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

[Tao Cui <cui.tao@linux.dev>]

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@linux.dev> 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.

>> +
>> +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