Gilles Chanteperdrix wrote: > Jan Kiszka wrote: > > Gilles Chanteperdrix wrote: > > > Hi, > > > > > > here comes, for review, a patch which reduces the overhead of > > > clock_gettime by directly reading the tsc in user-space for > > > architectures that support it. > > > > Highly welcome. But I have one concern: How and when do you propagate > > wallclock_offset changes to user space? > > Since clock_settime is not implemented, never, but if clock_settime was > implemented, clock_settime would re-issue the __xn_sys_info syscall.
This excludes automatic clock adjustment, something I'm convinced we will have to provide in the future. > > > > > I think we need some vsyscall-alike approach for this, some read-only > > page that is mapped into every RT process, containing things like a > > regularly updated offset (seqlock fashioned) or other read-only > > information (shadow mode? cpu id?). > > As I said yesterday, having a page mapped at a different address in > kernel-space and user-space is not an option on ARM, because each > mapping of this page would have a separated cache. > Hmm, what about the kernel writing directly to the (current) user address? We then only need to keep track of the per-mm address and have not fixed kernel equivalent. At least on arm, other archs may handle this differently at compile time. Jan
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