Re: [PATCH 10/46] perf core: Introduce new ioctl options to pause and resume ring buffer

["Wangnan (F)" <wangnan0@huawei.com>]

Hi Peter,

  Patch 10/46 to 14/46 were sent separately to you and modified
follow your suggestion. Do you have further comment on it?

Thank you.

On 2016/2/29 23:39, Arnaldo Carvalho de Melo wrote:
> Em Fri, Feb 26, 2016 at 09:31:58AM +0000, Wang Nan escreveu:
>> 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.
> Peter, have you have the chance too look at this and the other kernel
> bits in this kit?
>
> - Arnaldo
>   
>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>> Cc: He Kuang <hekuang@huawei.com>
>> Cc: Alexei Starovoitov <ast@kernel.org>
>> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
>> Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
>> Cc: Jiri Olsa <jolsa@kernel.org>
>> Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
>> Cc: Namhyung Kim <namhyung@kernel.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Zefan Li <lizefan@huawei.com>
>> Cc: pi3orama@163.com
>> ---
>>   include/uapi/linux/perf_event.h |  1 +
>>   kernel/events/core.c            | 13 +++++++++++++
>>   kernel/events/internal.h        | 11 +++++++++++
>>   kernel/events/ring_buffer.c     |  7 ++++++-
>>   4 files changed, 31 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>> index 1afe962..a3c1903 100644
>> --- a/include/uapi/linux/perf_event.h
>> +++ b/include/uapi/linux/perf_event.h
>> @@ -401,6 +401,7 @@ struct perf_event_attr {
>>   #define PERF_EVENT_IOC_SET_FILTER	_IOW('$', 6, char *)
>>   #define PERF_EVENT_IOC_ID		_IOR('$', 7, __u64 *)
>>   #define PERF_EVENT_IOC_SET_BPF		_IOW('$', 8, __u32)
>> +#define PERF_EVENT_IOC_PAUSE_OUTPUT	_IOW('$', 9, __u32)
>>   
>>   enum perf_event_ioc_flags {
>>   	PERF_IOC_FLAG_GROUP		= 1U << 0,
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 94c47e3..a7075ae 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -4231,6 +4231,19 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
>>   	case PERF_EVENT_IOC_SET_BPF:
>>   		return perf_event_set_bpf_prog(event, arg);
>>   
>> +	case PERF_EVENT_IOC_PAUSE_OUTPUT: {
>> +		struct ring_buffer *rb;
>> +
>> +		rcu_read_lock();
>> +		rb = rcu_dereference(event->rb);
>> +		if (!event->rb) {
>> +			rcu_read_unlock();
>> +			return -EINVAL;
>> +		}
>> +		rb_toggle_paused(rb, !!arg);
>> +		rcu_read_unlock();
>> +		return 0;
>> +	}
>>   	default:
>>   		return -ENOTTY;
>>   	}
>> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
>> index 2bbad9c..6a93d1b 100644
>> --- a/kernel/events/internal.h
>> +++ b/kernel/events/internal.h
>> @@ -18,6 +18,7 @@ struct ring_buffer {
>>   #endif
>>   	int				nr_pages;	/* nr of data pages  */
>>   	int				overwrite;	/* can overwrite itself */
>> +	int				paused;		/* can write into ring buffer */
>>   
>>   	atomic_t			poll;		/* POLL_ for wakeups */
>>   
>> @@ -65,6 +66,16 @@ static inline void rb_free_rcu(struct rcu_head *rcu_head)
>>   	rb_free(rb);
>>   }
>>   
>> +static inline void
>> +rb_toggle_paused(struct ring_buffer *rb,
>> +		 bool pause)
>> +{
>> +	if (!pause && rb->nr_pages)
>> +		rb->paused = 0;
>> +	else
>> +		rb->paused = 1;
>> +}
>> +
>>   extern struct ring_buffer *
>>   rb_alloc(int nr_pages, long watermark, int cpu, int flags);
>>   extern void perf_event_wakeup(struct perf_event *event);
>> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
>> index 1faad2c..22e1a47 100644
>> --- a/kernel/events/ring_buffer.c
>> +++ b/kernel/events/ring_buffer.c
>> @@ -125,8 +125,11 @@ int perf_output_begin(struct perf_output_handle *handle,
>>   	if (unlikely(!rb))
>>   		goto out;
>>   
>> -	if (unlikely(!rb->nr_pages))
>> +	if (unlikely(rb->paused)) {
>> +		if (rb->nr_pages)
>> +			local_inc(&rb->lost);
>>   		goto out;
>> +	}
>>   
>>   	handle->rb    = rb;
>>   	handle->event = event;
>> @@ -244,6 +247,8 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
>>   	INIT_LIST_HEAD(&rb->event_list);
>>   	spin_lock_init(&rb->event_lock);
>>   	init_irq_work(&rb->irq_work, rb_irq_work);
>> +
>> +	rb->paused = rb->nr_pages ? 0 : 1;
>>   }
>>   
>>   static void ring_buffer_put_async(struct ring_buffer *rb)
>> -- 
>> 1.8.3.4