linux-kernel - Re: [PATCH RESEND 0/3] Teach perf_trace_##call() to check hlist_empty(perf

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Date:	Fri, 19 Jul 2013 16:27:21 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	David Ahern <dsahern@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 0/3] Teach perf_trace_##call() to check
 hlist_empty(perf_events)

On Fri, 2013-07-19 at 09:43 +0200, Ingo Molnar wrote:
> * Oleg Nesterov <oleg@...hat.com> wrote:
> 
> > Hello.
> > 
> > The patches are the same, I only tried to update the changelogs a bit.
> > I am also quoting my old email below, to explain what this hack tries
> > to do.
> > 
> > 	Say, "perf record -e sched:sched_switch -p1".
> > 
> > 	Every task except /sbin/init will do perf_trace_sched_switch() and
> > 	perf_trace_buf_prepare() + perf_trace_buf_submit for no reason(),
> > 	it doesn't have a counter.
> > 
> > 	So it makes sense to add the fast-path check at the start of
> > 	perf_trace_##call(),
> > 
> > 		if (hlist_empty(event_call->perf_events))
> > 			return;
> > 
> > 	The problem is, we should not do this if __task != NULL (iow, if
> > 	DECLARE_EVENT_CLASS() uses __perf_task()), perf_tp_event() has the
> > 	additional code for this case.
> > 
> > 	So we should do
> > 
> > 		if (!__task && hlist_empty(event_call->perf_events))
> > 			return;
> > 
> > 	But __task is changed by "{ assign; }" block right before
> > 	perf_trace_buf_submit(). Too late for the fast-path check,
> > 	we already called perf_trace_buf_prepare/fetch_regs.
> > 
> > 	So. After 2/3 __perf_task() (and __perf_count/addr) is called
> > 	when ftrace_get_offsets_##call(args) evaluates the arguments,
> > 	and we can check !__task && hlist_empty() right after that.
> > 
> > Oleg.
> 
> Nice improvement.
> 
> Peter, Steve, any objections?
> 

Yep, agreed.


The whole series...

Reviewed-and-Acked-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve


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