linux-kernel - Re: [PATCH 1/3] tracing/kprobes: Avoid perf_trace_buf_*() if ->perf

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Date:	Mon, 17 Jun 2013 15:48:05 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] tracing/kprobes: Avoid perf_trace_buf_*() if
	->perf_events is empty

On 06/17, zhangwei(Jovi) wrote:
>
> On 2013/6/17 1:21, Oleg Nesterov wrote:
> > perf_trace_buf_prepare() + perf_trace_buf_submit() make no sense
> > if this task/CPU has no active counters. Change kprobe_perf_func()
> > and kretprobe_perf_func() to check call->perf_events beforehand
> > and return if this list is empty.
> >
> > For example, "perf record -e some_probe -p1". Only /sbin/init will
> > report, all other threads which hit the same probe will do
> > perf_trace_buf_prepare/perf_trace_buf_submit just to realize that
> > nobody wants perf_swevent_event().
> >
> > Signed-off-by: Oleg Nesterov <oleg@...hat.com>
>
> Good point, I think we also need to change other places in below patch.
>
> After applied the patch, perf_tp_event() function call reduced a lots
> when using task based perf tracing.

Yes, I was going to do this, but this is not that simple.

> --- a/include/trace/ftrace.h
> +++ b/include/trace/ftrace.h
> @@ -659,6 +659,10 @@ perf_trace_##call(void *__data, proto)					\
>  	int __data_size;						\
>  	int rctx;							\
>  									\
> +	head = this_cpu_ptr(event_call->perf_events);			\
> +	if (hlist_empty(head))						\
> +		return;							\
> +									\

This is not right. Please note __perf_task() and
"if (task && task != current)" in perf_tp_event().

Oleg.

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