linux-kernel - Re: [PATCH 1/4] perf core: Introduce new ioctl options to pause and resume ring buffer

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Date:	Wed, 30 Mar 2016 09:57:02 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Alexei Starovoitov <ast@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	<linux-kernel@...r.kernel.org>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	He Kuang <hekuang@...wei.com>, Jiri Olsa <jolsa@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>, <pi3orama@....com>,
	Zefan Li <lizefan@...wei.com>
Subject: Re: [PATCH 1/4] perf core: Introduce new ioctl options to pause and
 resume ring buffer



On 2016/3/29 20:54, Peter Zijlstra wrote:
> On Mon, Mar 28, 2016 at 06:41:29AM +0000, Wang Nan wrote:
>> Add new ioctl() to pause/resume ring-buffer output.
>>
>> In some situations we want to read from ring buffer only when we
>> ensure nothing can write to the ring buffer during reading. Without
>> this patch we have to turn off all events attached to this ring buffer
>> to achieve this.
>>
>> This patch is for supporting overwrite ring buffer. Following
>> commits will introduce new methods support reading from overwrite ring
>> buffer. Before reading, caller must ensure the ring buffer is frozen, or
>> the reading is unreliable.
>>
>> Signed-off-by: Wang Nan <wangnan0@...wei.com>
> I made the below changes.

Can I add your SOB when I resend it?

> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4346,7 +4346,7 @@ static long _perf_ioctl(struct perf_even
>   
>   		rcu_read_lock();
>   		rb = rcu_dereference(event->rb);
> -		if (!event->rb) {
> +		if (!event->rb || !event->nr_pages) {
>   			rcu_read_unlock();
>   			return -EINVAL;
>   		}
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -66,9 +66,7 @@ static inline void rb_free_rcu(struct rc
>   	rb_free(rb);
>   }
>   
> -static inline void
> -rb_toggle_paused(struct ring_buffer *rb,
> -		 bool pause)
> +static inline void rb_toggle_paused(struct ring_buffer *rb, bool pause)
>   {
>   	if (!pause && rb->nr_pages)
>   		rb->paused = 0;
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -248,7 +248,12 @@ ring_buffer_init(struct ring_buffer *rb,
>   	spin_lock_init(&rb->event_lock);
>   	init_irq_work(&rb->irq_work, rb_irq_work);
>   
> -	rb->paused = rb->nr_pages ? 0 : 1;
> +	/*
> +	 * perf_output_begin() only checks rb->paused, therefore
> +	 * rb->paused must be true if we have no pages for output.
> +	 */
> +	if (!rb->nr_pages)
> +		rb->paused = 1;
>   }

I still think we need to explicitly set rb->paused to 0
when rb->nr_pages is non-zero to avoid further improvement
re-init an old 'struct ring_buffer':

         rb->paused = 0;
         if (unlikely(!rb->nr_pages))
                 rb->paused = 1;

Thank you.

>   
>   static void ring_buffer_put_async(struct ring_buffer *rb)