Re: [PATCH v5] selftests/cgroup: add PSI pressure trigger and validation tests

[Suren Baghdasaryan]

On Thu, Sep 3, 2026 at 12:24 AM Tao Cui <cui.tao@xxxxxxxxx> wrote:
>
> Hello Michal, Suren,
>
> 在 2026/9/3 02:47, Michal Koutný 写道:
> > Hello Tao.
> >
> > On Wed, Sep 02, 2026 at 12:07:25PM +0800, Tao Cui <cui.tao@xxxxxxxxx> wrote:
> >> +/* PSI triggers are written with a trailing NUL the kernel parser expects. */
> >> +static ssize_t write_trigger(int fd, const char *trigger)
> >> +{
> >> +    return write(fd, trigger, strlen(trigger) + 1);
> >> +}
> >
> > Hyrum's law. It all works for me: NUL, \n or just write(2) the exact
> > length of the string.
> > For conventionality, I'd prefer the simple literals and plain strlen() +
> > 0. (I reckon cg_write() cannot be used because of FD access.)
> >
>
> Your Hyrum's law point made me look at the parser, and I'm glad it
> did, because the behavior is more subtle than "all of them work".
> psi_write() does
>
>         buf[buf_size - 1] = '\0';
>
> i.e. it overwrites the last byte of whatever was written. With a plain
> strlen()-sized write that eats the last digit: "some 150000 2000000"
> silently arms a 200000us window when privileged, and fails with EINVAL
> for unprivileged users (200000 is not a multiple of the 2s minimum). I
> reproduced both on 7.0.0-28 here. I suspect your runs succeeded
> because a truncated window still makes a valid trigger for root, so
> nothing looked off.
>
> I went with your \n variant instead: the newline gets clobbered, the
> payload stays intact, and it is the conventional procfile form. So the
> reliance on the undocumented NUL is gone, even though not quite via
> strlen()+0.
>
> Two follow-ups this suggests, if there is interest (I'm not pushing
> either within this series):
>
> - psi.rst says nothing about the terminator while sysfs documents its
>   (append, not clobber) behavior explicitly; a sentence in psi.rst
>   would at least make the convention discoverable.
> - kernfs and sysfs both append the NUL after the written data, so the
>   user bytes survive. psi_write() could do the same with
>   buf_size = min(nbytes, sizeof(buf) - 1) and buf[buf_size] = '\0'.
>   Terminator-terminated writes keep their exact meaning, and a bare
>   strlen() write would parse in full instead of losing its last digit.
>   That would be the more principled fix, but it is a behavior change
>   for unterminated writes, so it needs a call from the PSI maintainers.

Yeah, I was expecting a NUL-terminated string with the NUL included
when we write, like shown in the psi.rst code example:

write(fds.fd, trig, strlen(trig) + 1)

but I agree that this might be unconventional.
I guess we could check if buf[buf_size - 1] is already '\0' then do
nothing (keeps current users happy) and otherwise we append the NUL at
the end. That way both write(fds.fd, trig, strlen(trig) + 1) and
write(fds.fd, trig, strlen(trig) + 0) would work. Would you agree or
did I miss anything?

>
> >> +
> >> +static int pressure_open(const char *resource)
> >> +{
> >> +    char path[PATH_MAX];
> >> +    int fd;
> >> +
> >> +    snprintf(path, sizeof(path), "/proc/pressure/%s", resource);
> >> +    fd = open(path, O_RDWR);
> >> +    if (fd < 0)
> >> +            ksft_perror(path);
> >
> > This outputs:
> > | # /proc/pressure/irq: No such file or directory (2)
> > | #      SKIP      /proc/pressure/irq unavailable
> >
> > I.e. similar message is printed twice.
> > Since strace is a companion of cgroup selftests, I'd keep this helper
> > silent.
> >
>
> Agreed, the helper is silent now. The duplicate was my own doing: I
> added the print in v5 on Suren's v4 request (it replaced a raw
> fprintf) without noticing the SKIP message right below it already
> carries the reason, so removing it satisfies both comments.
>
> >> +    return fd;
> >> +}
> >> +
> >> +FIXTURE(psi)
> >> +{
> >> +    char root[PATH_MAX];
> >> +    char *cg;
> >> +};
> >> +
> >> +FIXTURE_SETUP(psi)
> >> +{
> >> +    int psi_fd;
> >> +
> >> +    if (cg_find_unified_root(self->root, sizeof(self->root), NULL))
> >> +            SKIP(return, "cgroup v2 isn't mounted");
> >> +
> >> +    /* PSI must be enabled (CONFIG_PSI=y, not disabled on the cmdline). */
> >> +    psi_fd = open("/proc/pressure/memory", O_RDONLY);
> >> +    if (psi_fd < 0)
> >> +            SKIP(return, "PSI unavailable (CONFIG_PSI=n or psi=0)");
> >> +    close(psi_fd);
> >> +
> >> +    self->cg = cg_name(self->root, "psi_trigger_test");
> >> +    if (!self->cg)
> >> +            SKIP(return, "failed to allocate cgroup name");
> >> +    if (cg_create(self->cg))
> >> +            SKIP(return, "failed to create cgroup: %s", strerror(errno));
> >
> > Why are these two SKIPs (not failures)?
> >
>
> You're right, they are not environment problems. They are ASSERTs in
> FIXTURE_SETUP() now, so a run without privileges fails loudly instead
> of vanishing into skips.
>
> >> +TEST_F(psi, cgroup_trigger_fire)
> >> +{
> >> +    char *cpupress;
> >> +    struct pollfd pfd = { .events = POLLPRI };
> >> +    long ncpus;
> >> +    int fd;
> >> +    int i;
> >> +
> >> +    cpupress = cg_control(self->cg, "cpu.pressure");
> >> +    ASSERT_NE(NULL, cpupress);
> >> +    fd = open(cpupress, O_RDWR);
> >> +    free(cpupress);
> >> +    ASSERT_GE(fd, 0);
> >> +    pfd.fd = fd;
> >> +
> >> +    /*
> >> +     * 1usec threshold over a 2s window: any CPU stall fires it. The 2s
> >> +     * window is the smallest unprivileged users are allowed to arm.
> >> +     */
> >> +    ASSERT_GT(write_trigger(fd, "some 1 2000000"), 0);
> >
> > The selftest rarely can be run as unprivileged user (even test cgroup
> > creation needs privileges), so this comment is irrelevant. (But it's
> > fine to test with that value.)
> >
>
> Dropped.
>
> > On the more abstract level -- I was playing with this and thinking about
> > a value that'd test both sides, i.e. false triggers as well as false
> > non-triggers. I'd find that to be the half of the window and the number
> > of tasks should be then (3*ncpus + 1) / 2.
> > Or perhaps test two thresholds, one tiny like you did and one maximum
> > (whole window) with same amount tasks but expect trigger, no trigger
> > respectively.
> >
>
> I tried both of your variants before picking one. The half-window
> threshold with (3*ncpus+1)/2 hogs fired reliably, and the two-threshold
> variant also held up on the negative side: with the threshold equal to
> the whole window, ncpus+1 hogs never fired it (measured cpu.pressure
> "some" stays around 2-17% under that oversubscription, so the margin
> is comfortable rather than borderline). I kept the existing tiny-
> threshold test and added cgroup_trigger_no_fire for the whole-window
> side, which matches your second variant and keeps the hog setup shared.
>
> >
> >> +
> >> +    ncpus = sysconf(_SC_NPROCESSORS_ONLN);
> >> +    if (ncpus == -1)
> >> +            TH_LOG("sysconf(_SC_NPROCESSORS_ONLN): %s", strerror(errno));
> >> +    ASSERT_NE(-1, ncpus);
> >
> > Same as messages from pressure_open() above. Simply assert.
> >
> >> +
> >> +    /* ncpus+1 hogs guarantee CPU contention inside the cgroup. */
> >> +    for (i = 0; i < ncpus + 1; i++)
> >> +            ASSERT_GE(cg_run_nowait(self->cg, hog_cpu, NULL), 0);
> >> +
> >> +    ASSERT_EQ(1, poll(&pfd, 1, PSI_POLL_TIMEOUT_MS));
> >> +    ASSERT_NE(0, pfd.revents & POLLPRI);
> >> +    close(fd);
> >> +}
> >> +
> >> +TEST_HARNESS_MAIN
> >
> > All in all, this looks so much better than the initial version, well
> > done. Just a few polishing touches.
> >
>
> All 7 tests pass on a 2-CPU and a 16-CPU machine.
>
> Thank you both, this series learned a lot from you: Suren walked me
> through the harness conversion and the reporting details, and you kept
> the semantics honest, which is what surfaced the parser behavior. The
> test is in a much better shape than what I started with, and the
> parser finding above came out of your push on both directions of the
> trigger.
>
> Thanks
> Tao
>
> > Michal
>