linux-kernel - Re: [PATCH v8 01/13] tools/libperf: avoid moving of fds at fdarray_

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Message-ID: <20200625171405.GL2719003@krava>
Date:   Thu, 25 Jun 2020 19:14:05 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 01/13] tools/libperf: avoid moving of fds at
 fdarray__filter() call

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@...ux.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