linux-kernel - Re: [PATCH v4] perf record: collect user registers set jointly with dwarf stacks

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Date:   Thu, 30 May 2019 10:13:37 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        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 v4] perf record: collect user registers set jointly with
 dwarf stacks

Em Thu, May 30, 2019 at 11:24:49AM +0300, Alexey Budankov escreveu:
> On 29.05.2019 22:25, Arnaldo Carvalho de Melo wrote:
> > Em Wed, May 29, 2019 at 05:30:49PM +0300, Alexey Budankov escreveu:
> <SNIP>
> >> +++ b/tools/perf/util/evsel.c
> >> +#define DWARF_REGS_MASK ((1ULL << PERF_REG_IP) | \
> >> +			 (1ULL << PERF_REG_SP))
> >> +
> >>  static void __perf_evsel__config_callchain(struct perf_evsel *evsel,
> >>  					   struct record_opts *opts,
> >>  					   struct callchain_param *param)
> >> @@ -702,7 +705,13 @@ static void __perf_evsel__config_callchain(struct perf_evsel *evsel,
> >>  		if (!function) {
> >>  			perf_evsel__set_sample_bit(evsel, REGS_USER);
> >>  			perf_evsel__set_sample_bit(evsel, STACK_USER);
> >> -			attr->sample_regs_user |= PERF_REGS_MASK;
> >> +			if (opts->sample_user_regs) {
> > 
> > Where are you checking that opts->sample_user_regs doesn't have either
> > IP or SP?
> 
> Sure. The the intention was to avoid such a complication, merge two 
> masks and provide explicit warning that the resulting mask is extended.

s/is/may be/g
 
> If you still see the checking and auto detection of the exact mask 
> extension as essential it can be implemented.

perf, tracing, systems internals, etc are super complicated, full of
details, the more precise we can make the messages, the better.
 
> > So, __perf_evsel__config_callchain its the routine that sets up the
> > attr->sample_regs_user when callchains are asked for, and what was it
> > doing? Asking for _all_ user regs, right?
> > 
> > I.e. what you're saying is that when --callgraph-dwarf is asked for,
> > then only IP and BP are needed, and we should stop doing that, so that
> > would be a first patch, if that is the case. I.e. a patch that doesn't
> > even mention opts->sample_user_regs.
> > 
> > Then, a second patch would fix the opt->sample_user_regs request clash
> > with --callgraph dwarf, i.e. it would do something like:
> > 
> > 	      if ((opts->sample_regs_user & DWARF_REGS_MASK) != DWARF_REGS_MASK) {
> > 	      		char * ip = (opts->sample_regs_user & (1ULL << PERF_REG_IP)) ? NULL : "IP",
> > 	      		     * sp = (opts->sample_regs_user & (1ULL << PERF_REG_SP)) ? NULL : "SP",
> > 			     * all = (!ip && !sp) ?  "s" : "";
> > 
> > 			pr_warning("WARNING: specified --user-regs register set doesn't include register%s "
> > 				   "needed by also specified --call-graph=dwarf, auto adding %s%s%s register%s.\n",
> > 				   all, ip, all : ", " : "", sp, all);
> > 		}
> > 
> > This if and only if all the registers that are needed to do DWARF
> > unwinding are just IP and BP, which doesn't look like its true, since
> > when no --user_regs is set (i.e. opts->user_regs is not set) then we
> > continue asking for PERF_REGS_MASK...
> > 
> > Can you check where I'm missing something?
> 
> 1.  -g call-graph dwarf,K                         full_regs
> 2.  --user-regs=user_regs                         user_regs
> 3.  -g call-graph dwarf,K --user-regs=user_regs	  user_regs + dwarf_regs
> 
> The default behavior stays the same for cases 1, 2 above.
> For case 3 register set becomes the one asked using --user_regs option.
> If the option value misses IP or SP or the both then they are explicitly
> added to the option value and a warning message mentioning the exact 
> added registers is provided.
 
> > Jiri DWARF unwind uses just IP and SP? Looking at
> > tools/perf/util/unwind-libunwind-local.c's access_reg() I don't think
> > so, right?
 
> If you ask me, AFAIK, DWARF unwind rules sometimes can refer additional 
> general purpose registers for frames boundaries calculation.

:-) So that DWARF_REGS is misleading, should be something like
DWARF_MINIMAL_REGS, as we may need other registers, so the original code
was correct, right?

After all if the user asks for both --call-graph dwarf and --user-regs,
then probably we should require --force? I.e. the message then would be:

"
WARNING: The use of --call-graph=dwarf may require all the user
registers, specifying a subset with --user-regs may render DWARF
unwinding unreliable, please use --force if you're sure that the subset
specified via --user-regs is enough for your specific use case.
"

And then plain refuse, if the user _really_ wants it, then we have
--force/-f for those cases.

Does this sound better?

- Arnaldo