linux-kernel - Re: [PATCH] posix-cpu-timers: clear TICK_DEP_BIT_POSIX

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Message-ID: <xm26zfn2bifv.fsf@google.com>
Date: Thu, 17 Oct 2024 13:09:08 -0700
From: Benjamin Segall <bsegall@...gle.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org,  Anna-Maria Behnsen
 <anna-maria@...utronix.de>,  Chris Metcalf <cmetcalf@...hip.com>,  Thomas
 Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] posix-cpu-timers: clear TICK_DEP_BIT_POSIX_TIMER on clone

Frederic Weisbecker <frederic@...nel.org> writes:

> 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@...gle.com>
>> Cc: stable@...r.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?

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.

>
> 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);