From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Rafael J. Wysocki" Subject: Re: [PATCH] cpufreq: x86: Make scaling_cur_freq behave more as expected Date: Fri, 28 Jul 2017 14:26:56 +0200 Message-ID: <1577372.F0DYpzg9Ak@aspire.rjw.lan> References: <1498281114-3868-1-git-send-email-lenb@kernel.org> <006201d30595$e0a23870$a1e6a950$@net> <001901d30766$fc66a3c0$f533eb40$@net> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Return-path: Received: from cloudserver094114.home.net.pl ([79.96.170.134]:51216 "EHLO cloudserver094114.home.net.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751690AbdG1Me7 (ORCPT ); Fri, 28 Jul 2017 08:34:59 -0400 In-Reply-To: <001901d30766$fc66a3c0$f533eb40$@net> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Doug Smythies Cc: 'Len Brown' , rafael@kernel.org, tglx@linutronix.de, srinivas.pandruvada@linux.intel.com, peterz@infradead.org, linux-kernel@vger.kernel.org, 'Len Brown' , x86@kernel.org, linux-pm@vger.kernel.org On Thursday, July 27, 2017 11:01:39 PM Doug Smythies wrote: > On 2017.07.27 17:13 Rafael J. Wysocki wrote: > > > From: Rafael J. Wysocki > > > > After commit f8475cef9008 "x86: use common aperfmperf_khz_on_cpu() to > > calculate KHz using APERF/MPERF" the scaling_cur_freq policy attribute > > in sysfs only behaves as expected on x86 with APERF/MPERF registers > > available when it is read from at least twice in a row. > > > > The value returned by the first read may not be meaningful, because > > the computations in there use cached values from the previous > > aperfmperf_snapshot_khz() call which may be stale. However, the > > interface is expected to return meaningful values on every read, > > including the first one. > > > > To address this problem modify arch_freq_get_on_cpu() to call > > aperfmperf_snapshot_khz() twice, with a short delay between > > these calls, if the previous invocation of aperfmperf_snapshot_khz() > > was too far back in the past (specifically, more that 1s ago) and > > adjust aperfmperf_snapshot_khz() for that. > > > > Fixes: f8475cef9008 "x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF" > > Reported-by: Doug Smythies > > Signed-off-by: Rafael J. Wysocki > > --- > > arch/x86/kernel/cpu/aperfmperf.c | 36 +++++++++++++++++++++++++++++------- > > 1 file changed, 29 insertions(+), 7 deletions(-) > > > > Index: linux-pm/arch/x86/kernel/cpu/aperfmperf.c > > ...[deleted the rest]... > > This proposed patch would be good. However, I can only try it maybe by Sunday. > I think the maximum time span means that this code: > > /* > * if (cpu_khz * aperf_delta) fits into ULLONG_MAX, then > * khz = (cpu_khz * aperf_delta) / mperf_delta > */ > if (div64_u64(ULLONG_MAX, cpu_khz) > aperf_delta) > s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta); > else /* khz = aperf_delta / (mperf_delta / cpu_khz) */ > s->khz = div64_u64(aperf_delta, > div64_u64(mperf_delta, cpu_khz)); > > Could be reduced to this: > > s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta); > > Because it could never overflow anymore. Right, that's a good point. I'll send a v2 with this change included shortly. Thanks, Rafael