linux-kernel - Re: [RFC PATCH 1/2] perf: Add the flag sample

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Message-ID: <20151012121213.GL17308@twins.programming.kicks-ass.net>
Date:	Mon, 12 Oct 2015 14:12:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	Kaixu Xia <xiakaixu@...wei.com>, ast@...mgrid.com,
	davem@...emloft.net, acme@...nel.org, mingo@...hat.com,
	masami.hiramatsu.pt@...achi.com, jolsa@...nel.org,
	daniel@...earbox.net, linux-kernel@...r.kernel.org,
	pi3orama@....com, hekuang@...wei.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] perf: Add the flag sample_disable not to output
 data on samples

On Mon, Oct 12, 2015 at 08:05:20PM +0800, Wangnan (F) wrote:
> 
> 
> On 2015/10/12 20:02, Peter Zijlstra wrote:
> >On Mon, Oct 12, 2015 at 09:02:42AM +0000, Kaixu Xia wrote:
> >>--- a/include/linux/perf_event.h
> >>+++ b/include/linux/perf_event.h
> >>@@ -483,6 +483,8 @@ struct perf_event {
> >>  	perf_overflow_handler_t		overflow_handler;
> >>  	void				*overflow_handler_context;
> >>+	atomic_t			*sample_disable;
> >>+
> >>  #ifdef CONFIG_EVENT_TRACING
> >>  	struct trace_event_call		*tp_event;
> >>  	struct event_filter		*filter;
> >>diff --git a/kernel/events/core.c b/kernel/events/core.c
> >>index b11756f..f6ef45c 100644
> >>--- a/kernel/events/core.c
> >>+++ b/kernel/events/core.c
> >>@@ -6337,6 +6337,9 @@ static int __perf_event_overflow(struct perf_event *event,
> >>  		irq_work_queue(&event->pending);
> >>  	}
> >>+	if ((event->sample_disable) && atomic_read(event->sample_disable))
> >>+		return ret;
> >>+
> >>  	if (event->overflow_handler)
> >>  		event->overflow_handler(event, data, regs);
> >>  	else
> >Try and guarantee sample_disable lives in the same cacheline as
> >overflow_handler.
> 
> Could you please explain why we need them to be in a same cacheline?

Because otherwise you've just added a cacheline miss to this relatively
hot path.
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