[PATCH V2 bpf-next 1/2] bpf: add bpf_task_get_cgroup kfunc

[PATCH V2 bpf-next 1/2] bpf: add bpf_task_get_cgroup kfunc

[Jose Fernandez]

This patch enhances the BPF helpers by adding a kfunc to retrieve the
cgroup v2 of a task, addressing a previous limitation where only
bpf_task_get_cgroup1 was available for cgroup v1. The new kfunc is
particularly useful for scenarios where obtaining the cgroup ID of a
task other than the "current" one is necessary, which the existing
bpf_get_current_cgroup_id helper cannot accommodate. A specific use
case at Netflix involved the sched_switch tracepoint, where we had to
get the cgroup IDs of both the prev and next tasks.

The bpf_task_get_cgroup kfunc acquires and returns a reference to a
task's default cgroup, ensuring thread-safe access by correctly
implementing RCU read locking and unlocking. It leverages the existing
cgroup.h helper, and cgroup_tryget to safely acquire a reference to it.

Signed-off-by: Jose Fernandez <josef@netflix.com>
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
---
V1 -> V2: Return a pointer to the cgroup instead of the cgroup ID

 kernel/bpf/helpers.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a89587859571..bbd19d5eedb6 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2266,6 +2266,31 @@ bpf_task_get_cgroup1(struct task_struct *task, int hierarchy_id)
 		return NULL;
 	return cgrp;
 }
+
+/**
+ * bpf_task_get_cgroup - Acquire a reference to the default cgroup of a task.
+ * @task: The target task
+ *
+ * This function returns the task's default cgroup, primarily
+ * designed for use with cgroup v2. In cgroup v1, the concept of default
+ * cgroup varies by subsystem, and while this function will work with
+ * cgroup v1, it's recommended to use bpf_task_get_cgroup1 instead.
+ * A cgroup returned by this kfunc which is not subsequently stored in a
+ * map, must be released by calling bpf_cgroup_release().
+ *
+ * Return: On success, the cgroup is returned. On failure, NULL is returned.
+ */
+__bpf_kfunc struct cgroup *bpf_task_get_cgroup(struct task_struct *task)
+{
+	struct cgroup *cgrp;
+
+	rcu_read_lock();
+	cgrp = task_dfl_cgroup(task);
+	if (!cgroup_tryget(cgrp))
+		cgrp = NULL;
+	rcu_read_unlock();
+	return cgrp;
+}
 #endif /* CONFIG_CGROUPS */
 
 /**
@@ -2573,6 +2598,7 @@ BTF_ID_FLAGS(func, bpf_cgroup_ancestor, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_from_id, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_under_cgroup, KF_RCU)
 BTF_ID_FLAGS(func, bpf_task_get_cgroup1, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_task_get_cgroup, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 #endif
 BTF_ID_FLAGS(func, bpf_task_from_pid, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_throw)

base-commit: 4c8644f86c854c214aaabbcc24a27fa4c7e6a951
-- 
2.40.1

[PATCH V2 bpf-next 2/2] selftests/bpf: add selftest for bpf_task_get_cgroup

[Jose Fernandez]

This patch adds a selftest for the `bpf_task_get_cgroup` kfunc. The test
focuses on the use case of obtaining the cgroup ID of the previous task
in a `sched_switch` tracepoint.

The selftest involves creating a test cgroup, attaching a BPF program
that utilizes the `bpf_task_get_cgroup` during a `sched_switch`
tracepoint, and validating that the obtained cgroup ID for the previous
task matches the expected cgroup ID.

Signed-off-by: Jose Fernandez <josef@netflix.com>
Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
---
V1 -> V2: Refactor test to work with a cgroup pointer instead of the ID

 .../bpf/prog_tests/task_get_cgroup.c          | 58 +++++++++++++++++++
 .../bpf/progs/test_task_get_cgroup.c          | 37 ++++++++++++
 2 files changed, 95 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_task_get_cgroup.c

diff --git a/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
new file mode 100644
index 000000000000..67ed65d0c461
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright 2024 Netflix, Inc.
+
+#include <test_progs.h>
+#include <cgroup_helpers.h>
+#include "test_task_get_cgroup.skel.h"
+#include <unistd.h>
+
+#define TEST_CGROUP "/test-task-get-cgroup/"
+
+void test_task_get_cgroup(void)
+{
+	struct test_task_get_cgroup *skel;
+	int err, fd;
+	pid_t pid;
+	__u64 cgroup_id, expected_cgroup_id;
+	const struct timespec req = {
+		.tv_sec = 1,
+		.tv_nsec = 0,
+	};
+
+	fd = test__join_cgroup(TEST_CGROUP);
+	if (!ASSERT_OK(fd < 0, "test_join_cgroup_TEST_CGROUP"))
+		return;
+
+	skel = test_task_get_cgroup__open();
+	if (!ASSERT_OK_PTR(skel, "test_task_get_cgroup__open"))
+		goto cleanup;
+
+	err = test_task_get_cgroup__load(skel);
+	if (!ASSERT_OK(err, "test_task_get_cgroup__load"))
+		goto cleanup;
+
+	err = test_task_get_cgroup__attach(skel);
+	if (!ASSERT_OK(err, "test_task_get_cgroup__attach"))
+		goto cleanup;
+
+	pid = getpid();
+	expected_cgroup_id = get_cgroup_id(TEST_CGROUP);
+	if (!ASSERT_GT(expected_cgroup_id, 0, "get_cgroup_id"))
+		goto cleanup;
+
+	/* Trigger nanosleep to enter the sched_switch tracepoint */
+	/* The previous task should be this process */
+	syscall(__NR_nanosleep, &req, NULL);
+
+	err = bpf_map_lookup_elem(bpf_map__fd(skel->maps.pid_to_cgid_map), &pid,
+				  &cgroup_id);
+
+	if (!ASSERT_OK(err, "bpf_map_lookup_elem"))
+		goto cleanup;
+
+	ASSERT_EQ(cgroup_id, expected_cgroup_id, "cgroup_id");
+
+cleanup:
+	test_task_get_cgroup__destroy(skel);
+	close(fd);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
new file mode 100644
index 000000000000..580f8f0657d5
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright 2024 Netflix, Inc.
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+struct cgroup *bpf_task_get_cgroup(struct task_struct *task) __ksym;
+void bpf_cgroup_release(struct cgroup *cgrp) __ksym;
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 4096);
+	__type(key, __u32);
+	__type(value, __u64);
+} pid_to_cgid_map SEC(".maps");
+
+SEC("tp_btf/sched_switch")
+int BPF_PROG(sched_switch, bool preempt, struct task_struct *prev,
+	     struct task_struct *next)
+{
+	struct cgroup *cgrp;
+	u64 cgroup_id;
+	u32 pid;
+
+	cgrp = bpf_task_get_cgroup(prev);
+	if (cgrp == NULL)
+		return 0;
+	cgroup_id = cgrp->kn->id;
+	pid = prev->pid;
+	bpf_map_update_elem(&pid_to_cgid_map, &pid, &cgroup_id, BPF_ANY);
+
+	bpf_cgroup_release(cgrp);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.40.1

Re: [PATCH V2 bpf-next 2/2] selftests/bpf: add selftest for bpf_task_get_cgroup

[Ratheesh Kannoth]

On 2024-03-16 at 21:52:41, Jose Fernandez (josef@netflix.com) wrote:
> This patch adds a selftest for the `bpf_task_get_cgroup` kfunc. The test
> focuses on the use case of obtaining the cgroup ID of the previous task
> in a `sched_switch` tracepoint.
>
> The selftest involves creating a test cgroup, attaching a BPF program
> that utilizes the `bpf_task_get_cgroup` during a `sched_switch`
> tracepoint, and validating that the obtained cgroup ID for the previous
> task matches the expected cgroup ID.
>
> Signed-off-by: Jose Fernandez <josef@netflix.com>
> Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
> ---
> V1 -> V2: Refactor test to work with a cgroup pointer instead of the ID
>
>  .../bpf/prog_tests/task_get_cgroup.c          | 58 +++++++++++++++++++
>  .../bpf/progs/test_task_get_cgroup.c          | 37 ++++++++++++
>  2 files changed, 95 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
> new file mode 100644
> index 000000000000..67ed65d0c461
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
> @@ -0,0 +1,58 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright 2024 Netflix, Inc.
> +
> +#include <test_progs.h>
> +#include <cgroup_helpers.h>
> +#include "test_task_get_cgroup.skel.h"
> +#include <unistd.h>
> +
> +#define TEST_CGROUP "/test-task-get-cgroup/"
> +
> +void test_task_get_cgroup(void)
> +{
> +	struct test_task_get_cgroup *skel;
> +	int err, fd;
> +	pid_t pid;
> +	__u64 cgroup_id, expected_cgroup_id;
> +	const struct timespec req = {
> +		.tv_sec = 1,
> +		.tv_nsec = 0,
> +	};
Reverse Xmas tree.

> +
> +	fd = test__join_cgroup(TEST_CGROUP);
> +	if (!ASSERT_OK(fd < 0, "test_join_cgroup_TEST_CGROUP"))
> +		return;
> +
> +	skel = test_task_get_cgroup__open();
> +	if (!ASSERT_OK_PTR(skel, "test_task_get_cgroup__open"))
> +		goto cleanup;
> +
> +	err = test_task_get_cgroup__load(skel);
> +	if (!ASSERT_OK(err, "test_task_get_cgroup__load"))
> +		goto cleanup;
> +
> +	err = test_task_get_cgroup__attach(skel);
> +	if (!ASSERT_OK(err, "test_task_get_cgroup__attach"))
> +		goto cleanup;
> +
> +	pid = getpid();
> +	expected_cgroup_id = get_cgroup_id(TEST_CGROUP);
> +	if (!ASSERT_GT(expected_cgroup_id, 0, "get_cgroup_id"))
> +		goto cleanup;
> +
> +	/* Trigger nanosleep to enter the sched_switch tracepoint */
> +	/* The previous task should be this process */
> +	syscall(__NR_nanosleep, &req, NULL);
> +
> +	err = bpf_map_lookup_elem(bpf_map__fd(skel->maps.pid_to_cgid_map), &pid,
> +				  &cgroup_id);
> +
> +	if (!ASSERT_OK(err, "bpf_map_lookup_elem"))
> +		goto cleanup;
> +
> +	ASSERT_EQ(cgroup_id, expected_cgroup_id, "cgroup_id");
> +
> +cleanup:
> +	test_task_get_cgroup__destroy(skel);
> +	close(fd);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
> new file mode 100644
> index 000000000000..580f8f0657d5
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
> @@ -0,0 +1,37 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright 2024 Netflix, Inc.
> +
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +struct cgroup *bpf_task_get_cgroup(struct task_struct *task) __ksym;
> +void bpf_cgroup_release(struct cgroup *cgrp) __ksym;
> +
> +struct {
> +	__uint(type, BPF_MAP_TYPE_HASH);
> +	__uint(max_entries, 4096);
> +	__type(key, __u32);
> +	__type(value, __u64);
> +} pid_to_cgid_map SEC(".maps");
> +
> +SEC("tp_btf/sched_switch")
> +int BPF_PROG(sched_switch, bool preempt, struct task_struct *prev,
> +	     struct task_struct *next)
> +{
> +	struct cgroup *cgrp;
> +	u64 cgroup_id;
> +	u32 pid;
> +
> +	cgrp = bpf_task_get_cgroup(prev);
> +	if (cgrp == NULL)
> +		return 0;
> +	cgroup_id = cgrp->kn->id;
> +	pid = prev->pid;
> +	bpf_map_update_elem(&pid_to_cgid_map, &pid, &cgroup_id, BPF_ANY);
> +
> +	bpf_cgroup_release(cgrp);
> +	return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> --
> 2.40.1
>

Re: [PATCH V2 bpf-next 2/2] selftests/bpf: add selftest for bpf_task_get_cgroup

[Alexei Starovoitov]

On Sun, Mar 17, 2024 at 8:10 PM Ratheesh Kannoth <rkannoth@marvell.com> wrote:
>
> On 2024-03-16 at 21:52:41, Jose Fernandez (josef@netflix.com) wrote:
> > This patch adds a selftest for the `bpf_task_get_cgroup` kfunc. The test
> > focuses on the use case of obtaining the cgroup ID of the previous task
> > in a `sched_switch` tracepoint.
> >
> > The selftest involves creating a test cgroup, attaching a BPF program
> > that utilizes the `bpf_task_get_cgroup` during a `sched_switch`
> > tracepoint, and validating that the obtained cgroup ID for the previous
> > task matches the expected cgroup ID.
> >
> > Signed-off-by: Jose Fernandez <josef@netflix.com>
> > Reviewed-by: Tycho Andersen <tycho@tycho.pizza>
> > ---
> > V1 -> V2: Refactor test to work with a cgroup pointer instead of the ID
> >
> >  .../bpf/prog_tests/task_get_cgroup.c          | 58 +++++++++++++++++++
> >  .../bpf/progs/test_task_get_cgroup.c          | 37 ++++++++++++
> >  2 files changed, 95 insertions(+)
> >  create mode 100644 tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
> > new file mode 100644
> > index 000000000000..67ed65d0c461
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c
> > @@ -0,0 +1,58 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +// Copyright 2024 Netflix, Inc.
> > +
> > +#include <test_progs.h>
> > +#include <cgroup_helpers.h>
> > +#include "test_task_get_cgroup.skel.h"
> > +#include <unistd.h>
> > +
> > +#define TEST_CGROUP "/test-task-get-cgroup/"
> > +
> > +void test_task_get_cgroup(void)
> > +{
> > +     struct test_task_get_cgroup *skel;
> > +     int err, fd;
> > +     pid_t pid;
> > +     __u64 cgroup_id, expected_cgroup_id;
> > +     const struct timespec req = {
> > +             .tv_sec = 1,
> > +             .tv_nsec = 0,
> > +     };
> Reverse Xmas tree.

NO. We don't do it in bpf trees.

Re: [PATCH V2 bpf-next 2/2] selftests/bpf: add selftest for bpf_task_get_cgroup

[Jiri Olsa]

On Sat, Mar 16, 2024 at 10:22:41AM -0600, Jose Fernandez wrote:

SNIP

> +void test_task_get_cgroup(void)
> +{
> +	struct test_task_get_cgroup *skel;
> +	int err, fd;
> +	pid_t pid;
> +	__u64 cgroup_id, expected_cgroup_id;
> +	const struct timespec req = {
> +		.tv_sec = 1,
> +		.tv_nsec = 0,
> +	};
> +
> +	fd = test__join_cgroup(TEST_CGROUP);
> +	if (!ASSERT_OK(fd < 0, "test_join_cgroup_TEST_CGROUP"))
> +		return;
> +
> +	skel = test_task_get_cgroup__open();
> +	if (!ASSERT_OK_PTR(skel, "test_task_get_cgroup__open"))
> +		goto cleanup;
> +
> +	err = test_task_get_cgroup__load(skel);
> +	if (!ASSERT_OK(err, "test_task_get_cgroup__load"))
> +		goto cleanup;

nit, you could call test_task_get_cgroup__open_and_load

> +
> +	err = test_task_get_cgroup__attach(skel);
> +	if (!ASSERT_OK(err, "test_task_get_cgroup__attach"))
> +		goto cleanup;
> +
> +	pid = getpid();
> +	expected_cgroup_id = get_cgroup_id(TEST_CGROUP);
> +	if (!ASSERT_GT(expected_cgroup_id, 0, "get_cgroup_id"))
> +		goto cleanup;
> +
> +	/* Trigger nanosleep to enter the sched_switch tracepoint */
> +	/* The previous task should be this process */
> +	syscall(__NR_nanosleep, &req, NULL);

would smaller sleep do? also we have our own usleep (in test_progs.c)
that calls nanosleep

> +
> +	err = bpf_map_lookup_elem(bpf_map__fd(skel->maps.pid_to_cgid_map), &pid,
> +				  &cgroup_id);
> +
> +	if (!ASSERT_OK(err, "bpf_map_lookup_elem"))
> +		goto cleanup;
> +
> +	ASSERT_EQ(cgroup_id, expected_cgroup_id, "cgroup_id");
> +
> +cleanup:
> +	test_task_get_cgroup__destroy(skel);
> +	close(fd);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
> new file mode 100644
> index 000000000000..580f8f0657d5
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
> @@ -0,0 +1,37 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright 2024 Netflix, Inc.
> +
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +struct cgroup *bpf_task_get_cgroup(struct task_struct *task) __ksym;
> +void bpf_cgroup_release(struct cgroup *cgrp) __ksym;
> +
> +struct {
> +	__uint(type, BPF_MAP_TYPE_HASH);
> +	__uint(max_entries, 4096);
> +	__type(key, __u32);
> +	__type(value, __u64);
> +} pid_to_cgid_map SEC(".maps");
> +
> +SEC("tp_btf/sched_switch")
> +int BPF_PROG(sched_switch, bool preempt, struct task_struct *prev,
> +	     struct task_struct *next)
> +{
> +	struct cgroup *cgrp;
> +	u64 cgroup_id;
> +	u32 pid;
> +

could you filter for your pid in here like we do in other places,
(eg in progs/kprobe_multi.c)

in which case you won't need hash map, but just a single value
to store the cgroup id to

jirka

> +	cgrp = bpf_task_get_cgroup(prev);
> +	if (cgrp == NULL)
> +		return 0;
> +	cgroup_id = cgrp->kn->id;
> +	pid = prev->pid;
> +	bpf_map_update_elem(&pid_to_cgid_map, &pid, &cgroup_id, BPF_ANY);
> +
> +	bpf_cgroup_release(cgrp);
> +	return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> -- 
> 2.40.1
> 

Re: [PATCH V2 bpf-next 1/2] bpf: add bpf_task_get_cgroup kfunc

[Yonghong Song]

On 3/16/24 9:22 AM, Jose Fernandez wrote:
> This patch enhances the BPF helpers by adding a kfunc to retrieve the
> cgroup v2 of a task, addressing a previous limitation where only
> bpf_task_get_cgroup1 was available for cgroup v1. The new kfunc is
> particularly useful for scenarios where obtaining the cgroup ID of a
> task other than the "current" one is necessary, which the existing
> bpf_get_current_cgroup_id helper cannot accommodate. A specific use
> case at Netflix involved the sched_switch tracepoint, where we had to
> get the cgroup IDs of both the prev and next tasks.
>
> The bpf_task_get_cgroup kfunc acquires and returns a reference to a
> task's default cgroup, ensuring thread-safe access by correctly
> implementing RCU read locking and unlocking. It leverages the existing
> cgroup.h helper, and cgroup_tryget to safely acquire a reference to it.
>
> Signed-off-by: Jose Fernandez <josef@netflix.com>
> Reviewed-by: Tycho Andersen <tycho@tycho.pizza>

Acked-by: Yonghong Song <yonghong.song@linux.dev>

Re: [PATCH V2 bpf-next 1/2] bpf: add bpf_task_get_cgroup kfunc

[Stanislav Fomichev]

On 03/16, Jose Fernandez wrote:
> This patch enhances the BPF helpers by adding a kfunc to retrieve the
> cgroup v2 of a task, addressing a previous limitation where only
> bpf_task_get_cgroup1 was available for cgroup v1. The new kfunc is
> particularly useful for scenarios where obtaining the cgroup ID of a
> task other than the "current" one is necessary, which the existing
> bpf_get_current_cgroup_id helper cannot accommodate. A specific use
> case at Netflix involved the sched_switch tracepoint, where we had to
> get the cgroup IDs of both the prev and next tasks.
> 
> The bpf_task_get_cgroup kfunc acquires and returns a reference to a
> task's default cgroup, ensuring thread-safe access by correctly
> implementing RCU read locking and unlocking. It leverages the existing
> cgroup.h helper, and cgroup_tryget to safely acquire a reference to it.
> 
> Signed-off-by: Jose Fernandez <josef@netflix.com>
> Reviewed-by: Tycho Andersen <tycho@tycho.pizza>

Acked-by: Stanislav Fomichev <sdf@google.com>

Re: [PATCH V2 bpf-next 2/2] selftests/bpf: add selftest for bpf_task_get_cgroup

[Jose Fernandez]

On 24/03/18 10:58AM, Jiri Olsa wrote:
> On Sat, Mar 16, 2024 at 10:22:41AM -0600, Jose Fernandez wrote:
> 
> SNIP
> 
> > +void test_task_get_cgroup(void)
> > +{
> > +	struct test_task_get_cgroup *skel;
> > +	int err, fd;
> > +	pid_t pid;
> > +	__u64 cgroup_id, expected_cgroup_id;
> > +	const struct timespec req = {
> > +		.tv_sec = 1,
> > +		.tv_nsec = 0,
> > +	};
> > +
> > +	fd = test__join_cgroup(TEST_CGROUP);
> > +	if (!ASSERT_OK(fd < 0, "test_join_cgroup_TEST_CGROUP"))
> > +		return;
> > +
> > +	skel = test_task_get_cgroup__open();
> > +	if (!ASSERT_OK_PTR(skel, "test_task_get_cgroup__open"))
> > +		goto cleanup;
> > +
> > +	err = test_task_get_cgroup__load(skel);
> > +	if (!ASSERT_OK(err, "test_task_get_cgroup__load"))
> > +		goto cleanup;
> 
> nit, you could call test_task_get_cgroup__open_and_load

I'll rename.

> > +
> > +	err = test_task_get_cgroup__attach(skel);
> > +	if (!ASSERT_OK(err, "test_task_get_cgroup__attach"))
> > +		goto cleanup;
> > +
> > +	pid = getpid();
> > +	expected_cgroup_id = get_cgroup_id(TEST_CGROUP);
> > +	if (!ASSERT_GT(expected_cgroup_id, 0, "get_cgroup_id"))
> > +		goto cleanup;
> > +
> > +	/* Trigger nanosleep to enter the sched_switch tracepoint */
> > +	/* The previous task should be this process */
> > +	syscall(__NR_nanosleep, &req, NULL);
> 
> would smaller sleep do? also we have our own usleep (in test_progs.c)
> that calls nanosleep

Yes a smaller sleep should be fine.
I'll reduce the sleep and use the usleep helper.

> > +
> > +	err = bpf_map_lookup_elem(bpf_map__fd(skel->maps.pid_to_cgid_map), &pid,
> > +				  &cgroup_id);
> > +
> > +	if (!ASSERT_OK(err, "bpf_map_lookup_elem"))
> > +		goto cleanup;
> > +
> > +	ASSERT_EQ(cgroup_id, expected_cgroup_id, "cgroup_id");
> > +
> > +cleanup:
> > +	test_task_get_cgroup__destroy(skel);
> > +	close(fd);
> > +}
> > diff --git a/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
> > new file mode 100644
> > index 000000000000..580f8f0657d5
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c
> > @@ -0,0 +1,37 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +// Copyright 2024 Netflix, Inc.
> > +
> > +#include "vmlinux.h"
> > +#include <bpf/bpf_helpers.h>
> > +#include <bpf/bpf_tracing.h>
> > +
> > +struct cgroup *bpf_task_get_cgroup(struct task_struct *task) __ksym;
> > +void bpf_cgroup_release(struct cgroup *cgrp) __ksym;
> > +
> > +struct {
> > +	__uint(type, BPF_MAP_TYPE_HASH);
> > +	__uint(max_entries, 4096);
> > +	__type(key, __u32);
> > +	__type(value, __u64);
> > +} pid_to_cgid_map SEC(".maps");
> > +
> > +SEC("tp_btf/sched_switch")
> > +int BPF_PROG(sched_switch, bool preempt, struct task_struct *prev,
> > +	     struct task_struct *next)
> > +{
> > +	struct cgroup *cgrp;
> > +	u64 cgroup_id;
> > +	u32 pid;
> > +
> 
> could you filter for your pid in here like we do in other places,
> (eg in progs/kprobe_multi.c)
> 
> in which case you won't need hash map, but just a single value
> to store the cgroup id to
> 
> jirka

I'll apply this suggestion as well and include it in V3. 
Thanks for the feedback.

> 
> > +	cgrp = bpf_task_get_cgroup(prev);
> > +	if (cgrp == NULL)
> > +		return 0;
> > +	cgroup_id = cgrp->kn->id;
> > +	pid = prev->pid;
> > +	bpf_map_update_elem(&pid_to_cgid_map, &pid, &cgroup_id, BPF_ANY);
> > +
> > +	bpf_cgroup_release(cgrp);
> > +	return 0;
> > +}
> > +
> > +char _license[] SEC("license") = "GPL";
> > -- 
> > 2.40.1
> >