Messages in this thread | | | From | Rob Herring <> | Date | Tue, 20 Oct 2020 09:38:13 -0500 | Subject | Re: [PATCH v4 4/9] libperf: Add libperf_evsel__mmap() |
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On Mon, Oct 19, 2020 at 3:15 PM Jiri Olsa <jolsa@redhat.com> wrote: > > On Fri, Oct 16, 2020 at 04:39:15PM -0500, Rob Herring wrote: > > On Wed, Oct 14, 2020 at 6:05 AM Jiri Olsa <jolsa@redhat.com> wrote: > > > > > > On Thu, Oct 01, 2020 at 09:01:11AM -0500, Rob Herring wrote: > > > > > > SNIP > > > > > > > > > > > +void *perf_evsel__mmap(struct perf_evsel *evsel, int pages) > > > > +{ > > > > + int ret; > > > > + struct perf_mmap *map; > > > > + struct perf_mmap_param mp = { > > > > + .prot = PROT_READ | PROT_WRITE, > > > > + }; > > > > + > > > > + if (FD(evsel, 0, 0) < 0) > > > > + return NULL; > > > > + > > > > + mp.mask = (pages * page_size) - 1; > > > > + > > > > + map = zalloc(sizeof(*map)); > > > > + if (!map) > > > > + return NULL; > > > > + > > > > + perf_mmap__init(map, NULL, false, NULL); > > > > + > > > > + ret = perf_mmap__mmap(map, &mp, FD(evsel, 0, 0), 0); > > > > > > hum, so you map event for FD(0,0) but later in perf_evsel__read > > > you allow to read any cpu/thread combination ending up reading > > > data from FD(0,0) map: > > > > > > int perf_evsel__read(struct perf_evsel *evsel, int cpu, int thread, > > > struct perf_counts_values *count) > > > { > > > size_t size = perf_evsel__read_size(evsel); > > > > > > memset(count, 0, sizeof(*count)); > > > > > > if (FD(evsel, cpu, thread) < 0) > > > return -EINVAL; > > > > > > if (evsel->mmap && !perf_mmap__read_self(evsel->mmap, count)) > > > return 0; > > > > > > > > > I think we should either check cpu == 0, thread == 0, or make it > > > general and store perf_evsel::mmap in xyarray as we do for fds > > > > The mmapped read will actually fail and then we fallback here. My main > > concern though is adding more overhead on a feature that's meant to be > > low overhead (granted, it's not much). Maybe we could add checks on > > the mmap that we've opened the event with pid == 0 and cpu == -1 (so > > only 1 FD)? > > but then you limit this just for single fd.. having mmap as xyarray > would not be that bad and perf_evsel__mmap will call perf_mmap__mmap > for each defined cpu/thread .. so it depends on user how fast this > will be - how many maps needs to be created/mmaped
Given userspace access fails for anything other than the calling thread and all cpus, how would more than 1 mmap be useful here?
If we did want multiple mmaps, wouldn't we just use the evlist API in that case? It already does all that.
Rob
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