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

Jiri Olsa
2020-06-26T09:37:56Z

On Thu, Jun 25, 2020 at 10:32:29PM +0300, Alexey Budankov wrote:
>
> On 25.06.2020 20:14, Jiri Olsa wrote:
> > 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?
>
> Currently it is several hundreds so default of 1K is easily hit and
> "Profile a Large Number of PMU Events on Multi-Core Systems" section [1]
> recommends:
>
> soft nofile 65535
> hard nofile 65535

I'm confused, are you talking about file descriptors limit now?
this wont be affected by epoll change.. what do I miss?

I thought your concern was fs.epoll.max_user_watches, which has
default value that seems to be enough:

$ cat /proc/sys/fs/epoll/max_user_watches
3169996

jirka

>
> for for /etc/security/limits.conf settings.
>
> ~Alexey
>
> [1] https://software.intel.com/content/www/us/en/develop/documentation/vtune-cookbook/top/configuration-recipes/profiling-hardware-without-sampling-drivers.html
>
> >
> > jirka
> >
>