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

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Date:   Fri, 26 Jun 2020 11:37:45 +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 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@...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?
> 
> 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
> > 
>