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Date: Thu, 18 Jul 2013 20:30:18 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>
Cc: David Ahern <dsahern@gmail.com>, Ingo Molnar <mingo@redhat.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        "zhangwei(Jovi)" <jovi.zhangwei@huawei.com>,
        linux-kernel@vger.kernel.org
Subject: [PATCH RESEND 0/3] Teach perf_trace_##call() to check
	hlist_empty(perf_events)
Message-ID: <20130718183018.GA4043@redhat.com>
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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.