On Fri, Aug 18, 2017 at 08:17:32PM +0300, Andy Shevchenko wrote: > +PeterZ (since I mentioned his name) > Since we're talking C States, +Rafael and +Len > On Fri, Aug 18, 2017 at 5:58 PM, Rajneesh Bhardwaj > <rajneesh.bhardwaj@xxxxxxxxx> wrote: > > On Fri, Aug 18, 2017 at 03:57:34PM +0300, Andy Shevchenko wrote: > >> On Fri, Aug 18, 2017 at 3:37 PM, Rajneesh Bhardwaj > >> <rajneesh.bhardwaj@xxxxxxxxx> wrote: > >> > This patch introduces a new debugfs entry to read current Package C-state > >> > residency values and, one new kernel API to read the Package C-10 residency > >> > counter. > >> > > >> > Package C-state residency MSRs provide useful debug information about system > >> > idle states. In idle states system must enter deeper Package C-states. > > >> Why this patch is needed? > > > > Andy, I'll try to give some background for this. > > > > This is needed to enhance the S0ix failure debug capabilities from within > > the kernel. On ChromeOS we have S0ix failsafe kernel framework that is used > > to validate S0ix and report the blockers in case of a failure. > > https://patchwork.kernel.org/patch/9148999/ > > (It's not part of upstream) > > > So far only intel_pmc_slp_s0_counter_read is called by this framework to > > check whether the previous attempt to enter S0ix was success or not. > > I harder see even a single user of that API in current kernel. It > should be unexported and removed I think. > > > Having > > another PC10 counter related exported function enhances the S0ix debug since > > PC10 state is a prerequisite to enter S0ix. > > > >> See, we have turbostat and cpupower user space tools which do this > >> without any additional code to be written in kernel. What prevents > >> your user space application do the same? > >> > >> Moreover, we have events for cstate, I assume perf or something alike > >> can monitor those counters as well. I tend to agree. > > > > You're right, perhaps the debugfs is redundant when we have those user space > > tools but such tools are not available readily for all platforms/distros. > > Interfaces like /dev/cpu/*/msr that turbostat uses are not available on all > > the platforms. Can these be exposed in this way for *this* platform though? > > PMC driver is a debug driver so i thought its better to show Package C-state > > related info for low power debug here. > > > >> > >> Sorry, NAK. > > > > This patch has two parts i.e. exported PC10 API and the debugfs. Based on > > the above explanation, if the patch is not good as is, please let me know if > > i should drop the debugfs part and respin a v2 with just the exported API or > > drop this totally. > > > > Thanks for the feedback and thanks for taking time to review! > > Reading above makes me think that entire design of this is misguided. > Since the most of values are counters they better to be accessed in a > way how perf does. > > In case you need *in-kernel* facility, do some APIs (if it's not done > yet) for events drivers first. > cstate event driver is already in upstream. > > Sorry, NAK for entire patch until it would be blessed by people like Peter Z. > > -- > With Best Regards, > Andy Shevchenko > -- Darren Hart VMware Open Source Technology Center
