| Message ID | 20240319050302.1085006-1-josef@netflix.com |
|---|---|
| State | New |
| Headers | show |
| Series | [V3,bpf-next,1/2] bpf: add bpf_task_get_cgroup kfunc | expand |
On Mon, Mar 18, 2024 at 11:03:02PM -0600, Jose Fernandez 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> Acked-by: Jiri Olsa <jolsa@kernel.org> jirka > --- > V2 -> v3: Use _open_and_load(), usleep helper, and drop map usage > V1 -> V2: Refactor test to work with a cgroup pointer instead of the ID > > .../bpf/prog_tests/task_get_cgroup.c | 43 +++++++++++++++++++ > .../bpf/progs/test_task_get_cgroup.c | 32 ++++++++++++++ > 2 files changed, 75 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..031623067e7e > --- /dev/null > +++ b/tools/testing/selftests/bpf/prog_tests/task_get_cgroup.c > @@ -0,0 +1,43 @@ > +// 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; > + __u64 expected_cgroup_id; > + > + fd = test__join_cgroup(TEST_CGROUP); > + if (!ASSERT_OK(fd < 0, "test_join_cgroup_TEST_CGROUP")) > + return; > + > + skel = test_task_get_cgroup__open_and_load(); > + if (!ASSERT_OK_PTR(skel, "test_task_get_cgroup__open_and_load")) > + goto cleanup; > + > + err = test_task_get_cgroup__attach(skel); > + if (!ASSERT_OK(err, "test_task_get_cgroup__attach")) > + goto cleanup; > + > + skel->bss->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 */ > + usleep(100); > + > + ASSERT_EQ(skel->bss->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..30d4499c6bc5 > --- /dev/null > +++ b/tools/testing/selftests/bpf/progs/test_task_get_cgroup.c > @@ -0,0 +1,32 @@ > +// 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; > + > +int pid = 0; > +u64 cgroup_id = 0; > + > +SEC("tp_btf/sched_switch") > +int BPF_PROG(sched_switch, bool preempt, struct task_struct *prev, > + struct task_struct *next) > +{ > + struct cgroup *cgrp; > + > + if (prev->pid != pid) > + return 0; > + > + cgrp = bpf_task_get_cgroup(prev); > + if (cgrp == NULL) > + return 0; > + cgroup_id = cgrp->kn->id; > + > + bpf_cgroup_release(cgrp); > + return 0; > +} > + > +char _license[] SEC("license") = "GPL"; > -- > 2.40.1 >
On Mon, Mar 18, 2024 at 10:04 PM Jose Fernandez <josef@netflix.com> 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> > Acked-by: Stanislav Fomichev <sdf@google.com> > --- > V2 -> V3: No changes > 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; > +} Tejun, Does this patch look good to you?
On Mon, Mar 18, 2024 at 11:03:01PM -0600, 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> > Acked-by: Stanislav Fomichev <sdf@google.com> Acked-by: Tejun Heo <tj@kernel.org> but some questions below > +__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; > +} So, as this is a lot easier in cgroup2, the above can probably be written directly in BPF (untested and not sure the necessary annotations are in place, so please take it with a big grain of salt): bpf_rcu_read_lock(); cgrp = task->cgroups->dfl_cgrp; cgrp = bpf_cgroup_from_id(cgrp->kn.id); bpf_rcu_read_unlock(); If all you need is ID, it's even simpler: bpf_rcu_read_lock(); cgrp_id = task->cgroups->dfl_cgrp->kn.id; bpf_rcu_read_unlock(); In the first example, it's not great that we go from task pointer to cgroup pointer to ID and then back to acquired cgroup pointer. I wonder whether what we really want is to support something like the following: bpf_rcu_read_lock(); cgrp = bpf_cgroup_tryget(task->cgroups->dfl_cgrp); bpf_rcu_read_unlock(); What do you think? Thanks.
On Wed, Mar 20, 2024 at 7:41 AM Tejun Heo <tj@kernel.org> wrote: > > On Mon, Mar 18, 2024 at 11:03:01PM -0600, 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> > > Acked-by: Stanislav Fomichev <sdf@google.com> > > Acked-by: Tejun Heo <tj@kernel.org> > > but some questions below > > > +__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; > > +} > > So, as this is a lot easier in cgroup2, the above can probably be written > directly in BPF (untested and not sure the necessary annotations are in > place, so please take it with a big grain of salt): > > bpf_rcu_read_lock(); > cgrp = task->cgroups->dfl_cgrp; > cgrp = bpf_cgroup_from_id(cgrp->kn.id); > bpf_rcu_read_unlock(); > > If all you need is ID, it's even simpler: > > bpf_rcu_read_lock(); > cgrp_id = task->cgroups->dfl_cgrp->kn.id; > bpf_rcu_read_unlock(); Good point. Looks like this kfunc isn't really needed. We even have the following in one of the selftests: /* simulate bpf_get_current_cgroup_id() helper */ bpf_rcu_read_lock(); cgroups = task->cgroups; if (!cgroups) goto unlock; cgroup_id = cgroups->dfl_cgrp->kn->id; unlock: bpf_rcu_read_unlock(); > In the first example, it's not great that we go from task pointer to cgroup > pointer to ID and then back to acquired cgroup pointer. I wonder whether > what we really want is to support something like the following: > > bpf_rcu_read_lock(); > cgrp = bpf_cgroup_tryget(task->cgroups->dfl_cgrp); > bpf_rcu_read_unlock(); this one should work already. that's existing bpf_cgroup_acquire(). pw-bot: cr
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)