Re: [PATCH v2 2/2]: perf record: enable asynchronous trace writing

[Alexey Budankov]

On 27.08.2018 13:38, Jiri Olsa wrote:
> On Mon, Aug 27, 2018 at 01:25:35PM +0300, Alexey Budankov wrote:
>> Hi Namhyung,
>>
>> On 27.08.2018 13:05, Namhyung Kim wrote:
>>> Hello,
>>>
>>> On Mon, Aug 27, 2018 at 12:33:07PM +0300, Alexey Budankov wrote:
>>>> Hi,
>>>>
>>>> On 27.08.2018 11:38, Jiri Olsa wrote:
>>>>> On Thu, Aug 23, 2018 at 07:47:01PM +0300, Alexey Budankov wrote:
>>>>>
>>>>> SNIP
>>>>>
>>>>>>  static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evlist,
>>>>>>  				    bool overwrite)
>>>>>>  {
>>>>>>  	u64 bytes_written = rec->bytes_written;
>>>>>> -	int i;
>>>>>> -	int rc = 0;
>>>>>> +	int i, rc = 0;
>>>>>>  	struct perf_mmap *maps;
>>>>>> +	int trace_fd = rec->session->data->file.fd;
>>>>>> +	struct aiocb **mmap_aio = rec->evlist->mmap_aio;
>>>>>> +	int mmap_aio_size = 0;
>>>>>> +	off_t off;
>>>>>>  
>>>>>>  	if (!evlist)
>>>>>>  		return 0;
>>>>>> @@ -546,14 +620,17 @@ static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evli
>>>>>>  	if (overwrite && evlist->bkw_mmap_state != BKW_MMAP_DATA_PENDING)
>>>>>>  		return 0;
>>>>>>  Hi
>>>>>> +	off = lseek(trace_fd, 0, SEEK_CUR);
>>>>>> +
>>>>>
>>>>> with async write, do we need to query/set the offset like this
>>>>> all the time?
>>>>
>>>> It looks like we need it this way. Internally glibc AIO implements writes 
>>>> using pwrite64 syscall in our case. The sycall requires offset as a parameter 
>>>> and doesn't update file position on the completion.
>>>>
>>>>>
>>>>> could we just keep/update the offset value in the 'struct perf_data_file'
>>>>> and skip both lseek calls?
>>>>
>>>> Don't see how it is possible. offset is different for every enqeued write 
>>>> operation and write areas don't intersect for the whole writing loop. 
>>>> To know the final file position it is required to iterate thru 
>>>> the loop.
>>>
>>> But as far as I can see the offset is linearly updated in
>>> perf_mmap__push() and I guess those two lseek() calls will return
>>> a same value as the last updated offset, no?
>>
>> Yes, offset is linearly calculated by perf_mmap__push() code for 
>> the next possible write operation, but file position is update by 
>> the kernel only in the second lseek() syscall after the loop. 
>> The first lseek() syscall reads that file position for 
>> the next loop iterations.
> 
> does the file's offset need to get updated with lseek at all?
> the async write gets offset.. so I'd think as long as we keep
> offset value, we don't need to call lseek at all

Yep, I expected the same from pwrite64() syscall which eventually 
gets this offset but that is not the case.

> 
> jirka
>