netdev - Re: [RFC PATCH 1/2] perf: Add the flag sample_disable not to output data on samples

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Date:	Mon, 12 Oct 2015 14:02:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kaixu Xia <xiakaixu@...wei.com>
Cc:	ast@...mgrid.com, davem@...emloft.net, acme@...nel.org,
	mingo@...hat.com, masami.hiramatsu.pt@...achi.com,
	jolsa@...nel.org, daniel@...earbox.net, wangnan0@...wei.com,
	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 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.

I think we should at the very least replace the kzalloc() currently used
with a cacheline aligned alloc, and check the structure layout to verify
these two do in fact share a cacheline.
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