Re: [External] : Re: [PATCH 0/1] fsnotify: Check parent watches without child dentry flags

[Partha Satapathy]

On 22-10-2026 13:17, Partha Satapathy wrote:
> On 13-10-2026 12:08, Partha Satapathy wrote:
>> On 12-10-2026 14:10, Jan Kara wrote:
>>> On Fri 09-10-26 17:18:17, Partha Satapathy wrote:
>>>> Hi Honza,
>>>>
>>>> Thanks for suggesting this workload.
>>>> The setup and before/after timing comparisons are below.
>>>>
>>>> Architecture:                x86_64
>>>> CPU(s):                      512
>>>>   On-line CPU(s) list:       0-511
>>>>
>>>> NUMA:
>>>>   NUMA node(s):              2
>>>>   NUMA node0 CPU(s):         0-127,256-383
>>>>   NUMA node1 CPU(s):         128-255,384-511
>>>>
>>>> Testcase :
>>>> Using /dev/shm  tmpfs to create  private test directory.
>>>> Create 1,024 files each fully populated with 4kb.
>>>> Add IN_MODIFY inotify watch to the directory.
>>>> Start a listener thread to read notifications and count queue overflows.
>>>> Start X(1,2,4..1024) client threads; each opens a different file once.
>>>> Wait until every client is ready, then release them together.
>>>> Each client starts its timer and performs 1000000
>>>> one-byte pwrite() at offset0 on its own file.
>>>> Each client stops its timer and closes its file.
>>>> Wait for all clients to finish, drain remaining notifications,
>>>> and stop the listener.
>>>> Print write times, throughput, notification count, and queue-overflow count.
>>>> Close inotify and delete the test files and directory.
>>>>
>>>> Note : File creation, open, close, and cleanup are outside the timed
>>>> write loops.
>>>>
>>>> Run 1:
>>>> -------
>>>>
>>>> | Clients | Before average time | After average time | Time change |
>>>> |---:|---:|---:|---:|
>>>> | 1 | 1.392 s | 1.484 s | +6.6% |
>>>> | 2 | 2.205 s | 2.402 s | +8.9% |
>>>> | 4 | 3.969 s | 3.644 s | −8.2% |
>>>> | 8 | 5.851 s | 5.804 s | −0.8% |
>>>> | 16 | 10.607 s | 11.538 s | +8.8% |
>>>> | 32 | 21.093 s | 22.492 s | +6.6% |
>>>> | 64 | 48.698 s | 49.576 s | +1.8% |
>>>> | 128 | 100.208 s | 100.467 s | +0.3% |
>>>> | 256 | 198.773 s | 197.669 s | −0.6% |
>>>> | 512 | 400.056 s | 399.550 s | −0.1% |
>>>>
>>>>
>>>> Run2 :
>>>> --------
>>>>
>>>> | Clients | Before average time | After average time | Time change |
>>>> |---:|---:|---:|---:|
>>>> | 1 | 1.491 s | 1.484 s | −0.4% |
>>>> | 2 | 2.391 s | 2.402 s | +0.5% |
>>>> | 4 | 4.050 s | 3.644 s | −10.0% |
>>>> | 8 | 6.630 s | 5.804 s | −12.5% |
>>>> | 16 | 11.931 s | 11.538 s | −3.3% |
>>>> | 32 | 22.513 s | 22.492 s | −0.1% |
>>>> | 64 | 50.231 s | 49.576 s | −1.3% |
>>>> | 128 | 100.699 s | 100.467 s | −0.2% |
>>>> | 256 | 198.771 s | 197.669 s | −0.6% |
>>>> | 512 | 401.860 s | 399.550 s | −0.6% |
>>>> | 1,024 | 803.812 s | 806.593 s | +0.3% |
>>>>
>>>>
>>>> Positive means After Fix took longer; negative means faster.
>>>>
>>>>
>>>> The measured write performance is broadly comparable before and after
>>>> the fix, with no consistent substantial regression or progressively
>>>> worsening slowdown as concurrency increases.
>>>>
>>>> The differences vary between baseline batches: the increases at 16
>>>> and 32 clients in the first comparison disappear or reverse in the
>>>> second. At 128–1024 clients, average write times differ by approximately
>>>> 0.6% or less. These shifts are consistent with run-to-run variation;
>>>> the results do not show a persistent substantial slowdown from the fix.
>>>
>>> Thanks for running these tests. Results are very interesting. Can you
>>> please share your benchmarking program with me?  I'll check it and try to
>>> play with it on my end. Thanks!
>>>
>>> 								Honza
>>> -- 
>>> Jan Kara <jack@xxxxxxxx>
>>> SUSE Labs, CR
>>>
>>
>> $ cat run_parent_ref_scale.sh
>> #!/usr/bin/env bash
>> set -u -o pipefail
>>
>> bench=./fsnotify_parent_ref_bench
>> if [[ ! -x "$bench" ]]; then
>>         printf 'Build %s in the current directory first.\n' "$bench" >&2
>>         exit 1
>> fi
>>
>> results_dir=$(mktemp -d ./parent-ref-results-XXXXXXXX) || exit 1
>> printf
>> 'kernel=%s\nbenchmark=%s\ntmpfs=/dev/shm\nwrites_per_client=1000000\nmode=watch\n'
>> \
>>         "$(uname -r)" "$bench" > "$results_dir/run-info.txt"
>> printf 'Results: %s\n' "$results_dir"
>>
>> for i in 1 2 3 4 5; do
>> for clients in 1 2 4 8 16 32 64 128 256 512 1024; do
>>         log="$results_dir/clients-${clients}.log"
>>         printf '\n=== clients=%d ===\n' "$clients"
>>         "$bench" /dev/shm "$clients" 1000000 watch 2>&1 | tee "$log"
>>         status=${PIPESTATUS[0]}
>>         printf 'exit_status=%d\n' "$status" | tee -a "$log"
>> done
>> done
>>
>> printf '\nAll results saved in %s\n' "$results_dir"
>>
>>
>>
>> $ cat fsnotify_parent_ref_bench.c
>> #define _GNU_SOURCE
>> #include <errno.h>
>> #include <fcntl.h>
>> #include <inttypes.h>
>> #include <limits.h>
>> #include <poll.h>
>> #include <pthread.h>
>> #include <stdatomic.h>
>> #include <stdbool.h>
>> #include <stdint.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <string.h>
>> #include <sys/inotify.h>
>> #include <sys/stat.h>
>> #include <sys/statfs.h>
>> #include <time.h>
>> #include <unistd.h>
>> #include <linux/magic.h>
>>
>> #define FILES 1024
>> #define FILE_SIZE 4096
>> #define DEFAULT_WRITES 1000000UL
>>
>> struct gate {
>>         pthread_mutex_t mutex;
>>         pthread_cond_t cond;
>>         unsigned int ready;
>>         bool start;
>>         bool abort;
>> };
>>
>> struct worker {
>>         char path[PATH_MAX];
>>         unsigned long writes;
>>         unsigned int id;
>>         struct gate *gate;
>>         uint64_t begin_ns;
>>         uint64_t end_ns;
>>         int error;
>> };
>>
>> struct listener {
>>         int fd;
>>         atomic_bool stop;
>>         uint64_t events;
>>         uint64_t overflows;
>>         int error;
>> };
>>
>> static uint64_t now_ns(void)
>> {
>>         struct timespec ts;
>>
>>         if (clock_gettime(CLOCK_MONOTONIC_RAW, &ts)) {
>>                 perror("clock_gettime");
>>                 exit(EXIT_FAILURE);
>>         }
>>         return (uint64_t)ts.tv_sec * 1000000000ULL + ts.tv_nsec;
>> }
>>
>> static unsigned long parse_count(const char *s, const char *name)
>> {
>>         char *end;
>>         unsigned long value;
>>
>>         errno = 0;
>>         value = strtoul(s, &end, 10);
>>         if (errno || !*s || *end || !value) {
>>                 fprintf(stderr, "Invalid %s: %s\n", name, s);
>>                 exit(EXIT_FAILURE);
>>         }
>>         return value;
>> }
>>
>> static void file_path(char *out, size_t size, const char *dir, unsigned
>> int i)
>> {
>>         int n = snprintf(out, size, "%s/file-%04u", dir, i);
>>
>>         if (n < 0 || (size_t)n >= size) {
>>                 fputs("File path is too long\n", stderr);
>>                 exit(EXIT_FAILURE);
>>         }
>> }
>>
>> static void *run_worker(void *arg)
>> {
>>         struct worker *w = arg;
>>         int fd;
>>         char byte = (char)w->id;
>>         unsigned long i;
>>         bool abort;
>>
>>         fd = open(w->path, O_WRONLY | O_CLOEXEC);
>>         if (fd < 0)
>>                 w->error = errno;
>>         pthread_mutex_lock(&w->gate->mutex);
>>         w->gate->ready++;
>>         pthread_cond_broadcast(&w->gate->cond);
>>         while (!w->gate->start)
>>                 pthread_cond_wait(&w->gate->cond, &w->gate->mutex);
>>         abort = w->gate->abort;
>>         pthread_mutex_unlock(&w->gate->mutex);
>>         if (abort || w->error)
>>                 goto out;
>>
>>         w->begin_ns = now_ns();
>>         for (i = 0; i < w->writes; i++) {
>>                 if (pwrite(fd, &byte, 1, 0) != 1) {
>>                         w->error = errno ? errno : EIO;
>>                         break;
>>                 }
>>         }
>>         w->end_ns = now_ns();
>> out:
>>         if (fd >= 0 && close(fd) && !w->error)
>>                 w->error = errno;
>>         return NULL;
>> }
>>
>> static void *drain_events(void *arg)
>> {
>>         struct listener *l = arg;
>>         char buf[65536] __attribute__((aligned(8)));
>>         struct pollfd pfd = { .fd = l->fd, .events = POLLIN };
>>
>>         for (;;) {
>>                 int ready = poll(&pfd, 1, atomic_load(&l->stop) ? 0 : 50);
>>
>>                 if (ready < 0) {
>>                         if (errno == EINTR)
>>                                 continue;
>>                         l->error = errno;
>>                         break;
>>                 }
>>                 if (!ready) {
>>                         if (atomic_load(&l->stop))
>>                                 break;
>>                         continue;
>>                 }
>>                 for (;;) {
>>                         ssize_t len = read(l->fd, buf, sizeof(buf));
>>                         size_t pos = 0;
>>
>>                         if (len < 0) {
>>                                 if (errno == EINTR)
>>                                         continue;
>>                                 if (errno == EAGAIN)
>>                                         break;
>>                                 l->error = errno;
>>                                 return NULL;
>>                         }
>>                         while (pos + sizeof(struct inotify_event) <=
>> (size_t)len) {
>>                                 struct inotify_event *ev =
>>                                         (struct inotify_event *)(buf + pos);
>>                                 size_t step = sizeof(*ev) + ev->len;
>>
>>                                 if (step > (size_t)len - pos) {
>>                                         l->error = EPROTO;
>>                                         return NULL;
>>                                 }
>>                                 l->events++;
>>                                 if (ev->mask & IN_Q_OVERFLOW)
>>                                         l->overflows++;
>>                                 pos += step;
>>                         }
>>                         if (pos != (size_t)len) {
>>                                 l->error = EPROTO;
>>                                 return NULL;
>>                         }
>>                 }
>>         }
>>         return NULL;
>> }
>>
>> static void cleanup(const char *dir)
>> {
>>         char path[PATH_MAX];
>>         unsigned int i;
>>
>>         for (i = 0; i < FILES; i++) {
>>                 file_path(path, sizeof(path), dir, i);
>>                 if (unlink(path) && errno != ENOENT)
>>                         perror(path);
>>         }
>>         if (rmdir(dir))
>>                 perror(dir);
>> }
>>
>> int main(int argc, char **argv)
>> {
>>         char dir[PATH_MAX], path[PATH_MAX];
>>         unsigned long writes = DEFAULT_WRITES;
>>         unsigned long count;
>>         unsigned int clients, i, created = 0;
>>         struct statfs fs;
>>         struct gate gate = {
>>                 .mutex = PTHREAD_MUTEX_INITIALIZER,
>>                 .cond = PTHREAD_COND_INITIALIZER,
>>         };
>>         struct listener listener = { .fd = -1 };
>>         struct worker *workers;
>>         pthread_t *threads, listener_thread;
>>         bool watched = true, listener_started = false;
>>         uint64_t first = UINT64_MAX, last = 0, sum = 0;
>>         uint64_t shortest = UINT64_MAX, longest = 0;
>>         char zeroes[FILE_SIZE] = { 0 };
>>         int rc = EXIT_FAILURE, err;
>>
>>         if (argc < 3 || argc > 5) {
>>                 fprintf(stderr, "Usage: %s TMPFS_ROOT CLIENTS
>> [WRITES_PER_CLIENT] [watch|none]\n", argv[0]);
>>                 return rc;
>>         }
>>         clients = parse_count(argv[2], "clients");
>>         if (clients > FILES) {
>>                 fprintf(stderr, "Clients must be at most %d\n", FILES);
>>                 return rc;
>>         }
>>         if (argc > 3)
>>                 writes = parse_count(argv[3], "writes per client");
>>         if (writes > ULONG_MAX / clients) {
>>                 fputs("Total write count is too large\n", stderr);
>>                 return rc;
>>         }
>>         if (argc > 4) {
>>                 if (!strcmp(argv[4], "none"))
>>                         watched = false;
>>                 else if (strcmp(argv[4], "watch")) {
>>                         fputs("Mode must be watch or none\n", stderr);
>>                         return rc;
>>                 }
>>         }
>>         if (statfs(argv[1], &fs)) {
>>                 perror(argv[1]);
>>                 return rc;
>>         }
>>         if (fs.f_type != TMPFS_MAGIC) {
>>                 fprintf(stderr, "%s must be a tmpfs directory\n", argv[1]);
>>                 return rc;
>>         }
>>         if (snprintf(dir, sizeof(dir), "%s/fsnotify-parent-ref-XXXXXX",
>> argv[1]) >=
>>             (int)sizeof(dir) || !mkdtemp(dir)) {
>>                 perror("mkdtemp");
>>                 return rc;
>>         }
>>         workers = calloc(clients, sizeof(*workers));
>>         threads = calloc(clients, sizeof(*threads));
>>         if (!workers || !threads) {
>>                 fputs("Cannot allocate worker state\n", stderr);
>>                 goto out;
>>         }
>>         for (i = 0; i < FILES; i++) {
>>                 int fd;
>>
>>                 file_path(path, sizeof(path), dir, i);
>>                 fd = open(path, O_CREAT | O_EXCL | O_WRONLY | O_CLOEXEC,
>> 0600);
>>                 if (fd < 0) {
>>                         perror(path);
>>                         goto out;
>>                 }
>>                 /* Allocate the tmpfs page before the measured write
>> loop. */
>>                 if (write(fd, zeroes, sizeof(zeroes)) != sizeof(zeroes)) {
>>                         perror(path);
>>                         close(fd);
>>                         goto out;
>>                 }
>>                 if (close(fd)) {
>>                         perror(path);
>>                         goto out;
>>                 }
>>         }
>>         if (watched) {
>>                 listener.fd = inotify_init1(IN_NONBLOCK | IN_CLOEXEC);
>>                 if (listener.fd < 0 ||
>>                     inotify_add_watch(listener.fd, dir, IN_MODIFY) < 0) {
>>                         perror("inotify watch");
>>                         goto out;
>>                 }
>>                 err = pthread_create(&listener_thread, NULL,
>> drain_events, &listener);
>>                 if (err) {
>>                         errno = err;
>>                         perror("listener thread");
>>                         goto out;
>>                 }
>>                 listener_started = true;
>>         }
>>         for (i = 0; i < clients; i++) {
>>                 workers[i].id = i;
>>                 workers[i].writes = writes;
>>                 workers[i].gate = &gate;
>>                 file_path(workers[i].path, sizeof(workers[i].path), dir, i);
>>                 err = pthread_create(&threads[i], NULL, run_worker,
>> &workers[i]);
>>                 if (err) {
>>                         errno = err;
>>                         perror("worker thread");
>>                         gate.abort = true;
>>                         goto release;
>>                 }
>>                 created++;
>>         }
>>         pthread_mutex_lock(&gate.mutex);
>>         while (gate.ready < clients)
>>                 pthread_cond_wait(&gate.cond, &gate.mutex);
>>         pthread_mutex_unlock(&gate.mutex);
>>         for (i = 0; i < clients; i++) {
>>                 if (workers[i].error) {
>>                         fprintf(stderr, "Worker %u open: %s\n", i,
>>                                 strerror(workers[i].error));
>>                         gate.abort = true;
>>                         break;
>>                 }
>>         }
>>         printf("test_dir=%s files=%d file_size=%d clients=%u
>> writes_per_client=%lu mode=%s\n",
>>                dir, FILES, FILE_SIZE, clients, writes,
>>                watched ? "watch" : "none");
>>         fflush(stdout);
>> release:
>>         pthread_mutex_lock(&gate.mutex);
>>         gate.start = true;
>>         pthread_cond_broadcast(&gate.cond);
>>         pthread_mutex_unlock(&gate.mutex);
>>         for (i = 0; i < created; i++)
>>                 pthread_join(threads[i], NULL);
>>         if (listener_started) {
>>                 atomic_store(&listener.stop, true);
>>                 pthread_join(listener_thread, NULL);
>>                 printf("inotify_events=%" PRIu64 " queue_overflows=%"
>> PRIu64 "\n",
>>                        listener.events, listener.overflows);
>>                 if (listener.error)
>>                         fprintf(stderr, "Inotify listener: %s\n",
>> strerror(listener.error));
>>                 if (listener.overflows)
>>                         fputs("Inotify queue overflowed; event delivery
>> load changed during the run\n", stderr);
>>         }
>>         if (gate.abort || listener.error)
>>                 goto out;
>>         for (i = 0; i < clients; i++) {
>>                 uint64_t duration;
>>
>>                 if (workers[i].error) {
>>                         fprintf(stderr, "Worker %u write: %s\n", i,
>>                                 strerror(workers[i].error));
>>                         goto out;
>>                 }
>>                 duration = workers[i].end_ns - workers[i].begin_ns;
>>                 if (workers[i].begin_ns < first)
>>                         first = workers[i].begin_ns;
>>                 if (workers[i].end_ns > last)
>>                         last = workers[i].end_ns;
>>                 if (duration < shortest)
>>                         shortest = duration;
>>                 if (duration > longest)
>>                         longest = duration;
>>                 sum += duration;
>>         }
>>         count = (unsigned long)clients * writes;
>>         printf("total_writes=%lu wall_seconds=%.6f
>> throughput_writes_per_sec=%.0f mean_worker_ns_per_write=%.1f
>> shortest_worker_seconds=%.6f longest_worker_seconds=%.6f\n",
>>                count, (last - first) / 1e9, count * 1e9 / (last - first),
>>                (double)sum / count, shortest / 1e9, longest / 1e9);
>>         printf("result=%s\n", listener.overflows ? "QUEUE_OVERFLOW" :
>> "PASS");
>>         rc = listener.overflows ? EXIT_FAILURE : EXIT_SUCCESS;
>> out:
>>         if (listener.fd >= 0)
>>                 close(listener.fd);
>>         cleanup(dir);
>>         free(workers);
>>         free(threads);
>>         return rc;
>> }
>>
>>
>> Please compile : fsnotify_parent_ref_bench.c -o
>> fsnotify_parent_ref_bench. After that
>> run the script ./run_parent_ref_scale.sh
>>
>> Thanks
>> Partha
> 
> Hi Honza,
> 
> A gentle ping on the benchmark program, Have you had a chance to try it
> on your setup? Please let me know if any further tests or information
> would be helpful.
> 
> Thanks,
> Partha



Hi Honza,

Have you had a chance to try the benchmark on your setup? Please let me
know if you need any further tests from my side to move this patch forward.

Thanks,
Partha