From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Lezcano Subject: Re: [PATCH] Revert "cpuidle: Replace ktime_get() with local_clock()" Date: Thu, 20 Apr 2017 15:07:13 +0200 Message-ID: <20170420130713.GD2523@mai> References: <20170420124447.13716-1-ville.syrjala@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: <20170420124447.13716-1-ville.syrjala@linux.intel.com> Sender: stable-owner@vger.kernel.org To: ville.syrjala@linux.intel.com Cc: linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Peter Zijlstra , "Rafael J . Wysocki" List-Id: linux-pm@vger.kernel.org On Thu, Apr 20, 2017 at 03:44:47PM +0300, ville.syrjala@linux.intel.com wrote: > From: Ville Syrjälä > > This reverts commit e93e59ce5b85e6c2b444f09fd1f707274ec066dc. > > The TSC stops in deeper C states, so using local_clock() in cpuidle > to track the C state residency seems like a bad idea. With local_clock() > powertop is reporting mostly 0% residency for C states here. Presumably > the core is still spending most of its time in some deep C-state since > the totals typically add up to only 5% or so, so perhaps the governor > isn't getting totally confused by these bogus numbers. But let's go > back to using ktime_get() as that at least works correctly across the > board. The local clock is faster, more accurate and more stable. We saw ktime_get() can be expensive, especially on slower CPUs. Why not add flag for the idle state to tell the local clocksource stops and use in this case ktime_get() ? This flag can be set on the idle state at init time in intel_idle.c around: ... if (((mwait_cstate + 1) > 2) && !boot_cpu_has(X86_FEATURE_NONSTOP_TSC)) mark_tsc_unstable("TSC halts in idle" " states deeper than C2"); ... and in processor_idlec.c around: ... tsc_check_state(cx->type); ... So we keep using local_clock() in most of the cases, for most of the boards. > Note that the code has changed somewhat since the regression happened, > so this isn't a 1:1 revert of the offending commit. > > Cc: stable@vger.kernel.org > Cc: Daniel Lezcano > Cc: Peter Zijlstra (Intel) > Cc: Rafael J. Wysocki > Signed-off-by: Ville Syrjälä > --- > drivers/cpuidle/cpuidle.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c > index 548b90be7685..24a52805527f 100644 > --- a/drivers/cpuidle/cpuidle.c > +++ b/drivers/cpuidle/cpuidle.c > @@ -213,13 +213,13 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv, > sched_idle_set_state(target_state); > > trace_cpu_idle_rcuidle(index, dev->cpu); > - time_start = ns_to_ktime(local_clock()); > + time_start = ktime_get(); > > stop_critical_timings(); > entered_state = target_state->enter(dev, drv, index); > start_critical_timings(); > > - time_end = ns_to_ktime(local_clock()); > + time_end = ktime_get(); > trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu); > > /* The cpu is no longer idle or about to enter idle. */ > -- > 2.10.2 > -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog