Hello, On 15 August 2014 10:41, Peter Zijlstra <pet...@infradead.org> wrote: > On Fri, Aug 15, 2014 at 10:37:32AM -0400, Ashwin Chaugule wrote: >> On 15 August 2014 10:07, Peter Zijlstra <pet...@infradead.org> wrote: >> > On Fri, Aug 15, 2014 at 09:08:50AM -0400, Ashwin Chaugule wrote: >> >> If the OS only looks at Highest, Lowest, Delivered registers and only >> >> writes to Desired, then we're not really any different than how we do >> >> things today in the CPUFreq layer. >> > >> > The thing is; we're already struggling to make 'sense' of x86 as it >> > stands today. And it looks like this CPPC stuff makes the behaviour even >> > less certain. >> >> I think its still better than the "p-state" thing we have going today, >> where the algorithms are making their decisions based on the incorrect >> assumption that the CPU got what it requested for. (among other things >> listed earlier.) CPPC at least gives you a guarantee that the >> delivered performance will be within a range you requested. It can >> even force the platform to deliver a specific performance value if you >> choose over a specific time window. > > Maybe; the guarantee and interrupt on change might be useful indeed. But > which ever way we need aperf/mperf ratios somewhere.
Right. Regarding aperf/mperf; some ARMs will re-use existing performance counters, while others may have dedicated registers to count the equivalent. So I'm suggesting the use of the CPPC descriptors to unify the various implementations. In CPPC terms, aperf maps to the Delivered ctr and mperf maps to the Nominal ctr. Until we have rest of the plumbing in the scheduler to make use of the other CPPC specified regs, we can at least start off with having a common lower level driver as suggested in this patchset and reusing the PID controller as the "governor". Thats the only one that actually makes use of aperf/mperf today AFAIK. The driver can then be modified to include the Freq domain awareness as well, so that it works well on non-X86 platforms. Unless anyone strongly thinks that we should fix the shortcomings in the CPUfreq layer(broken pstate and freq domain assumptions) rather than enhancing PID? Thoughts? Cheers, Ashwin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
