linux-kernel - Re: [PATCH 4/8] perf daemon: Add daemon command

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Date:   Wed, 16 Dec 2020 09:14:39 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Budankov <abudankov@...wei.com>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Petlan <mpetlan@...hat.com>,
        Ian Rogers <irogers@...gle.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 4/8] perf daemon: Add daemon command

On Wed, Dec 16, 2020 at 10:54:43AM +0300, Alexei Budankov wrote:
> 
> On 15.12.2020 22:43, Jiri Olsa wrote:
> > On Tue, Dec 15, 2020 at 06:40:26PM +0300, Alexei Budankov wrote:
> >> Hi,
> >>
> >> On 12.12.2020 13:43, Jiri Olsa wrote:
> >>> Adding daemon command that allows to run record sessions
> >>> on background. Each session represents one perf record
> >>> process and is configured in config file.
> >>>
> >>> Example:
> >>>
> >>>   # cat config.daemon
> >>>   [daemon]
> >>>   base=/opt/perfdata
> >>
> >> It could probably make sense to consider using locations at /var/
> >> directory, similar to other already existing daemon processes in
> >> system so admin and user experience would be easily reusabe for
> >> performance monitoring daemon (service).
> > 
> > hm, you can specify any /var path in there if you like,
> > do you suggest to hardcode it?
> 
> This thing: https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard
> Since Perf is a part of OS it would better use some standardized locations.

sure, user is free to configure that

SNIP

> >>> +	start = current = time(NULL);
> >>> +
> >>> +	do {
> >>> +		usleep(500);
> >>
> >> This polling design is actually sub-optimal because it induces redundant
> >> noise in a system. Ideally it should be implemented in async fashion so
> >> kernel would atomically notify daemon process on event happened in some
> >> of record processes e.g. using of poll-like() system call.
> > 
> > ok, any suggestion?
> 
> Possibly, checking SIGCHLDs via signalfd [1] OR using pidfd [2] on kernel v5.3+
> 
> [1] https://man7.org/linux/man-pages/man2/signalfd.2.html
> [2] https://man7.org/linux/man-pages/man2/pidfd_open.2.html

will check, thanks

jirka