lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Tue, 25 Aug 2020 08:06:09 -0700 From: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com> To: "Rafael J. Wysocki" <rafael@...nel.org>, Artem Bityutskiy <dedekind1@...il.com> Cc: "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, 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. Thanks, Srinivas > > In other words, in my usage scenarios at list, I prefer kernel > > telling > > the true EPP value, not some "cached, but not used" value. > > An alternative is to fail writes to energy_performance_preference if > the driver works in the active mode and the scaling algorithm for the > scaling CPU is performance and *then* to make reads from it return > the > value in the register. > > Accepting a write and returning a different value in a subsequent > read > is confusing. > > Thanks!
Powered by blists - more mailing lists