On 2015.06.10 16:46 Rafael J. Wysocki wrote: > On Wednesday, June 10, 2015 09:18:45 AM Prarit Bhargava wrote: >> I looked into switching to div64_s64() instead of the 32-bit version in >> div_fp(), however, this would result in sample_ratio and core_busy returning >> 0 which is something we don't want.
??? Due to a great many overflow related issues, div_fp() was changed to div64_s64() a long time ago. I have not found the actual commit to reference, but it was about a year ago. And the math in general was all changed to 64 bit, over a few commits. > > P. > > ---8<--- > > The kernel may delay interrupts for a long time which can result in timers > being delayed. If this occurs the intel_pstate driver will crash with > a divide by zero error: More recent versions will not crash. Long timer delays are extremely common, and this is a fundamental flaw in the duration method. Patch sets have been submitting dealing with this, and other, issues. >> >> which results in the time between samples = last_sample_time - sample.time >> = 4063149215234118 - 4063132438017305 = 16777216813 which is 16.777 seconds. I have never seen anything over 4 seconds before, and I study this stuff (with respect to the intel_pstate driver operation) a lot. Due to help from others, I have data from a variety of processors. 4 seconds not unusual, even under load. >> >> The duration between reads of the APERF and MPERF registers overflowed a s32 >> sized integer in intel_pstate_get_scaled_busy()'s call to div_fp(). The >> result >> is that int_tofp(duration_us) == 0, and the kernel attempts to divide by 0. >> >> While the kernel shouldn't be delaying for a long time, it can and does >> happen, and the intel_pstate driver should not panic in this situation. This >> patch checks for an overflow and ignores the current calculation cycle by >> returning -EINVAL. Since intel_pstate_sample() has been called, subsequent >> timer function calls will then again pick up the correct calculations and the >> system will continue functioning properly. That would run the risk that the correct calculation would never be done. It is fairly easy (I do it all the time) to create a scenario where there is high load on a CPU, but also a very very high duration value, for each and every duration. (and O.K., in that scenario the calculation is always wrong anyhow, due to the long duration check engaging.) -- 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/
