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Mon, 29 Jul 2024 14:19:11 -0700 (PDT) Received: from ghost ([50.145.13.30]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-70ead6e12f4sm7248364b3a.32.2024.07.29.14.19.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Jul 2024 14:19:10 -0700 (PDT) Date: Mon, 29 Jul 2024 14:19:08 -0700 From: Charlie Jenkins To: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Atish Patra , Al Viro Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/2] perf: Add PERF_EVENT_IOC_SET_EVENT_LIMIT Message-ID: References: <20240724-perf_set_event_limit-v1-0-e680c93eca55@rivosinc.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240724-perf_set_event_limit-v1-0-e680c93eca55@rivosinc.com> On Wed, Jul 24, 2024 at 03:54:10PM -0700, Charlie Jenkins wrote: > My motivation here is to have a program that is able to detect when an > arbitrary other program has reviewed some number of PMU overflows. > PERF_EVENT_IOC_REFRESH is not useful in the scenario since that ioctl > enables the event immediately. This new ioctl flag > PERF_EVENT_IOC_SET_EVENT_LIMIT allows a program to setup a event_limit > and then enable the event when ready at some later time. > > This solves the first problem. The second problem that I am debating a > solution for is that the refresh SIGIO signals are marked as duplicate > signals and not always delivered to userspace. I will attach a simple > program below that can be ran a handful of times to catch this behavior. > REFRESH is supposed to send a SIGIO for every overflow, with an si_code > of POLL_IN for each signal except the last one which has a si_code of > POLL_HUP. However, __send_signal_locked() in kernel/signal.c will > occasionally catch this POLL_HUP signal before all POLL_IN signals have > been processed, thus treating this signal as a duplicate and dropping > with this logic: > > pending = (type != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending; > /* > * Short-circuit ignored signals and support queuing > * exactly one non-rt signal, so that we can get more > * detailed information about the cause of the signal. > */ > result = TRACE_SIGNAL_ALREADY_PENDING; > if (legacy_queue(pending, sig)) > goto ret; > + Al Viro Do you have any input into why signals with the same signal number but different si_code are marked as duplicates of each other? Does it need to work this way? - Charlie > I am sending this patch to begin a conversation on this issue with > REFRESH. One solution could be to only treat a signal as a duplicate in > __send_signal_locked() if the si_code is the same. Another solution is > to only use event_limits with a value of one, as any other number will > run into this race condition. If consistent signals are desired, sigtrap > can be used, but that also doesn't support enable_on_exec. > > Here is a minimal test program for dropped signals. Can be ran on a loop until > program halts waiting for signal that will never arrive. > > while ./signal 1 1; do sleep .01; done > > // signal.c > > int fd; > pid_t pid; > > struct read_format { > unsigned long long value; > unsigned long long time_enabled; > unsigned long long time_running; > }; > > int counter; > > static inline int sys_perf_event_open(struct perf_event_attr *attr, int pid, int cpu, int group_fd, > unsigned long flags) > { > return syscall(__NR_perf_event_open, attr, pid, cpu, group_fd, flags); > } > > static void sig_handler(int signum, siginfo_t *siginfo, void *user_context) > { > struct read_format rd; > counter++; > read(fd, &rd, sizeof(rd)); > printf("fd: %d, signum: %d, si_code: %d, counter: %llu, time to overflow: %llu. time enabled: %llu\n", signum, siginfo->si_fd, siginfo->si_code, rd.value, rd.time_running, rd.time_enabled); > if (counter == REFRESH_COUNT) { > printf("Hit refresh count\n"); > close(fd); > kill(pid, SIGKILL); > if (siginfo->si_code == POLL_HUP) { > exit(EXIT_SUCCESS); > } else { > printf("INCORRECT COUNT! Expected POLL_HUP on second signal.\n"); > exit(EXIT_FAILURE); > } > } > } > > int main(int argc, char *argv[]) > { > char sbuf[128]; > > if (argc < 3) > goto invalid_args; > > unsigned long long num_instructions = atoll(argv[1]); > unsigned int num_samples = atoi(argv[2]); > unsigned long long sample_periods = num_instructions / num_samples; > > pid = fork(); > > if (pid) { > struct sigaction sa = { > .sa_sigaction = (void *) sig_handler, > .sa_flags = SA_SIGINFO > }; > struct perf_event_attr pe = { > .type = PERF_TYPE_HARDWARE, > .size = sizeof(pe), > .config = PERF_COUNT_HW_INSTRUCTIONS, > .sample_period = sample_periods, > .wakeup_events = 1, > .disabled = 1, > .sample_type = PERF_SAMPLE_IP | PERF_SAMPLE_TID | PERF_SAMPLE_CPU | PERF_SAMPLE_PERIOD, > .read_format = PERF_FORMAT_TOTAL_TIME_RUNNING | PERF_FORMAT_TOTAL_TIME_ENABLED, > .exclude_kernel = 1, > .exclude_idle = 1, > .exclude_hv = 1 > }; > > if (sigaction(SIGIO, &sa, NULL) < 0) { > fprintf(stderr, "FAILED setting up signal handler\n"); > exit(EXIT_FAILURE); > } > > fd = sys_perf_event_open(&pe, pid, -1, -1, PERF_FLAG_FD_CLOEXEC); > > if (fd == -1) { > fprintf(stderr, "FAILED opening perf: %s\n", strerror_r(errno, sbuf, sizeof(sbuf))); > exit(EXIT_FAILURE); > } > > ioctl(fd, PERF_EVENT_IOC_RESET, 0); > ioctl(fd, PERF_EVENT_IOC_REFRESH, REFRESH_COUNT); > > fcntl(fd, F_SETFL, O_RDWR | O_NONBLOCK | O_ASYNC); > fcntl(fd, F_SETSIG, SIGIO); > fcntl(fd, F_SETOWN, getpid()); > > while (1); > } else { > while (1); > } > > invalid_args: > printf("FAILED: Please provide at least 2 args: number of instructions, number of samples. eg: %s 10000 5\n", argv[0]); > exit(EXIT_FAILURE); > } > > Signed-off-by: Charlie Jenkins > --- > Charlie Jenkins (2): > perf: Add PERF_EVENT_IOC_INC_EVENT_LIMIT > perf: Document PERF_EVENT_IOC_SET_EVENT_LIMIT > > include/linux/perf_event.h | 4 ++-- > include/uapi/linux/perf_event.h | 1 + > kernel/events/core.c | 17 +++++++++++------ > tools/include/uapi/linux/perf_event.h | 1 + > tools/perf/design.txt | 5 +++++ > 5 files changed, 20 insertions(+), 8 deletions(-) > --- > base-commit: 0c3836482481200ead7b416ca80c68a29cfdaabd > change-id: 20240724-perf_set_event_limit-079f1b996376 > -- > - Charlie >