On Sun, Feb 16, 2020 at 5:09 PM Hayden Livingston <halivings...@gmail.com> wrote: > > Thanks. I had to re-read your reply and the kernel code multiple > times, but I think I get it now. Please confirm. > > It is this call is made by user mode code: > > fd = bpf_create_map(BPF_MAP_TYPE_PERF_EVENT_ARRAY, /*key_size*/ > sizeof(int), /*value_size*/ sizeof(int), NUM_POSSIBLE_CPUS, 0); > > key is smp_processor_id. value is perf_events fd. This is why the map > is both is key integer and value integer. > > Why so many indirections? Is it to support pinning where user program > can different ring buffers?
Perf event ring buffer is per cpu. > > To me it seems the kernel code that uses cpu index to look into array > could just to told fd directly. Yes, it is what it did in the kernel. Each array element holds one ring buffer. > > On Sun, Feb 16, 2020 at 1:50 PM Y Song <ys114...@gmail.com> wrote: > > > > PERF_EVENT_OUTPUT map is to hold per cpu ring buffers created by > > perf_event_open. > > That is why its typical size is the number of cpus on the host. > > > > On Sun, Feb 16, 2020 at 1:52 AM Hayden Livingston > > <halivings...@gmail.com> wrote: > > > > > > I'm very confused why BCC creates a map of number of processors for > > > the perf_events output map. > > > > > > I can imagine it being 1 since all it does is act as a kernel-user > > > mode intermediary and it is true that the code cannot be preempted. > > > > > > Or if it can be preempted then I can imagine that since there can't be > > > more than processor count it is the max depth one has to worry about. > > > > > > Is my thinking flawed? Or maybe there is a completely different reason? > > > > > > > > > > > > -=-=-=-=-=-=-=-=-=-=-=- Links: You receive all messages sent to this group. View/Reply Online (#1803): https://lists.iovisor.org/g/iovisor-dev/message/1803 Mute This Topic: https://lists.iovisor.org/mt/71322089/21656 Group Owner: iovisor-dev+ow...@lists.iovisor.org Unsubscribe: https://lists.iovisor.org/g/iovisor-dev/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
