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

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 6 Dec 2018 09:34:00 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> > 
> > I don't understand this.. why are we using schedule_timeout() and all
> > that?  
> 
> Urgh.. in fact, the more I look at this the more I hate it.
> 
> We want to block in __perf_output_begin(), but we cannot because both
> tracepoints and perf will have preemptability disabled down there.
> 
> So what we do is fail the event, fake the lost count and go all the way
> up that callstack, detect the failure and then poll-wait and retry.
> 
> And only do this for a few special events...  *yuck*

Since this is a special case, we should add a new option to the perf
system call that, 1 states that it wants the traced process to block
(and must have PTRACE permission to do so) and 2, after it reads from
the buffer, it needs to check a bit that says "this process is blocked,
please wake it up" and then do another perf call to kick the process to
continue.

I really dislike the polling too. But because this is not a default
case, and is a new feature, we can add more infrastructure to make it
work properly, instead of trying to hack the current method into
something that does something poorly.

-- Steve