On Sat, 2014-09-27 at 18:14 +0100, Frederic Weisbecker wrote: > 2014-09-25 20:33 GMT+02:00 Ingo Molnar <mingo@xxxxxxxxxx>: > > > > * Pawel Moll <pawel.moll@xxxxxxx> wrote: > > > >> On Wed, 2014-09-24 at 08:49 +0100, Ingo Molnar wrote: > >> > * Pawel Moll <pawel.moll@xxxxxxx> wrote: > >> > > >> > > On Thu, 2014-09-18 at 15:34 +0100, Pawel Moll wrote: > >> > > > This patch adds a PERF_COUNT_SW_USERSPACE_EVENT type, > >> > > > which can be generated by user with PERF_EVENT_IOC_ENTRY > >> > > > ioctl command, which injects an event of said type into > >> > > > the perf buffer. > >> > > > >> > > It occurred to me last night that currently perf doesn't handle "write" > >> > > syscall at all, while this seems like the most natural way of > >> > > "injecting" userspace events into perf buffer. > >> > > > >> > > An ioctl would still be needed to set a type of the following events, > >> > > something like: > >> > > > >> > > ioctl(SET_TYPE, 0x42); > >> > > write(perf_fd, binaryblob, size); > >> > > ioctl(SET_TYPE, 0); > >> > > dprintf(perf_fd, "String"); > >> > > > >> > > which is fine for use cases when the type doesn't change often, > >> > > but would double the amount of syscalls when every single event > >> > > is of a different type. Perhaps there still should be a > >> > > "generating ioctl" taking both type and data/size in one go? > >> > > >> > Absolutely, there should be a single syscall. > >> > >> Yeah, it's my gut feeling as well. I just wonder if we still want to > >> keep write() handler for operations on perf fds? This seems natural - > >> takes data buffer and its size. The only issue is the type. > >> > >> > I'd even argue it should be a new prctl(): that way we could both > >> > generate user events for specific perf fds, but also into any > >> > currently active context (that allows just generation/injection > >> > of user events). In the latter case we might have no fd to work > >> > off from. > >> > >> When Arnaldo suggested that the "user events" could be used by perf > >> trace, it was exactly my first thought. I just didn't have answer how to > >> present it to the user (an extra syscall didn't seem like a good idea), > >> but prctl seems interesting, something like this? > >> > >> prctl(PR_TRACE_UEVENT, type, size, data, 0); > > > > Exactly! > > > >> How would we select tasks that can write to a given buffer? Maybe an > >> ioctl() on a perf fd? Something like this? > >> > >> ioctl(perf_fd, PERF_EVENT_IOC_ENABLE_UEVENT, pid); > >> ioctl(perf_fd, PERF_EVENT_IOC_DISABLE_UEVENT, pid); > > > > No, I think there's a simpler way: this should be a regular > > perf_attr flag, which defaults to '0' (tasks cannot do this), but > > which can be set to 1 if the profiler explicitly allows such > > event injection. > > Maybe we just don't even need any permission at all. Which harm can > that do if this only ever generate events to those interested in the > relevant perf context? It could be a simple tracepoint BTW. Yeah, Ingo already pointed it out (that non-root task can't trace root tasks anyway). > Oh and I really like the fact we don't use a syscall that requires an > fd. The tracee really shouldn't be aware of the tracer. Agreed, I'll look at solution with prctl() this week. Pawel -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
