Hey Peter, On Wed, Jun 12, 2019 at 11:03 AM Peter Zijlstra <pet...@infradead.org> wrote: > How quasi? Do the comments in kernel/sched/clock.c look like something > you could use? > > As already mentioned in the other tasks, anything ktime will be > horrifically crap when it ends up using the HPET, the code in > kernel/sched/clock.c is a best effort to keep using TSC even when it is > deemed unusable for timekeeping.
Thanks for pointing that out. Indeed the HPET path is a bummer and I'd like to just escape using ktime all together. In fact, my accuracy requirements are very lax. I could probably even deal with an inaccuracy as huge as ~200 milliseconds. But what I do need is 64-bit, so that it doesn't wrap, allowing me to compare two stamps taken a long time apart, and for it to take into account sleep time, like CLOCK_BOOTTIME does, which means get_jiffies_64() doesn't fit the bill. I was under the impression that I could only get this with ktime_get_boot & co, because those add the sleep offset. It looks like, though, kernel/sched/clock.c keeps track of some offsets too -- __sched_clock_offset and __gtod_offset, and the comment at the top mentions explicit sleep hooks. I wasn't sure which function to use from here, though. sched_clock() seems based on jiffies, which has the 32-bit wraparound issue, and the base implementation doesn't seem to take into account sleeptime. The x86 implementation seems use rdtsc and then adds cyc2ns_offset which looks to be based on cyc2ns_suspend, which I assume is what I want. But there's still the issue of the 32-bit wraparound on the base implementation. I guess you know this code better than my quick perusal. Is there some clock in here that doesn't have a wrap around issue and takes into account sleeptime, without being super slow like ktime/hpet? Jason
