Re: [PATCH v3 2/2]: perf record: enable asynchronous trace writing

Jiri Olsa
2018-08-28T12:21:21Z

On Tue, Aug 28, 2018 at 02:31:04PM +0300, Alexey Budankov wrote:
> Hi,
>
> On 28.08.2018 11:57, Jiri Olsa wrote:
> > On Mon, Aug 27, 2018 at 09:16:55PM +0300, Alexey Budankov wrote:
> >
> > SNIP
> >
> >> + 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;
> >> @@ -528,14 +632,17 @@ 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);
> >> +
> >> for (i = 0; i < evlist->nr_mmaps; i++) {
> >> struct auxtrace_mmap *mm = &maps[i].auxtrace_mmap;
> >>
> >> if (maps[i].base) {
> >> - if (perf_mmap__push(&maps[i], rec, record__pushfn) != 0) {
> >> - rc = -1;
> >> + rc = perf_mmap__push(&maps[i], rec, record__pushfn, &off);
> >> + if (rc < 0)
> >> goto out;
> >> - }
> >> + else if (rc > 0)
> >> + mmap_aio[mmap_aio_size++] = &maps[i].cblock;
> >
> > I understand the purpose of mmap_aio array, but I don't see a reason
> > to fill it in every time we call record__mmap_read_evlist
>
> The cycle trips the same number of iterations over kernel buffers
> for every call of record__mmap_read_evlist(). Called perf_mmap__push()
> checks if there is data ready for spill in the corresponding buffer
> and if there is no such data returns 0. So every time we execute
> the cycle we get different set of buffers to spill and in this
> circumstances dynamic filling of mmap_aio looks preferable.
> Lifetime management of perf_mmap object and referenced memory
> is not related another thing.

argh.. the perf_mmap__push returns 0 for empty buffer, ok

jirka