Re: [PATCH] posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Oct 16, 2024 at 04:59:08PM -0700, Benjamin Segall wrote:
> 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 <bsegall@google.com>
> Cc: stable@vger.kernel.org
> ---
>  kernel/fork.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index df8e4575ff01..b57cd63cfcd1 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -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.

So something like this?

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 72744638c5b0..99c9c5a7252a 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -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,
diff --git a/kernel/fork.c b/kernel/fork.c
index 89ceb4a68af2..6fa9fe62e01e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -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);