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Date: Tue, 25 Aug 2020 17:14:39 +0200 From: "Rafael J. Wysocki" <rafael@...nel.org> To: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com> Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Artem Bityutskiy <dedekind1@...il.com>, "Rafael J. Wysocki" <rjw@...ysocki.net>, Linux PM <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Doug Smythies <dsmythies@...us.net> Subject: Re: [PATCH v2 2/5] cpufreq: intel_pstate: Always return last EPP value from sysfs On Tue, Aug 25, 2020 at 5:06 PM Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com> wrote: > > On Tue, 2020-08-25 at 16:51 +0200, Rafael J. Wysocki wrote: > > On Tue, Aug 25, 2020 at 8:20 AM Artem Bityutskiy <dedekind1@...il.com > > > wrote: > > > On Mon, 2020-08-24 at 19:42 +0200, Rafael J. Wysocki wrote: > > > > From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com> > > > > > > > > Make the energy_performance_preference policy attribute in sysfs > > > > always return the last EPP value written to it instead of the one > > > > currently in the HWP Request MSR to avoid possible confusion when > > > > the performance scaling algorithm is used in the active mode with > > > > HWP enabled (in which case the EPP is forced to 0 regardless of > > > > what value it has been set to via sysfs). > > > > > > Why is this a good idea, I wonder. If there was a prior discussion, > > > please, point to it. > > > > > > The general approach to changing settings via sysfs is often like > > > this: > > > > > > 1. Write new value. > > > 2. Read it back and verify that it is the same. Because there is no > > > better way to verify that the kernel "accepted" the value. > > > > If the write is successful (ie. no errors returned and the value > > returned is equal to the number of written characters), the kernel > > *has* accepted the written value, but it may not have taken effect. > > These are two different things. > > > > The written value may take an effect immediately or it may take an > > effect later, depending on the current configuration etc. If you > > don't see the effect of it immediately, it doesn't matter that there > > was a failure of some sort. > > > > > Let's say I write 'balanced' to energy_performance_preference. I > > > read > > > it back, and it contains 'balanced', so I am happy, I trust the > > > kernel > > > changed EPP to "balanced". > > > > > > If the kernel, in fact, uses something else, I want to know about > > > it > > > and have my script fail. > > > > Why do you want it to fail then? > > > > > Why caching the value and making my script _think_ it succeeded is > > > a good idea. > > > > Because when you change the scaling algorithm or the driver's > > operation mode, the value you have written will take effect. > > > > In this particular case it is explained in the driver documentation > > that the performance scaling algorithm in the active mode overrides > > the sysfs value and that's the only case when it can be overridden. > > So whatever you write to this attribute will not take effect > > immediately anyway, but it may take an effect later. > > In some cases without even changing active/passive this is happening > when there was some error previously. For example: > > #cat energy_performance_preference > 127 > [root@...pl-perf-test-skx-i9 cpufreq]# rdmsr -p 1 0x774 > 8000ff00 > > I think we should show reality. In mode change can be a special case > and use the stored value to restore in new mode. OK, so I'll make it fail on attempts to change the EPP from 0 (performance) in the active mode with the performance "governor". Cheers!
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