linux-kernel - Re: "struct perf_sample

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Message-ID: <20210306131442.GA552533@gmail.com>
Date:   Sat, 6 Mar 2021 14:14:42 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: "struct perf_sample_data" alignment


* Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> > This seems to be it... (completely untested)
> >
> > ---
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 3f7f89ea5e51..918a296d2ca2 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -1032,7 +1032,9 @@ struct perf_sample_data {
> >         u64                             cgroup;
> >         u64                             data_page_size;
> >         u64                             code_page_size;
> > -} ____cacheline_aligned;
> > +};
> > +
> > +typedef struct perf_sample_data perf_sample_data_t ____cacheline_aligned;
> >
> >  /* default value for data source */
> >  #define PERF_MEM_NA (PERF_MEM_S(OP, NA)   |\
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index b0c45d923f0f..f32c623abef6 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -923,7 +923,7 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
> >   * bpf_perf_event_output
> >   */
> >  struct bpf_trace_sample_data {
> > -       struct perf_sample_data sds[3];
> > +       perf_sample_data_t sds[3];
> 
> bpf side doesn't care about about cacheline aligned.
> No need to add new typedef just for that.

So this structure is not supposed to be exposed to any ABI anywhere.

I did a (non-exhaustive) search of tooling, and there doesn't appear 
to be any accidental exposure.

The in-kernel ABI interaction appears to be the following:

 - In __perf_event_header_size() we only use fields within 
   perf_sample_data to size the header. Alignment won't change any of 
   the output.

 - Ditto in perf_event__id_header_size().

I.e. I think we should just zap it per the patch below (untested).

Thanks,

	Ingo

============>

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 3f7f89ea5e51..d75e03ff31ea 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1032,7 +1032,7 @@ struct perf_sample_data {
 	u64				cgroup;
 	u64				data_page_size;
 	u64				code_page_size;
-} ____cacheline_aligned;
+};
 
 /* default value for data source */
 #define PERF_MEM_NA (PERF_MEM_S(OP, NA)   |\