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

[Namhyung Kim]

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;
> >>  
> >> +	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?

Thanks,
Namhyung