On Wed, Jun 12, 2019 at 11:44:35AM +0200, Jason A. Donenfeld wrote: > 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,
Right, those are used to keep the clock values coherent (as best as possible) when we switch modes. When the TSC is stable sched_clock_cpu() is mapped directly to sched_clock() for performance reasons. The moment the TSC is detected to be unsuitable, we switch to the unstable mode, where we take a GTOD timestamp every tick and add resolution with the CPU local TSC (plus filters etc..). To make this mode-switch as smooth as possible, we track those offsets. > and the comment at the top mentions explicit sleep hooks. I wasn't > sure which function to use from here, though. Either local_clock() or cpu_clock(cpu). The sleep hooks are not something the consumer has to worry about. > 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. Yes. > But there's still the > issue of the 32-bit wraparound on the base implementation. If an architecture doesn't provide a sched_clock(), you're on a seriously handicapped arch. It wraps in ~500 days, and aside from changing jiffies_lock to a latch, I don't think we can do much about it. (the scheduler too expects sched_clock() to not wrap short of the u64 and so having those machines online for 500 days will get you 'funny' results) AFAICT only: alpha, h8300, hexagon, m68knommu, nds32, nios2, openrisc are lacking any form of sched_clock(), the rest has it either natively or through sched_clock_register(). > 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? You probably want to use local_clock().
