Incorrect CGroup v1 Resource Detection

[talbii]

Let's assume that cat /proc/1/cgroup outputs:

...
5:cpu,cpuacct:/some_group
...
3:memory:/some_other_group
...

moreover, assume we are running under some container (e.g., Kube). Then it might be the case that the CGroup /memory/some_group doesn't exist, causing us to skip over reading memory values. On the other hand, the container's limits are also present in /memory/memory.limit_in_bytes (for v1 only!) (in fact, this is the system's memory limit so it applies globally).

The same thing of course can happen with CPU values.

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This issue was raised by a Discord member, link here.

[talbii]

I already have a general idea of how to fix this. In this line we detect the memory cgroup path:

dragonfly/src/server/dfly_main.cc

Lines 538 to 540 in b9e8a2c

	if (entry[1] == "memory")
	*memory_path = absl::StrCat("/sys/fs/cgroup/memory/", entry[2]);

(using the example above, memory = "/sys/fs/cgroup/memory/some_group")

And we later read that file, on read failure, continue:

dragonfly/src/server/dfly_main.cc

Lines 562 to 568 in b9e8a2c

	/* Update memory limits */
	auto mlimit = io::ReadFileToString(mem_path + "/memory.limit_in_bytes");
	DVLOG(1) << "memory/memory.limit_in_bytes: " << mlimit.value_or("N/A");
	if (mlimit && !absl::StartsWith(*mlimit, "max")) {
	CHECK(absl::SimpleAtoi(*mlimit, &mdata->mem_total));
	}

which by itself is OK, since we might need to read a different file (the function handles v1 and v2 at the same time)

---

With this in mind, the fix would be to pre-load mdata with the contents of /memory/memory.limit_in_bytes.

[romange]

@talbii can you provide evidence that you need to concat paths like this:
absl::StrCat("/sys/fs/cgroup/memory/", entry[2]); for cgroups v1 ?

[talbii]

In v1 each system resource (memory, CPU, IO, Network) has its own CGroup, which is at location /sys/fs/cgroup/<resource>. To access my own CGroup, you access /sys/fs/cgroup/<resource>/my_cgroup

An example for this is available here: https://docs.kernel.org/admin-guide/cgroup-v1/cgroups.html#how-do-i-use-cgroups

(Charlie is a CGroup which limits CPU resources)