Re: [PATCH 2/2] perf: Userspace software event and ioctl

[Pawel Moll <pawel.moll@arm.com>]

On Sat, 2014-09-27 at 18:14 +0100, Frederic Weisbecker wrote:
> 2014-09-25 20:33 GMT+02:00 Ingo Molnar <mingo@kernel.org>:
> >
> > * Pawel Moll <pawel.moll@arm.com> wrote:
> >
> >> On Wed, 2014-09-24 at 08:49 +0100, Ingo Molnar wrote:
> >> > * Pawel Moll <pawel.moll@arm.com> 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