Re: [RFC/PATCH] perf trace: Add --syscall-period option

From: Howard Chu
Date: Wed Dec 18 2024 - 00:40:44 EST

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Hello Namhyung,

Thanks for the patch.

On Wed, Dec 11, 2024 at 2:21 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> This option is to implement the event sampling for system calls.
> The period is given in msec as it shows the time in msec.
>
> # perf trace -C 0 --syscall-period 100 sleep 1
> ? ( ): fleetspeakd/1828559 ... [continued]: futex()) = -1 ETIMEDOUT (Connection timed out)
> 0.050 (100.247 ms): gnome-shell/572531 recvmsg(fd: 10<socket:[3355761]>, msg: 0x7ffef8b39d20) = 40
> 100.357 (100.149 ms): pipewire-pulse/572245 read(fd: 5<anon_inode:[eventfd]>, buf: 0x7ffc0b9dc8f0, count: 8) = 8
> 200.553 (100.268 ms): NetworkManager/3424 epoll_wait(epfd: 19<anon_inode:[eventpoll]>, events: 0x5607b85bb880, maxevents: 6) = 0
> 300.876 ( ): mon/4932 poll(ufds: 0x7fa392784df0, nfds: 1, timeout_msecs: 100) ...
> 400.901 ( 0.025 ms): TaskCon~ller #/620145 futex(uaddr: 0x7f3fc596fa00, op: WAKE|PRIVATE_FLAG, val: 1) = 0
> 300.876 (100.123 ms): mon/4932 ... [continued]: poll()) = 0 (Timeout)
> 500.901 ( 0.012 ms): evdefer/2/2335122 futex(uaddr: 0x5640baac5198, op: WAKE|PRIVATE_FLAG, val: 1) = 0
> 602.701 ( 0.017 ms): Compositor/1992200 futex(uaddr: 0x7f1a51dfdd40, op: WAKE|PRIVATE_FLAG, val: 1) = 0
> 705.589 ( 0.017 ms): JS Watchdog/947933 futex(uaddr: 0x7f4cac1d4240, op: WAKE|PRIVATE_FLAG, val: 1) = 0
> 812.667 ( 0.027 ms): fix/1985151 futex(uaddr: 0xc0008f7148, op: WAKE|PRIVATE_FLAG, val: 1) = 1
> 912.807 ( 0.017 ms): Xorg/572315 setitimer(value: 0x7ffc375d6ba0) = 0
>
> The timestamp is kept in a per-cpu array and the allowed task is saved
> in a hash map.
>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
> tools/perf/Documentation/perf-trace.txt | 6 ++
> tools/perf/builtin-trace.c | 5 ++
> .../bpf_skel/augmented_raw_syscalls.bpf.c | 67 ++++++++++++++++++-
> 3 files changed, 76 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/Documentation/perf-trace.txt b/tools/perf/Documentation/perf-trace.txt
> index 6e0cc50bbc13fc7f..9f338a8c5357a67e 100644
> --- a/tools/perf/Documentation/perf-trace.txt
> +++ b/tools/perf/Documentation/perf-trace.txt
> @@ -241,6 +241,12 @@ the thread executes on the designated CPUs. Default is to monitor all CPUs.
> printing using the existing 'perf trace' syscall arg beautifiers to map integer
> arguments to strings (pid to comm, syscall id to syscall name, etc).
>
> +--syscall-period::
> + Trace a system call in the given period (in msec). This implements
> + sampling for syscalls in order to reduce the monitoring overhead.
> + For example, setting the sysall period to 100 (msec) means it will
> + sample a syscall and next one after 100 msec.
> +
>
> PAGEFAULTS
> ----------
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index 3c46de1a8d79bfe6..789eb0ffd5f90b61 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -185,6 +185,7 @@ struct trace {
> } stats;
> unsigned int max_stack;
> unsigned int min_stack;
> + unsigned long sample_period_ms;
> int raw_augmented_syscalls_args_size;
> bool raw_augmented_syscalls;
> bool fd_path_disabled;
> @@ -5219,6 +5220,7 @@ int cmd_trace(int argc, const char **argv)
> "start"),
> OPT_BOOLEAN(0, "force-btf", &trace.force_btf, "Prefer btf_dump general pretty printer"
> "to customized ones"),
> + OPT_ULONG(0, "syscall-period", &trace.sample_period_ms, "syscall sampling period in ms"),
> OPTS_EVSWITCH(&trace.evswitch),
> OPT_END()
> };
> @@ -5326,6 +5328,9 @@ int cmd_trace(int argc, const char **argv)
> bpf_program__set_autoattach(prog, /*autoattach=*/false);
> }
>
> + if (trace.sample_period_ms)
> + trace.skel->rodata->sample_period = trace.sample_period_ms * NSEC_PER_MSEC;
> +
> err = augmented_raw_syscalls_bpf__load(trace.skel);
>
> if (err < 0) {
> diff --git a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> index 4a62ed593e84edf8..12272620dcd73700 100644
> --- a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> +++ b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> @@ -113,6 +113,22 @@ struct pids_filtered {
> __uint(max_entries, 64);
> } pids_filtered SEC(".maps");
>
> +struct sample_timestamp {
> + __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> + __type(key, int);
> + __type(value, __u64);
> + __uint(max_entries, 1);
> +} sample_timestamp SEC(".maps");
> +
> +struct sample_filtered {
> + __uint(type, BPF_MAP_TYPE_HASH);
> + __type(key, pid_t);
> + __type(value, bool);
> + __uint(max_entries, MAX_CPUS);
> +} sample_filtered SEC(".maps");

Do you think MAX_CPUS provides enough entries? If there are MAX_CPUS
syscalls that have entered but haven't exited, new syscalls cannot
write to the map, which results in the new syscall's sys_exit not
being collected. This can lead to trace logs like the following:

210.947 ( ): tmux: server/4399 poll(ufds: 0x579c819eac40,
nfds: 51, timeout_msecs: 9)

which doesn't have a duration, because perf trace doesn't know when it
exits. For example, a poll system call can take a long time to exit,
if there are MAX_CPUS number of polls, they will occupy all the space
in the sample_filtered map.

> + if (prev_ts) {
> + if ((*prev_ts + sample_period) > timestamp)
> + return false;
> + *prev_ts = timestamp;
> + } else {
> + bpf_map_update_elem(&sample_timestamp, &idx, &timestamp, BPF_ANY);
> + }
> +
> + bpf_map_update_elem(&sample_filtered, &pid, &ok, BPF_ANY);

and during polling, when a new syscall enters, this
bpf_map_update_elem() will return an error and fail to insert, and in
sys_exit(), this:

> + if (!bpf_map_lookup_elem(&sample_filtered, &pid))
> + return false;

returns false, and creates a sample with no duration.

In order to eliminate samples with no duration, there should be no
more than MAX_CPUS simultaneous syscalls, but I haven't yet created a
scenario where there are MAX_CPUS polls at the same time. (such
creation needs some tricks, because --syscall-period=1 allows maximum
<number-of-cpus> number of syscalls every millisecond)

That said, I agree that MAX_CPUS (4096) is generally sufficient for
the sample_filtered map in most situations.

Aside from this edge case, this patch looks good to me, I tested it with:

perf trace --syscall-period=500

544.863 ( 0.008 ms): sshd/869528 rt_sigprocmask(how: UNBLOCK, nset:
0x7fffef5f71f0, oset: 0x7fffef5f7270, sigsetsize: 8) = 0
545.034 ( 0.007 ms): tmux: server/4399 ioctl(fd: 54</dev/ptmx>,
cmd: FIONREAD, arg: 0x7ffe020ec758) = 0
593.054 (100.214 ms): gnome-shell/2419 poll(ufds: 0x6236d31eb850,
nfds: 15, timeout_msecs: 100) = 0 (Timeout)
1073.251 ( 0.059 ms): gmain/38955 inotify_add_watch(fd:
4<anon_inode:inotify>, pathname:
"/var/lib/fwupd/metadata/lvfs-testing", mask: 16789454) = -1 ENOENT
(No such file or directory)
1073.641 ( 0.014 ms): tmux: server/4399 ioctl(fd: 54</dev/ptmx>,
cmd: FIONREAD, arg: 0x7ffe020ec758) = 0
1074.588 ( 0.013 ms): sshd/869528 rt_sigprocmask(how: UNBLOCK, nset:
0x7fffef5f71f0, oset: 0x7fffef5f7270, sigsetsize: 8) = 0
1094.651 (100.370 ms): gnome-shell/2419 poll(ufds: 0x6236d31eb850,
nfds: 15, timeout_msecs: 100) = 0 (Timeout)
1596.817 (100.363 ms): gnome-shell/2419 poll(ufds: 0x6236d31eb850,
nfds: 15, timeout_msecs: 100)

The timestamp makes sense for the 500 ms period.

Thanks,
Howard

Next message: Wei Fang: "RE: [PATCH v8 net-next 3/4] net: enetc: add LSO support for i.MX95 ENETC PF"
Previous message: Jiayuan Chen: "[PATCH bpf v3 2/2] selftests/bpf: add strparser test for bpf"
Next in thread: Namhyung Kim: "Re: [RFC/PATCH] perf trace: Add --syscall-period option"
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