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

[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.

perf-trace might want to set this flag by default.

I.e. whether user-events are allowed is controlled by the 
profiling/tracing context, via the regular perf syscall. It would 
propagate into the perf context, so it would be easy to check at 
event generation time.

Thanks,

	Ingo