On Sun, Oct 19, 2014 at 03:57:54PM -0700, Andy Lutomirski wrote: > > Maybe, but at that point we commit to yet another ABI... I'd rather just > > put a 'sane' implementation in a library or so. > > This cuts both ways, though. For vdso timekeeping, the underlying > data structure has changed repeatedly, sometimes to add features, and > sometimes for performance, and the vdso has done a good job insulating > userspace from it. (In fact, until 3.16, even the same exact kernel > version couldn't be relied on to have the same data structure with > different configs, and even now, no one really wants to teach user > libraries how to parse the pvclock data structures.)
Fair enough, but as it stands we've already committed to the data structure exposed to userspace. > I would certainly not suggest putting anything beyond the bare minimum > into the vdso. Depends on what you really want to do I suppose, if you've got a pinned event and know there cannot be multiplexing, not doing the time reads the multiplications and all that saves a ton of cycles. But in generic I suppose you have to do all that. > FWIW, something should probably specify exactly when it's safe to try > a userspace rdpmc. I think that the answer is that, for a perf event > watching a pid, only that pid can do it (in particular, other threads > must not try). For a perf event monitoring a whole cpu, the answer is > less clear to me. This all was really only meant to be used for self-monitoring, so where an event is attached to the very same task, anything else and I'm find disabling it. -- 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/
