Commit d6f3e7d5 authored by Tejun Heo's avatar Tejun Heo
Browse files

sched_ext: Fix incorrect autogroup migration detection 

scx_move_task() is called from sched_move_task() and tells the BPF scheduler
that cgroup migration is being committed. sched_move_task() is used by both
cgroup and autogroup migrations and scx_move_task() tried to filter out
autogroup migrations by testing the destination cgroup and PF_EXITING but
this is not enough. In fact, without explicitly tagging the thread which is
doing the cgroup migration, there is no good way to tell apart
scx_move_task() invocations for racing migration to the root cgroup and an
autogroup migration.

This led to scx_move_task() incorrectly ignoring a migration from non-root
cgroup to an autogroup of the root cgroup triggering the following warning:

  WARNING: CPU: 7 PID: 1 at kernel/sched/ext.c:3725 scx_cgroup_can_attach+0x196/0x340
  ...
  Call Trace:
  <TASK>
    cgroup_migrate_execute+0x5b1/0x700
    cgroup_attach_task+0x296/0x400
    __cgroup_procs_write+0x128/0x140
    cgroup_procs_write+0x17/0x30
    kernfs_fop_write_iter+0x141/0x1f0
    vfs_write+0x31d/0x4a0
    __x64_sys_write+0x72/0xf0
    do_syscall_64+0x82/0x160
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fix it by adding an argument to sched_move_task() that indicates whether the
moving is for a cgroup or autogroup migration. After the change,
scx_move_task() is called only for cgroup migrations and renamed to
scx_cgroup_move_task().

Link: https://github.com/sched-ext/scx/issues/370


Fixes: 81951366 ("sched_ext: Add cgroup support")
Cc: stable@vger.kernel.org # v6.12+
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent e9fe1827

kernel/sched/autogroup.c

+2 −2
Original line number Diff line number Diff line
@@ -150,7 +150,7 @@ void sched_autogroup_exit_task(struct task_struct *p) 
	 * see this thread after that: we can no longer use signal->autogroup.

	 * See the PF_EXITING check in task_wants_autogroup().

	 */

	sched_move_task(p);

	sched_move_task(p, true);

}



static void
@@ -182,7 +182,7 @@ autogroup_move_group(struct task_struct *p, struct autogroup *ag) 
	 * sched_autogroup_exit_task().

	 */

	for_each_thread(p, t)

		sched_move_task(t);

		sched_move_task(t, true);



	unlock_task_sighand(p, &flags);

	autogroup_kref_put(prev);

kernel/sched/core.c

+4 −3
Original line number Diff line number Diff line
@@ -9042,7 +9042,7 @@ static void sched_change_group(struct task_struct *tsk, struct task_group *group 
 * now. This function just updates tsk->se.cfs_rq and tsk->se.parent to reflect

 * its new group.

 */

void sched_move_task(struct task_struct *tsk)

void sched_move_task(struct task_struct *tsk, bool for_autogroup)

{

	int queued, running, queue_flags =

		DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
@@ -9071,7 +9071,8 @@ void sched_move_task(struct task_struct *tsk) 
		put_prev_task(rq, tsk);



	sched_change_group(tsk, group);

	scx_move_task(tsk);

	if (!for_autogroup)

		scx_cgroup_move_task(tsk);



	if (queued)

		enqueue_task(rq, tsk, queue_flags);
@@ -9172,7 +9173,7 @@ static void cpu_cgroup_attach(struct cgroup_taskset *tset) 
	struct cgroup_subsys_state *css;



	cgroup_taskset_for_each(task, css, tset)

		sched_move_task(task);

		sched_move_task(task, false);



	scx_cgroup_finish_attach();

}

kernel/sched/ext.c

+1 −14
Original line number Diff line number Diff line
@@ -4323,24 +4323,11 @@ int scx_cgroup_can_attach(struct cgroup_taskset *tset) 
	return ops_sanitize_err("cgroup_prep_move", ret);

}



void scx_move_task(struct task_struct *p)

void scx_cgroup_move_task(struct task_struct *p)

{

	if (!scx_cgroup_enabled)

		return;



	/*

	 * We're called from sched_move_task() which handles both cgroup and

	 * autogroup moves. Ignore the latter.

	 *

	 * Also ignore exiting tasks, because in the exit path tasks transition

	 * from the autogroup to the root group, so task_group_is_autogroup()

	 * alone isn't able to catch exiting autogroup tasks. This is safe for

	 * cgroup_move(), because cgroup migrations never happen for PF_EXITING

	 * tasks.

	 */

	if (task_group_is_autogroup(task_group(p)) || (p->flags & PF_EXITING))

		return;



	/*

	 * @p must have ops.cgroup_prep_move() called on it and thus

	 * cgrp_moving_from set.

kernel/sched/ext.h

+2 −2
Original line number Diff line number Diff line
@@ -73,7 +73,7 @@ static inline void scx_update_idle(struct rq *rq, bool idle, bool do_notify) {} 
int scx_tg_online(struct task_group *tg);

void scx_tg_offline(struct task_group *tg);

int scx_cgroup_can_attach(struct cgroup_taskset *tset);

void scx_move_task(struct task_struct *p);

void scx_cgroup_move_task(struct task_struct *p);

void scx_cgroup_finish_attach(void);

void scx_cgroup_cancel_attach(struct cgroup_taskset *tset);

void scx_group_set_weight(struct task_group *tg, unsigned long cgrp_weight);
@@ -82,7 +82,7 @@ void scx_group_set_idle(struct task_group *tg, bool idle); 
static inline int scx_tg_online(struct task_group *tg) { return 0; }

static inline void scx_tg_offline(struct task_group *tg) {}

static inline int scx_cgroup_can_attach(struct cgroup_taskset *tset) { return 0; }

static inline void scx_move_task(struct task_struct *p) {}

static inline void scx_cgroup_move_task(struct task_struct *p) {}

static inline void scx_cgroup_finish_attach(void) {}

static inline void scx_cgroup_cancel_attach(struct cgroup_taskset *tset) {}

static inline void scx_group_set_weight(struct task_group *tg, unsigned long cgrp_weight) {}

kernel/sched/sched.h

+1 −1
Original line number Diff line number Diff line
@@ -572,7 +572,7 @@ extern void sched_online_group(struct task_group *tg, 
extern void sched_destroy_group(struct task_group *tg);

extern void sched_release_group(struct task_group *tg);



extern void sched_move_task(struct task_struct *tsk);

extern void sched_move_task(struct task_struct *tsk, bool for_autogroup);



#ifdef CONFIG_FAIR_GROUP_SCHED

extern int sched_group_set_shares(struct task_group *tg, unsigned long shares);