posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

When we clone a new thread, we do not inherit its posix_cputimers, and clear them with posix_cputimers_init. However, this does not clear the tick dependency it creates in tsk->tick_dep_mask, and the handler does not reach the code to clear the dependency if there were no timers to begin with.

Thus if a thread has a cputimer running before cloneing/forking, that hierarchy will prevent nohz_full unless they create a cputimer of their own.

Process-wide timers do not have this problem because fork does not copy signal_struct as a baseline, it creates one from scratch.

Fixes b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")

Signed-off-by Ben Segall <[email protected]>

Cc [email protected]

---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

kernel/fork.c +0 -0

index df8e4575ff01..b57cd63cfcd1 100644

Quoted 29 lines · Benjamin Segall

When we clone a new thread, we do not inherit its posix_cputimers, and clear them with posix_cputimers_init. However, this does not clear the tick dependency it creates in tsk->tick_dep_mask, and the handler does not reach the code to clear the dependency if there were no timers to begin with.

Thus if a thread has a cputimer running before cloneing/forking, that hierarchy will prevent nohz_full unless they create a cputimer of their own.

Process-wide timers do not have this problem because fork does not copy signal_struct as a baseline, it creates one from scratch.

Fixes b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")

Signed-off-by Ben Segall <[email protected]>

Cc [email protected]

---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

kernel/fork.c +0 -0

index df8e4575ff01..b57cd63cfcd1 100644
@@ -2290,10 +2290,11 @@ __latent_entropy struct task_struct *copy_process(
 
 	task_io_accounting_init(&p->ioac);
 	acct_clear_integrals(p);
 

kernel/fork.c __latent_entropy struct task_struct *copy_process( · line 2295

Quoted 2 lines

kernel/fork.c +1 -0

 	posix_cputimers_init(&p->posix_cputimers);
+	tick_dep_clear_task(p, TICK_DEP_BIT_POSIX_TIMER);

Yes but we don't need the expensive atomic_fetch_andnot(). Also more generally the task tick dependency should be 0 upon creation.

So something like this?

include/linux/tick.h +8 -0

index 72744638c5b0..99c9c5a7252a 100644
@@ -251,12 +251,19 @@ static inline void tick_dep_set_task(struct task_struct *tsk,
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_set_task(tsk, bit);
 }
+
 static inline void tick_dep_clear_task(struct task_struct *tsk,
 				       enum tick_dep_bits bit)
 {
 	if (tick_nohz_full_enabled())
 		tick_nohz_dep_clear_task(tsk, bit);
 }
+
+static inline void tick_dep_init_task(struct task_struct *tsk)
+{
+	atomic_set(&tsk->tick_dep_mask, 0);
+}
+
 static inline void tick_dep_set_signal(struct task_struct *tsk,
 				       enum tick_dep_bits bit)
 {

@@ -290,6 +297,7 @@ static inline void tick_dep_set_task(struct task_struct *tsk,
 				     enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_task(struct task_struct *tsk,
 				       enum tick_dep_bits bit) { }
+static inline void tick_dep_init_task(struct task_struct *tsk) { }
 static inline void tick_dep_set_signal(struct task_struct *tsk,
 				       enum tick_dep_bits bit) { }
 static inline void tick_dep_clear_signal(struct signal_struct *signal,

kernel/fork.c +2 -0

index 89ceb4a68af2..6fa9fe62e01e 100644
@@ -105,6 +105,7 @@
 #include <linux/rseq.h>
 #include <uapi/linux/pidfd.h>
 #include <linux/pidfs.h>
+#include <linux/tick.h>
 
 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>

@@ -2292,6 +2293,7 @@ __latent_entropy struct task_struct *copy_process(
 	acct_clear_integrals(p);
 
 	posix_cputimers_init(&p->posix_cputimers);
+	tick_dep_init_task(p);
 
 	p->io_context = NULL;
 	audit_set_context(p, NULL);

Quoted 36 lines · Frederic Weisbecker

Quoted 25 lines · Benjamin Segall

When we clone a new thread, we do not inherit its posix_cputimers, and clear them with posix_cputimers_init. However, this does not clear the tick dependency it creates in tsk->tick_dep_mask, and the handler does not reach the code to clear the dependency if there were no timers to begin with.

Thus if a thread has a cputimer running before cloneing/forking, that hierarchy will prevent nohz_full unless they create a cputimer of their own.

Process-wide timers do not have this problem because fork does not copy signal_struct as a baseline, it creates one from scratch.

Fixes b78783000d5c ("posix-cpu-timers: Migrate to use new tick dependency mask model")

Signed-off-by Ben Segall <[email protected]>

Cc [email protected]

---
 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

kernel/fork.c +0 -0

index df8e4575ff01..b57cd63cfcd1 100644

kernel/fork.c __latent_entropy struct task_struct *copy_process( · line 2295

Quoted 6 lines

kernel/fork.c +1 -0

@@ -2290,10 +2290,11 @@ __latent_entropy struct task_struct *copy_process(
 
 	task_io_accounting_init(&p->ioac);
 	acct_clear_integrals(p);
 
 	posix_cputimers_init(&p->posix_cputimers);
+	tick_dep_clear_task(p, TICK_DEP_BIT_POSIX_TIMER);

Yes but we don't need the expensive atomic_fetch_andnot(). Also more generally the task tick dependency should be 0 upon creation.

Quoted 1 line

So something like this?

Yeah, the only other uses are contained in rcu_do_batch and rcutorture tests, which won't end up here anyways.

Up to you if you want to send this or I can send out a v2.