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

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Dec 06, 2018 at 09:34:00AM +0100, Peter Zijlstra wrote:
> On Thu, Dec 06, 2018 at 09:10:28AM +0100, Peter Zijlstra wrote:
> > On Wed, Dec 05, 2018 at 05:05:02PM +0100, Jiri Olsa wrote:
> > > +static void trace_block_syscall(struct pt_regs *regs, bool enter)
> > > +{
> > > +	current->perf_blocked = true;
> > > +
> > > +	do {
> > > +		schedule_timeout(100 * HZ);
> > > +		current->perf_blocked_cnt = 0;
> > > +
> > > +		if (enter) {
> > > +			/* perf syscalls:* enter */
> > > +			perf_trace_syscall_enter(regs);
> > > +
> > > +			/* perf raw_syscalls:* enter */
> > > +			perf_trace_sys_enter(&event_sys_enter, regs, regs->orig_ax);
> > > +		} else {
> > > +			/* perf syscalls:* enter */
> > > +			perf_trace_syscall_exit(regs);
> > > +
> > > +			/* perf raw_syscalls:* enter */
> > > +			perf_trace_sys_exit(&event_sys_exit, regs, regs->ax);
> > > +		}
> > > +	} while (current->perf_blocked_cnt);
> > > +
> > > +	current->perf_blocked = false;
> > > +}
> > 
> > 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.

right

> 
> And only do this for a few special events...  *yuck*

yes ;-)

jirka