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Date: Fri, 20 Nov 2020 20:35:01 +0100 From: Jiri Olsa <jolsa@...hat.com> To: Ian Rogers <irogers@...gle.com> Cc: Peter Zijlstra <peterz@...radead.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>, Namhyung Kim <namhyung@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Andi Kleen <ak@...ux.intel.com>, Jin Yao <yao.jin@...ux.intel.com>, John Garry <john.garry@...wei.com>, Paul Clarke <pc@...ibm.com>, kajoljain <kjain@...ux.ibm.com>, Stephane Eranian <eranian@...gle.com>, Sandeep Dasgupta <sdasgup@...gle.com>, linux-perf-users <linux-perf-users@...r.kernel.org> Subject: Re: [PATCH v2 5/5] perf metric: Don't compute unused events. On Thu, Nov 19, 2020 at 01:37:15PM -0800, Ian Rogers wrote: > On Thu, Nov 19, 2020 at 12:59 PM Jiri Olsa <jolsa@...hat.com> wrote: > > > On Tue, Nov 17, 2020 at 09:03:35PM -0800, Ian Rogers wrote: > > > > SNIP > > > > > + ids__free($1.ids); > > > + ids__free($3.ids); > > > + } > > > + } else { > > > + $$.val = NAN; > > > + $$.ids = ids__union($1.ids, $3.ids); > > > + } > > > +} > > > +| expr '*' expr > > > +{ > > > + if (!compute_ids || (isfinite($1.val) && isfinite($3.val))) { > > > + $$.val = $1.val * $3.val; > > > + $$.ids = NULL; > > > + if (compute_ids) { > > > + ids__free($1.ids); > > > + ids__free($3.ids); thanks for the explanation below, really nice, one more question ;-) why do we need to call ids__free in here, for compute_ids and constants case in here it should be always NULL, no? > > > + } > > > + } else { > > > + $$.val = NAN; > > > + $$.ids = ids__union($1.ids, $3.ids); > > > + } > > > +} > > > +| expr '/' expr > > > +{ > > > + if (fpclassify($3.val) == FP_ZERO) { > > > + pr_debug("division by zero\n"); > > > + YYABORT; > > > + } else if (!compute_ids || (isfinite($1.val) && isfinite($3.val))) > > { > > > + $$.val = $1.val / $3.val; > > > + $$.ids = NULL; > > > > hum, I'm confused with this.. compute_ids with finite values? > > why do we erase ids then? also val should be NAN then, no? > > could you please put in some comment with reasoning? > > > > Each expr parsing step needs to create a val and an ids. If we're not > computing ids then we know we don't need ids. If the values are both > constants (aka finite), again we don't need ids as we can just divide. It > is invariant that if you have ids then the value is NAN which isn't finite. > So when computing ids we may want to constant evaluate: > > event1 if 0.5 > 1.0/3.0 else event2 > > which is quite hypothetical but the idea is to have a general approach for > all the operators. The simple lattice is something like: > > Constants: 0.0, 1.0, .... > \. |. / > Bottom: NAN > > Bottom means we really have a set of values and we don't know which it > could be. So: > > 3.0 if event1 > 10.0 else 4.0 > > has possible values of 3.0 or 4.0 which we could represent with the set > {3.0, 4.0}, but because the lattice is simple we just say bottom, meaning > the set of all values - which is conservatively correct as 3.0 and 4.0 are > in the set of all values. It would be incorrect to say the value was 3.0. > Even with a simple lattice we could represent that: > > 3.0 if event1 > 10.0 else 3.0 > > evaluates to 3.0 and so there is no need to measure event1. Note, this > peephole optimization isn't performed here, just the peephole optimization > that if the condition is true or false we can remove events. > > The code in general needs to handle the compute_ids case and the evaluation > case. So what the code is trying to do is propagate constants with sets of > ids in the compute_ids case, or just evaluate in the other case. NAN is > used as a bottom just for simplicity and to avoid inventing a type lattice > abstraction. could you please put it in comment, will be helpful for future ;-) thanks, jirka
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