Re: Rough idea of implementing blocking perf calls for system call tracepoints

[Jiri Olsa]

On Wed, Nov 28, 2018 at 02:18:08PM -0500, Steven Rostedt wrote:
> 
> Adding Masami and Namhyung to this as well.
> 
> -- Steve
> 
> 
> On Wed, 28 Nov 2018 13:47:00 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > [
> >  Sorry for the late reply on this, when I got back from Plumbers, my
> >  work was really piled up, and then Turkey day came and just added more
> >  to the chaos.
> > ]
> > 
> > From our discussion at the Linux Plumbers strace talk about
> > implementing strace with perf. As strace requires to be lossless, it
> > currently can not be implemented with perf because there's always a
> > chance to lose events. The idea here is to have a way to instrument a
> > way to record system calls from perf but also block when the perf ring
> > buffer is full.
> > 
> > Below is a patch I wrote that gives an idea of what needs to be done.
> > It is by no means a real patch (wont even compile). And I left out the
> > wake up part, as I'm not familiar enough with how perf works to
> > implement it. But hopefully someone on this list can :-)
> > 
> > The idea here is that we set the tracepoints sys_enter and sys_exit
> > with a new flag called TRACE_EVENT_FL_BLOCK. When the perf code records
> > the event, if the buffer is full, it will set a "perf_block" field in
> > the current task structure to point to the tp_event, if the tp_event
> > has the BLOCK flag set.
> > 
> > Then on the exit of the syscall tracepoints, the perf_block field is
> > checked, and if it is set, it knows that the event was dropped, and
> > will add itself to a wait queue. When the reader reads the perf buffer
> > and hits a water mark, it can wake whatever is on the queue (not sure
> > where to put this queue, but someone can figure it out).
> > 
> > Once woken, it will try to write to the perf system call tracepoint
> > again (notice that it only tries perf and doesn't call the generic
> > tracepoint code, as only perf requires a repeat).
> > 
> > This is just a basic idea patch, to hopefully give someone else an idea
> > of what I envision. I think it can work, and if it does, I can imagine
> > that it would greatly improve the performance of strace!
> > 
> > Thoughts?
> > 
> > -- Steve
> > 
> > diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> > index 3b2490b81918..57fe95950a24 100644
> > --- a/arch/x86/entry/common.c
> > +++ b/arch/x86/entry/common.c
> > @@ -123,8 +123,22 @@ static long syscall_trace_enter(struct pt_regs *regs)
> >  	}
> >  #endif
> >  
> > -	if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
> > +	if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT))) {
> > +		current->perf_block = NULL;
> >  		trace_sys_enter(regs, regs->orig_ax);
> > +		while (current->perf_block) {
> > +			DECLARE_WAITQUEUE(wait, current);
> > +			struct trace_event_call *tp_event = current->perf_block;
> > +
> > +			current->perf_block = NULL;
> > +
> > +			set_current_state(TASK_INTERRUPTIBLE);
> > +			add_wait_queue(&tp_event->block_queue, &wait);
> > +			perf_trace_sys_enter(tp_event, regs, regs->orig_ax);
> > +			if (current->perf_block)
> > +				schedule();

the space gets freed up by user space moving the tail pointer
so I wonder we need actualy to do some polling in here

also how about making this ring buffer feature so it's not specific
just to sys_enter/sys_exit.. I'll check

jirka