From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dirk Brandewie Subject: Re: [PATCH] cpufreq: intel_pstate: Fix rounding of core_pct Date: Fri, 13 Jun 2014 06:48:55 -0700 Message-ID: <539B0147.8020407@gmail.com> References: <1402490012-19969-1-git-send-email-stratosk@semaphore.gr> <00b301cf85ba$40fe4290$c2fac7b0$@net> <5399BACF.6060201@semaphore.gr> <1815178.vjJU51ubGD@vostro.rjw.lan> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Received: from mail-pb0-f45.google.com ([209.85.160.45]:33536 "EHLO mail-pb0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751007AbaFMNs6 (ORCPT ); Fri, 13 Jun 2014 09:48:58 -0400 In-Reply-To: <1815178.vjJU51ubGD@vostro.rjw.lan> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: "Rafael J. Wysocki" , Stratos Karafotis Cc: dirk.brandewie@gmail.com, Doug Smythies , viresh.kumar@linaro.org, dirk.j.brandewie@intel.com, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org On 06/12/2014 01:03 PM, Rafael J. Wysocki wrote: > On Thursday, June 12, 2014 05:35:59 PM Stratos Karafotis wrote: >> On 12/06/2014 12:15 =CF=80=CE=BC, Doug Smythies wrote: >>> >>> >>> -----Original Message----- >>> From: Stratos Karafotis [mailto:stratosk@semaphore.gr] >>> Sent: June-11-2014 13:20 >>> To: Doug Smythies >>> Cc: linux-pm@vger.kernel.org; linux-kernel@vger.kernel.org; rjw@rjw= ysocki.net; viresh.kumar@linaro.org; dirk.j.brandewie@intel.com >>> Subject: Re: [PATCH] cpufreq: intel_pstate: Fix rounding of core_pc= t >>> >>> On 2014.06.11 13:20 Stratos Karafotis wrote: >>>> On 11/06/2014 06:02 =CE=BC=CE=BC, Doug Smythies wrote: >>>>> >>>>> On 2104.06.11 07:08 Stratos Karafotis wrote: >>>>>> On 11/06/2014 04:41 =CE=BC=CE=BC, Doug Smythies wrote: >>>>>> >>>>>> No. >>>>> >>>>>> The intent was only ever to round properly the pseudo floating p= oint result of the divide. >>>>>> It was much more important (ugh, well 4 times more) when FRACBIT= S was still 6, which also got changed to 8 in a recent patch. >>>>>> >>>>> >>>>> Are you sure? >>>>> >>>>> Yes. >>>>> >>>>>> This rounding was very recently added. >>>>>> As far as I can understand, I don't see the meaning of this roun= ding, as is. >>>>>> Even if FRAC_BITS was 6, I think it would have almost no improve= ment in >>>>>> calculations. >>>>> >>>>> Note: I had not seen this e-mail when I wrote a few minutes ago: >>>>> >>>>> You may be correct. >>>>> If Dirk agrees, I will re-analyse the entire driver for rounding = effects soon. >>>>> When FRACBITS was 6 there were subtle cases where the driver woul= d get stuck, and not make a final pstate change, with the default PID g= ains. >>>>> Other things have changed, and the analysis needs to be re-done. >>>>> >>> >>>> Could you please elaborate a little bit more what we need these 2 = lines below? >>>> Sorry for being MIA on this thread I have been up to my eyeballs. >>>> if ((rem << 1) >=3D int_tofp(sample->mperf)) >>>> core_pct +=3D 1; The rounding should have been core_pct +=3D (1 << (FRAC_BITS-1)); Since core_pct is is in fixeded point notation at this point. Adding .5= to core_pct to round up. As Stratos pointed out the the current code only adds 1/256 to core_pct Since core_pct_busy stays in fixed point through out the rest of the calculations ans we only do the rounding when the PID is returning an int I think we can safely remove these two lines. >>>> >>>> Because nothing is mentioned for them in commit's changelog. >>>> Do we need to round core_pct or not? >>>> Because if we try to round it, I think this patch should work. >>> >>> As mentioned originally, they are there just to round the pseudo fl= oating number, not the integer portion only. >>> Let us bring back the very numbers you originally gave and work thr= ough it. >>> >>> aperf =3D 5024 >>> mperf =3D 10619 >>> >>> core_pct =3D 47.31142292% >>> or 47 and 79.724267 256ths >>> or to the closest kept fractional part 47 and 80 256ths >>> or 12112 as a pseudo float. >>> The maximum error with this rounding will be 1 part in 512 and symm= etric instead of the 1 part in 256 always in one direction without. >>> >>> Now if FRACBITS was still 6: >>> core_pct =3D 47.31142292% >>> or 47 and 19.931 64ths >>> or to the closest kept fractional part 47 and 20 64ths >>> or 3028 as a pseudo float. >>> The maximum error with this rounding will be 1 part in 128 and symm= etric instead of the 1 part in 64 (1.6% !!!) always in one direction wi= thout. >>> >>> Hope this helps. >>> >> >> Yes, it helps. Thanks a lot! >> >> But please note that the maximum error without this rounding will be= 1.6% _only_ >> in fractional part. In the real number it will be much smaller: >> >> 47.19 instead of 47.20 >> >> And using FRAC_BITS 8: >> >> 47.79 instead of 47.80 >> >> This is a 0.0002% difference. I can't see how this is can affect the= calculations >> even with FRAC_BITS 6. >> >> Another thing is that this algorithm generally is used to round to t= he >> nearest integer. I'm not sure if it's valid to apply it for the roun= ding of >> the fractional part of fixed point number. > > Depending on the original reason, it may or may not be. > > In theory, it may help reduce numerical drift resulting from rounding= always in > one direction only, but I'm not really sure if that matters here. > > Doug seems to have carried out full analysis, though. > > Rafael > > -- > To unsubscribe from this list: send the line "unsubscribe linux-pm" i= n > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >