Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/deadline: fix inactive_task_timer splat with CONFIG_PREEMPT_RT | Date | Wed, 18 Jan 2023 15:57:38 +0000 |
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On 10/01/23 14:27, Paul E. McKenney wrote: > On Tue, Jan 10, 2023 at 05:52:03PM -0300, Wander Lairson Costa wrote: >> On Mon, Jan 9, 2023 at 10:40 PM Paul E. McKenney <paulmck@kernel.org> wrote: >> > >> > On Wed, Jan 04, 2023 at 03:17:01PM -0300, Wander Lairson Costa wrote: >> > > inactive_task_timer() executes in interrupt (atomic) context. It calls >> > > put_task_struct(), which indirectly acquires sleeping locks under >> > > PREEMPT_RT. >> > > >> > > Below is an example of a splat that happened in a test environment: >> > > >> > > CPU: 1 PID: 2848 Comm: life Kdump: loaded Tainted: G W --------- >> > > Hardware name: HP ProLiant DL388p Gen8, BIOS P70 07/15/2012 >> > > Call Trace: >> > > dump_stack_lvl+0x57/0x7d >> > > mark_lock_irq.cold+0x33/0xba >> > > ? stack_trace_save+0x4b/0x70 >> > > ? save_trace+0x55/0x150 >> > > mark_lock+0x1e7/0x400 >> > > mark_usage+0x11d/0x140 >> > > __lock_acquire+0x30d/0x930 >> > > lock_acquire.part.0+0x9c/0x210 >> > > ? refill_obj_stock+0x3d/0x3a0 >> > > ? rcu_read_lock_sched_held+0x3f/0x70 >> > > ? trace_lock_acquire+0x38/0x140 >> > > ? lock_acquire+0x30/0x80 >> > > ? refill_obj_stock+0x3d/0x3a0 >> > > rt_spin_lock+0x27/0xe0 >> > > ? refill_obj_stock+0x3d/0x3a0 >> > > refill_obj_stock+0x3d/0x3a0 >> > > ? inactive_task_timer+0x1ad/0x340 >> > > kmem_cache_free+0x357/0x560 >> > > inactive_task_timer+0x1ad/0x340 >> > > ? switched_from_dl+0x2d0/0x2d0 >> > > __run_hrtimer+0x8a/0x1a0 >> > > __hrtimer_run_queues+0x91/0x130 >> > > hrtimer_interrupt+0x10f/0x220 >> > > __sysvec_apic_timer_interrupt+0x7b/0xd0 >> > > sysvec_apic_timer_interrupt+0x4f/0xd0 >> > > ? asm_sysvec_apic_timer_interrupt+0xa/0x20 >> > > asm_sysvec_apic_timer_interrupt+0x12/0x20 >> > > RIP: 0033:0x7fff196bf6f5 >> > > >> > > Instead of calling put_task_struct() directly, we defer it using >> > > call_rcu(). A more natural approach would use a workqueue, but since >> > > in PREEMPT_RT, we can't allocate dynamic memory from atomic context, >> > > the code would become more complex because we would need to put the >> > > work_struct instance in the task_struct and initialize it when we >> > > allocate a new task_struct. >> > > >> > > Signed-off-by: Wander Lairson Costa <wander@redhat.com> >> > > Cc: Paul McKenney <paulmck@kernel.org> >> > > Cc: Thomas Gleixner <tglx@linutronix.de> >> > > --- >> > > kernel/sched/build_policy.c | 1 + >> > > kernel/sched/deadline.c | 24 +++++++++++++++++++++++- >> > > 2 files changed, 24 insertions(+), 1 deletion(-) >> > > >> > > diff --git a/kernel/sched/build_policy.c b/kernel/sched/build_policy.c >> > > index d9dc9ab3773f..f159304ee792 100644 >> > > --- a/kernel/sched/build_policy.c >> > > +++ b/kernel/sched/build_policy.c >> > > @@ -28,6 +28,7 @@ >> > > #include <linux/suspend.h> >> > > #include <linux/tsacct_kern.h> >> > > #include <linux/vtime.h> >> > > +#include <linux/rcupdate.h> >> > > >> > > #include <uapi/linux/sched/types.h> >> > > >> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c >> > > index 9ae8f41e3372..ab9301d4cc24 100644 >> > > --- a/kernel/sched/deadline.c >> > > +++ b/kernel/sched/deadline.c >> > > @@ -1405,6 +1405,13 @@ static void update_curr_dl(struct rq *rq) >> > > } >> > > } >> > > >> > > +static void delayed_put_task_struct(struct rcu_head *rhp) >> > > +{ >> > > + struct task_struct *task = container_of(rhp, struct task_struct, rcu); >> > > + >> > > + __put_task_struct(task); >> > >> > Please note that BH is disabled here. Don't you therefore >> > need to schedule a workqueue handler? Perhaps directly from >> > inactive_task_timer(), or maybe from this point. If the latter, one >> > way to skip the extra step is to use queue_rcu_work(). >> > >> >> My initial work was using a workqueue [1,2]. However, I realized I >> could reach a much simpler code with call_rcu(). >> I am afraid my ignorance doesn't allow me to get your point. Does >> disabling softirq imply atomic context? > > Given that this problem occurred in PREEMPT_RT, I am assuming that the > appropriate definition of "atomic context" is "cannot call schedule()". > And you are in fact not permitted to call schedule() from a bh-disabled > region. > > This also means that you cannot acquire a non-raw spinlock in a > bh-disabled region of code in a PREEMPT_RT kernel, because doing > so can invoke schedule. >
But per the PREEMPT_RT lock "replacement", non-raw spinlocks end up invoking schedule_rtlock(), which should be safe vs BH disabled (local_lock() + rcu_read_lock()):
6991436c2b5d ("sched/core: Provide a scheduling point for RT locks")
Unless I'm missing something else?
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