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

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Date:   Wed, 5 Dec 2018 18:56:38 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Luis Claudio R. Goncalves" <lclaudio@...g.org>, ldv@...linux.org,
        esyr@...hat.com, Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 1/8] perf: Allow to block process in syscall tracepoints

On Wed, Dec 05, 2018 at 12:35:03PM -0500, Steven Rostedt wrote:
> On Wed,  5 Dec 2018 17:05:02 +0100
> Jiri Olsa <jolsa@...nel.org> wrote:
> 
> > diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> > index 3b2490b81918..e55cf9169a03 100644
> > --- a/arch/x86/entry/common.c
> > +++ b/arch/x86/entry/common.c
> > @@ -60,6 +60,32 @@ static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
> >  	}
> >  }
> >  
> > +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);
> 
> I was thinking, if the process reading the perf buffer dies, how do we
> tell this task to continue on?

when the tracer process dies, its event will get uninstalled from the
trace_event_call::perf_events list, so the next iteration of above loop
won't get any blocked event and it will leave (assuming there's just
single event)

jirka