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Message-ID: <aJMUZTJ7FmB9FW9r@li-2b55cdcc-350b-11b2-a85c-a78bff51fc11.ibm.com>
Date: Wed, 6 Aug 2025 10:37:57 +0200
From: Sumanth Korikkar <sumanthk@...ux.ibm.com>
To: kan.liang@...ux.intel.com
Cc: peterz@...radead.org, mingo@...hat.com, namhyung@...nel.org,
irogers@...gle.com, mark.rutland@....com, linux-kernel@...r.kernel.org,
linux-perf-users@...r.kernel.org, eranian@...gle.com,
ctshao@...gle.com, tmricht@...ux.ibm.com, leo.yan@....com,
linux-s390@...r.kernel.org
Subject: Re: [PATCH V4 07/16] s390/perf: Remove driver-specific throttle
support
On Wed, Jul 23, 2025 at 10:06:26AM +0200, Sumanth Korikkar wrote:
> On Tue, May 20, 2025 at 11:16:35AM -0700, kan.liang@...ux.intel.com wrote:
> > From: Kan Liang <kan.liang@...ux.intel.com>
> >
> > The throttle support has been added in the generic code. Remove
> > the driver-specific throttle support.
> >
> > Besides the throttle, perf_event_overflow may return true because of
> > event_limit. It already does an inatomic event disable. The pmu->stop
> > is not required either.
> >
> > Tested-by: Thomas Richter <tmricht@...ux.ibm.com>
> > Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
> > Cc: Thomas Richter <tmricht@...ux.ibm.com>
> > Cc: linux-s390@...r.kernel.org
> > ---
> > arch/s390/kernel/perf_cpum_cf.c | 2 --
> > arch/s390/kernel/perf_cpum_sf.c | 5 +----
> > 2 files changed, 1 insertion(+), 6 deletions(-)
> >
> > diff --git a/arch/s390/kernel/perf_cpum_cf.c b/arch/s390/kernel/perf_cpum_cf.c
> > index e657fad7e376..6a262e198e35 100644
> > --- a/arch/s390/kernel/perf_cpum_cf.c
> > +++ b/arch/s390/kernel/perf_cpum_cf.c
> > @@ -980,8 +980,6 @@ static int cfdiag_push_sample(struct perf_event *event,
> > }
> >
> > overflow = perf_event_overflow(event, &data, ®s);
> > - if (overflow)
> > - event->pmu->stop(event, 0);
> >
> > perf_event_update_userpage(event);
> > return overflow;
> > diff --git a/arch/s390/kernel/perf_cpum_sf.c b/arch/s390/kernel/perf_cpum_sf.c
> > index ad22799d8a7d..91469401f2c9 100644
> > --- a/arch/s390/kernel/perf_cpum_sf.c
> > +++ b/arch/s390/kernel/perf_cpum_sf.c
> > @@ -1072,10 +1072,7 @@ static int perf_push_sample(struct perf_event *event,
> > overflow = 0;
> > if (perf_event_exclude(event, ®s, sde_regs))
> > goto out;
> > - if (perf_event_overflow(event, &data, ®s)) {
> > - overflow = 1;
> > - event->pmu->stop(event, 0);
> > - }
> > + overflow = perf_event_overflow(event, &data, ®s);
> > perf_event_update_userpage(event);
> > out:
> > return overflow;
> > --
> > 2.38.1
>
> Hi all,
>
> This seems to break POLL_HUP delivery to userspace - when event_limit reaches 0
>
> From perf_event_open man page:
> PERF_EVENT_IOC_REFRESH
> Non-inherited overflow counters can use this to enable a
> counter for a number of overflows specified by the
> argument, after which it is disabled. Subsequent calls of
> this ioctl add the argument value to the current count. An
> overflow notification with POLL_IN set will happen on each
> overflow until the count reaches 0; when that happens a
> notification with POLL_HUP set is sent and the event is
> disabled.
>
> When the event_limit reaches 0, the POLL_HUP signal is expected to be
> sent. Prior to this patch, an explicit call to event->stop() was made,
> which may have contributed to ensuring that the POLL_HUP signal was
> ultimately delivered. However, after this change, I often did not
> observe the POLL_HUP signal being delivered as expected in the end
>
> Example program:
> output:
> Computation result: 49951804672
> count.hup: 0 count.pollin: 22
>
> Expected output should be:
> count.hup: 1 in the end
>
> #define _GNU_SOURCE
> #include <time.h>
> #include <stdbool.h>
> #include <signal.h>
> #include <poll.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <unistd.h>
> #include <time.h>
>
> #include <sys/ioctl.h>
> #include <sys/syscall.h>
> #include <linux/perf_event.h>
>
> static struct signal_counts {
> int in;
> int out;
> int hup;
> int unknown;
> } count;
>
>
> static unsigned long sample_type = PERF_SAMPLE_IP | PERF_SAMPLE_TID |
> PERF_SAMPLE_TIME | PERF_SAMPLE_ADDR | PERF_SAMPLE_READ |
> PERF_SAMPLE_ID | PERF_SAMPLE_CPU |
> PERF_SAMPLE_PERIOD | PERF_SAMPLE_STREAM_ID | PERF_SAMPLE_RAW;
>
> static void sighandler(int signum, siginfo_t *info, void *uc)
> {
> switch(info->si_code) {
> case POLL_IN: count.in++; break;
> case POLL_OUT: count.out++; break;
> case POLL_HUP: count.hup++; break;
> default: count.unknown++; break;
> }
> }
>
> void generate_load(unsigned long long iterations) {
> unsigned long long sum = 0;
> srand(time(0));
>
> for (unsigned long long i = 0; i < iterations; ++i) {
> int rnd = rand();
> sum += (rnd ^ (rnd >> 3)) % 1000;
> }
> printf("Computation result: %llu\n", sum);
> }
>
> void perf_attr(struct perf_event_attr *pe,
> unsigned long config, unsigned long period, bool freq,
> unsigned long bits)
> {
> memset(pe, 0, sizeof(struct perf_event_attr));
> pe->size = sizeof(struct perf_event_attr);
> pe->type = PERF_TYPE_HARDWARE;
> pe->config = PERF_COUNT_HW_CPU_CYCLES;
> pe->exclude_kernel = 0;
> pe->sample_period = 50000;
> pe->freq = 1;
> pe->disabled = 1;
> pe->config = config;
> pe->freq = freq;
> pe->sample_type = bits;
> }
>
> int main(int argc, char **argv)
> {
> int fd, signo = SIGIO, rc = -1;
> struct sigaction sa, sa_old;
> struct perf_event_attr pe;
>
> perf_attr(&pe, PERF_COUNT_HW_CPU_CYCLES, 50000, 1, sample_type);
> /* Set up overflow handler */
> memset(&sa, 0, sizeof(struct sigaction));
> memset(&sa_old, 0, sizeof(struct sigaction));
> sa.sa_sigaction = sighandler;
> sa.sa_flags = SA_SIGINFO;
> if (sigaction(signo, &sa, &sa_old) < 0)
> goto out;
>
> fd = syscall(__NR_perf_event_open, &pe, 0, -1, -1, 0);
> if (fd < 0)
> return rc;
>
> rc = fcntl(fd, F_SETFL, O_RDWR | O_NONBLOCK | O_ASYNC);
> rc |= fcntl(fd, F_SETSIG, signo);
> rc |= fcntl(fd, F_SETOWN, getpid());
> if (rc)
> goto out;
>
> rc = ioctl(fd, PERF_EVENT_IOC_REFRESH, 2500);
> if (rc)
> goto out;
>
> generate_load(100000000ULL);
> sigaction(signo, &sa_old, NULL);
> printf("count.hup: %d count.pollin: %d\n", count.hup, count.in);
> close(fd);
> return 0;
> out:
> return rc;
> }
Hi Kan,
It would be great if you could share your feedback on this issue.
Thank you.
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