linux-kernel - Re: [PATCH v5 5/5] perf-stat: introduce bpf_counter

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Date:   Tue, 27 Apr 2021 14:33:26 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Song Liu <songliubraving@...com>
Cc:     Song Liu <song@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "acme@...nel.org" <acme@...nel.org>,
        "acme@...hat.com" <acme@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "jolsa@...nel.org" <jolsa@...nel.org>
Subject: Re: [PATCH v5 5/5] perf-stat: introduce bpf_counter_ops->disable()

On Mon, Apr 26, 2021 at 10:18:57PM +0000, Song Liu wrote:
> 
> 
> > On Apr 26, 2021, at 2:27 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> > 
> > On Sun, Apr 25, 2021 at 02:43:33PM -0700, Song Liu wrote:
> > 
> > SNIP
> > 
> >> +static inline int bpf_counter__disable(struct evsel *evsel __maybe_unused)
> >> +{
> >> +	return 0;
> >> +}
> >> +
> >> static inline int bpf_counter__read(struct evsel *evsel __maybe_unused)
> >> {
> >> 	return -EAGAIN;
> >> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> >> index d29a8a118973c..e71041c890102 100644
> >> --- a/tools/perf/util/evlist.c
> >> +++ b/tools/perf/util/evlist.c
> >> @@ -17,6 +17,7 @@
> >> #include "evsel.h"
> >> #include "debug.h"
> >> #include "units.h"
> >> +#include "bpf_counter.h"
> >> #include <internal/lib.h> // page_size
> >> #include "affinity.h"
> >> #include "../perf.h"
> >> @@ -421,6 +422,9 @@ static void __evlist__disable(struct evlist *evlist, char *evsel_name)
> >> 	if (affinity__setup(&affinity) < 0)
> >> 		return;
> >> 
> >> +	evlist__for_each_entry(evlist, pos)
> >> +		bpf_counter__disable(pos);
> > 
> > I was wondering why you don't check evsel__is_bpf like
> > for the enable case.. and realized that we don't skip
> > bpf evsels in __evlist__enable and __evlist__disable
> > like we do in read_affinity_counters
> > 
> > so I guess there's extra affinity setup and bunch of
> > wrong ioctls being called?
> 
> We actually didn't do wrong ioctls because the following check:
> 
>        if (... || !pos->core.fd)
>                 continue;
> 
> in __evlist__enable and __evlist__disable. That we don't allocate 
> core.fd for is_bpf events. 
> 
> It is probably good to be more safe with an extra check of 
> evsel__is_bpf(). But it is not required with current code. 

hum, but it will do all the affinity setup no? for no reason,
if there's no non-bpb event

jirka

> 
> Thanks,
> Song
> 
> [...]
>