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

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

-----Original Message-----
> Date: Wed, 11 Oct 2017 19:36:11 +0200
> From: Thomas Monjalon <thomas@monjalon.net>
> To: Gowrishankar <gowrishankar.m@linux.vnet.ibm.com>, Jerin Jacob
>  <jerin.jacob@caviumnetworks.com>
> Cc: dev@dpdk.org, Chao Zhu <chaozhu@linux.vnet.ibm.com>, Bruce Richardson
>  <bruce.richardson@intel.com>, Konstantin Ananyev
>  <konstantin.ananyev@intel.com>, viktorin@rehivetech.com,
>  jianbo.liu@linaro.org
> Subject: Re: [dpdk-dev] [PATCH v2 5/5] eal/timer: honor architecture
>  specific rdtsc hz function
> 
> 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. 

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

> 
> 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.