Re: [PATCH 1/8] perf: Allow to block process in syscall tracepoints

From: Peter Zijlstra
Date: Thu Dec 13 2018 - 05:02:08 EST

On Wed, Dec 12, 2018 at 08:26:39PM -0500, Steven Rostedt wrote:
> On Thu, 13 Dec 2018 03:39:38 +0300
> "Dmitry V. Levin" <ldv@xxxxxxxxxxxx> wrote:
> > btw, I didn't ask for the implementation to be ugly.
> > You don't have to introduce polling into the kernel if you don't want to,
> > userspace is perfectly capable of invoking wait4(2) in a loop.
> > Just block the tracee, notify the tracer, and let it pick up the pieces.
> Note, there's been some discussion offlist to only have perf set a flag
> when it dropped an event and have the ptrace code do the heavy lifting
> of blocking the task and waking it back up. I think that would be a
> cleaner solution and wont muck with perf as badly.

It's still really horrid -- the question is not if we can come up with
something, anything, to make strace work. The question is if we can
extend something in a sane and maintainable manner to allow this.

So there's a whole bunch of problems I see with all this, in no
particular order:

- we cannot block when writing to the actual buffer, and have to unroll
the callstack and bolt on the blocking manualy in a few specific
sites. This is ugly, inconsistent and maintenance heavy.

- it only works for some 'magic' events that got the treatment, but not
for many other you might expect it to work for with no real
indication which and why.

- the wakeups side is icky; the best I can come up with is making the
data page R/O and single stepping on write fault, but that isn't
multi-threading safe.

Another alternative would be keeping the whole page R/O and
using write(2) or an ioctl() to update the head pointer.

Again, if we're going to do this; it needs to be done well and
consistent and not as a special hack to enable strace-like
functionality. And without clean and sane solutions to the above I just
don't see it happening.

Note that the first 2 points are equally true for ftrace; so I don't see
how we could sanely add it there either.

One, very big maybe, would be to add a new tracepoint type that includes
a might_sleep() and we very carefully undo all the preempt_disable and
go sleep where we should. That also gives the tracepoint crud the
information it needs to publish the capability to userspace.

We also have to consider (and possibly forbid) mixing blocking and
!blocking events to the same buffer.