte_rdtsc_precise which adds a memory barrier.
> > 
> > The real question is what is the right abstraction for the application?
> > Do we want the fastest timer? the CPU timer? a precise timer?
> > 
> > I would like to see a real discussion on this topic, in order of building
> > a new timer API which would alias the old one for some time.
> 
> I guess, we may need to see to how abstract vmware TSC support also in
> proper way

Yes

> > If you don't want to bother with all these questions, I suggest to not
> > export the new function rte_rdtsc_arch_hz() and rename it to tsc_arch_hz.
> 
> If I understand it correctly, You would like to create a header file 
> in lib/librte_eal/common/include/arch/../ which should not be exported and change
> the name to tsc_arch_hz.

I had not think about the way to do this.
What about having internal headers in lib/librte_eal/common/arch/ ?