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