Re: [dpdk-dev] [EXT] Re: [PATCH v2 2/2] eal: roundup tsc frequency when estimating it

["Wiles, Keith" <keith.wiles@intel.com>]

> On Mar 16, 2019, at 12:56 PM, Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com> wrote:
> 
> On Sat, 2019-03-16 at 17:18 +0000, Wiles, Keith wrote:
>>> On Mar 16, 2019, at 10:06 AM, Pavan Nikhilesh Bhagavatula <
>>> pbhagavatula@marvell.com> wrote:
>>> 
>>> On Sat, 2019-03-16 at 14:42 +0000, Wiles, Keith wrote:
>>>>> On Mar 16, 2019, at 2:03 AM, Pavan Nikhilesh Bhagavatula <
>>>>> pbhagavatula@marvell.com> wrote:
>>>>> 
>>>>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>>>> 
>>>>> When estimating tsc frequency using sleep/gettime round it up
>>>>> to
>>>>> the
>>>>> nearest multiple of 10Mhz for more accuracy.
>> 
>> How does rounding up the TSC value become more accurate, If the value
>> is 1 cycles more then it should be then your macro would round down
>> and if it is 1 cycle greater than 1E7 it would round up.
> 
> Example in case of RTE_ARM_EAL_RDTSC_USE_PMU enabled 
> 
> Before roundup : 1400000979
> After roundup : 1400000000
> EAL: TSC frequency is ~1400000000 Hz
> 
> 
> Before roundup : 1399999060
> After roundup : 1400000000
> EAL: TSC frequency is ~1400000000 Hz
> 
>>>>> Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>>>> ---
>>>>> Useful in case of ARM64 if we enable RTE_ARM_EAL_RDTSC_USE_PMU,
>>>>> get_tsc_freq_arch() will return 0 as there is no instruction to
>>>>> determine
>>>>> the clk of PMU and eal falls back to sleep(1).
>>>>> 
>>>>> lib/librte_eal/common/eal_common_timer.c | 4 ++--
>>>>> lib/librte_eal/linuxapp/eal/eal_timer.c  | 2 +-
>>>>> 2 files changed, 3 insertions(+), 3 deletions(-)
>> 
>> It appears you did not use the head of the master as linuxapp is now
>> just linux and freebsdapp is freebsd. You need to rebase to the head
>> of master and send a new version.
>>>>> diff --git a/lib/librte_eal/common/eal_common_timer.c
>>>>> b/lib/librte_eal/common/eal_common_timer.c
>>>>> index dcf26bfea..1358bbed0 100644
>>>>> --- a/lib/librte_eal/common/eal_common_timer.c
>>>>> +++ b/lib/librte_eal/common/eal_common_timer.c
>>>>> @@ -69,7 +69,7 @@ estimate_tsc_freq(void)
>>>>> 	/* assume that the sleep(1) will sleep for 1 second */
>>>>> 	uint64_t start = rte_rdtsc();
>>>>> 	sleep(1);
>>>>> -	return rte_rdtsc() - start;
>>>>> +	return RTE_ALIGN_MUL_NEAR(rte_rdtsc() - start, 1E7);
>> 
>> The 1E7 is a magic number convert this to a meaningful define.
> 
> 1E7 ~ 10Mhz will convert to a macro.
> 
>>>>> }
>>>>> 
>>>>> void
>>>>> @@ -83,7 +83,7 @@ set_tsc_freq(void)
>>>>> 	if (!freq)
>>>>> 		freq = estimate_tsc_freq();
>>>>> 
>>>>> -	RTE_LOG(DEBUG, EAL, "TSC frequency is ~%" PRIu64 "
>>>>> KHz\n", freq
>>>>> / 1000);
>>>>> +	RTE_LOG(INFO, EAL, "TSC frequency is ~%" PRIu64 "
>>>>> Hz\n", freq);
>>>>> 	eal_tsc_resolution_hz = freq;
>>> 
>>> I missed this log will remove it in the next version.
>>> 
>>>>> }
>>>>> 
>>>>> diff --git a/lib/librte_eal/linuxapp/eal/eal_timer.c
>>>>> b/lib/librte_eal/linuxapp/eal/eal_timer.c
>>>>> index bc8f05199..864d6ef29 100644
>>>>> --- a/lib/librte_eal/linuxapp/eal/eal_timer.c
>>>>> +++ b/lib/librte_eal/linuxapp/eal/eal_timer.c
>>>>> @@ -248,7 +248,7 @@ get_tsc_freq(void)
>>>>> 
>>>>> 		double secs = (double)ns/NS_PER_SEC;
>>>>> 		tsc_hz = (uint64_t)((end - start)/secs);
>>>>> -		return tsc_hz;
>>>>> +		return RTE_ALIGN_MUL_NEAR(tsc_hz, 1E7);
>>>> 
>>>> Maybe I missed an email about this, but why would I want the TSC
>>>> hz
>>>> rounded here? I do not mind the macro just the fact that we are
>>>> changing TSC hz value. If the TSC value is wrong then we need to
>>>> fix
>>>> the value, but I do not see it being wrong here.
>>> 
>>> Since in this function nanosleep might not be cycle accurate we
>>> need to
>>> round it up.
>>> 
>>> Please note that estimation only applies when  get_tsc_freq_arch()
>>> fails. i.e there is no CPU instruction that specifies the cyc/sec.
>>> 
>>> As I mentioned in the patch notes
>>> "Useful in case of ARM64 if we enable RTE_ARM_EAL_RDTSC_USE_PMU,
>>> get_tsc_freq_arch() will return 0 as there is no instruction to
>>> determine the clock of PMU and eal falls back to
>>> sleep(1)/nanosleep.” 
>> 
>> OK, I looked at the changes and the code for setting the TSC again. I
>> would have not handled the setting of TSC in the way it was done, but
>> that is not your problem. I agree the changes do look ok, the only
>> problem I have is the new macro will roundup or down depending on the
>> value. In your statement you are wanting to roundup the values.
>> 
>> If the sleep/nanosleep is less than a second for some reason, then
>> your macro would round it down is that what we wanted? I guess my
>> point is you are assuming the TSC calculation will always be less
>> than a second (with sleep) and the macro would round up depending on
>> the value calculated using the sleep/nanosleep.
>> 
>> I was playing with these MUL macros and I am not sure they do what we
>> expect in the case of the multiple value is much closer to the value
>> passed.
>> 
>> If we have a v = 10001 and multiple to 1000 we have the following:
>> 
>> RTE_ALIGN_MUL_CEIL(10001, 1000)
>> 	(10001 + (1000 - 1)) / (1000 * 1000)
> ((10001 + (1000 - 1)) / 1000) * 1000
>> 	(10001 + 999)        / 1000000
>> 	20000                / 1000000
>> Result: 0
> 
> ((10001 + (1000 - 1) / 1000) * 1000
> ((10001 + 999) / 1000) * 1000
> (11000/1000) * 1000
> 11 * 1000 
> 
> Result : 11000
> 
>> 
>> RTE_ALIGN_MUL_FLOOR(10001, 1000)
>> 	(10001 / (1000 * 1000))
> (10001 / 1000) * 1000
>> 	(10001 / 1000000)
>> Result: 0
> 10.001 * 1000
> 
> Result : 1000

Ooops, too many parans and missed it.

Then we can get a new version and that should be OK.

I will add my $0.02 then:

Reviewed-by: Keith Wiles<keith.wiles>

> 
>> 
>> Unless I am wrong here the value v must be over a 1,000,000 to make
>> these macros work or the value v to be greater than (mul * mul) in
>> all cases or zero is the result. It may work for the TSC values as we
>> are using a small mul value compared to the TSC value. If DPDK was
>> ported to a slower machine it could be also zero.
> 
> Unless we have machines that run at freq < 10Mhz this scheme will
> always work.
> If we have such machines lets hope that they have a CPU instruction
> that tells us the cyc/sec.
> 
>> 
>> I think we need to fix the macros and rethink how TSC is set here.
>> 
>>>>> 	}
>>>>> #endif
>>>>> 	return 0;
>>>>> --
>>>>> 2.21.0
>>>>> 
>>>> 
>>>> Regards,
>>>> Keith
>> 
>> Regards,
>> Keith
>> 
> 
> Regards,
> Pavan.

Regards,
Keith