Re: [PATCH v8 01/13] tools/libperf: avoid moving of fds at fdarray__filter() call

Jiri Olsa
2020-06-25T17:14:16Z

On Wed, Jun 24, 2020 at 08:19:32PM +0300, Alexey Budankov wrote:
>
> On 17.06.2020 11:35, Alexey Budankov wrote:
> >
> > Skip fds with zeroed revents field from count and avoid fds moving
> > at fdarray__filter() call so fds indices returned by fdarray__add()
> > call stay the same and can be used for direct access and processing
> > of fd revents status field at entries array of struct fdarray object.
> >
> > Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
> > ---
> > tools/lib/api/fd/array.c | 11 +++++------
> > tools/perf/tests/fdarray.c | 20 ++------------------
> > 2 files changed, 7 insertions(+), 24 deletions(-)
> >
> > diff --git a/tools/lib/api/fd/array.c b/tools/lib/api/fd/array.c
> > index 58d44d5eee31..97843a837370 100644
> > --- a/tools/lib/api/fd/array.c
> > +++ b/tools/lib/api/fd/array.c
> > @@ -93,22 +93,21 @@ int fdarray__filter(struct fdarray *fda, short revents,
> > return 0;
> >
> > for (fd = 0; fd < fda->nr; ++fd) {
> > + if (!fda->entries[fd].revents)
> > + continue;
> > +
>
> So it looks like this condition also filters out non signaling events fds, not only
> control and others fds, and this should be somehow avoided so such event related fds
> would be counted. Several options have been proposed so far:
>
> 1) Explicit typing of fds via API extension and filtering based on the types:
> a) with separate fdarray__add_stat() call
> b) with type arg of existing fdarray__add() call
> c) various memory management design is possible
>
> 2) Playing tricks with fd positions inside entries and assumptions on fdarray API calls ordering
> - looks more like a hack than a designed solution
>
> 3) Rewrite of fdarray class to allocate separate object for every added fds
> - can be replaced with nonscrewing of fds by __filter()
>
> 4) Distinct between fds types at fdarray__filter() using .revents == 0 condition
> - seems to have corner cases and thus not applicable
>
> 5) Extension of fdarray__poll(, *arg_ptr, arg_size) with arg of fds array to atomically poll
> on fdarray_add()-ed fds and external arg fds and then external arg fds processing
>
> 6) Rewrite of fdarray class on epoll() call basis
> - introduces new scalability restrictions for Perf tool

hum, how many fds for polling do you expect in your workloads?

jirka