linux-kernel - Re: [PATCH] perf tools: Move precise_ip detection into perf_evsel_

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Date:   Sat, 23 Mar 2019 16:04:22 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Jonas Rabenstein <jonas.rabenstein@...dium.uni-erlangen.de>,
        Nageswara R Sastry <nasastry@...ibm.com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH] perf tools: Move precise_ip detection into
 perf_evsel__open

On Fri, Mar 15, 2019 at 03:52:25PM +0100, Jiri Olsa wrote:
> On Fri, Mar 15, 2019 at 11:35:04AM -0300, Arnaldo Carvalho de Melo wrote:
> 
> SNIP
> 
> > > +static void display_attr(struct perf_event_attr *attr)
> > > +{
> > > +	if (verbose >= 2) {
> > > +		fprintf(stderr, "%.60s\n", graph_dotted_line);
> > > +		fprintf(stderr, "perf_event_attr:\n");
> > > +		perf_event_attr__fprintf(stderr, attr, __open_attr__fprintf, NULL);
> > > +		fprintf(stderr, "%.60s\n", graph_dotted_line);
> > > +	}
> > > +}
> > > +
> > > +static int perf_event_open(struct perf_evsel *evsel,
> > > +			   pid_t pid, int cpu, int group_fd,
> > > +			   unsigned long flags)
> > 
> > 
> > The patch is ok, but I think the naming of this function is too generic,
> > so I'm renaming it to:
> > 
> > static int perf_evsel__open_adjust_precise_ip(struct perf_evsel *evsel,
> > 					      pid_t pid, int cpu, int group_fd,
> > 					      unsigned long flags)
> > 
> > Ok?
> 
> ok
> 
> > 
> > The perf_evsel__open() code is already complex with that fallback
> > mechanism, this is just one more way of fallbacking when asking the
> > kernel for something that may fail.
> > 
> > In fact what happens if the precise_ip that is being asked _is_
> > supported but sys_perf_event_open() fails because some other
> > perf_event_attr attribute that is set is not supported? 
> 
> it's outside the scope of standard feature fallback code,
> so we will try it for any possible fallback variant, so:
> 
> we will try all possible precise_ip (3,2,1,0) and they will
> all fail because of the unsupported attribute - so we will
> restore the precise_ip back and continue in standard fallback
> code that will eventualy switch that attribute off
> 
> > 
> > I see, it gets it back restored to what the user asked so that the
> > standard fallback is tried, ok, I'll apply with just the rename for this
> > function,
> 
> thanks,
> jirka

ping, there's rebased version in my perf/fixes branch

jirka