Messages in this thread | | | From | Arnd Bergmann <> | Date | Wed, 12 Jun 2019 16:01:04 +0200 | Subject | Re: infinite loop in read_hpet from ktime_get_boot_fast_ns |
| |
On Wed, Jun 12, 2019 at 11:46 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > On Wed, Jun 12, 2019 at 11:03 AM Peter Zijlstra <peterz@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.
Documentation/core-api/timekeeping.rst describes the timekeeping interfaces. I think what you want here is ktime_get_coarse_boottime().
Note that "coarse" means "don't access the hardware clocksource" here, which is faster than "fast", but less accurate.
This is updated as often as "jiffies_64", but is in nanosecond resolution and takes suspended time into account.
Arnd
| |