Re: [patch V2 5/9] tick/sched: Prevent false positive softirq pending warnings on RT

[Frederic Weisbecker]

On Fri, Dec 04, 2020 at 06:01:56PM +0100, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> On RT a task which has soft interrupts disabled can block on a lock and
> schedule out to idle while soft interrupts are pending. This triggers the
> warning in the NOHZ idle code which complains about going idle with pending
> soft interrupts. But as the task is blocked soft interrupt processing is
> temporarily blocked as well which means that such a warning is a false
> positive.
> 
> To prevent that check the per CPU state which indicates that a scheduled
> out task has soft interrupts disabled.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  include/linux/bottom_half.h |    6 ++++++
>  kernel/softirq.c            |   15 +++++++++++++++
>  kernel/time/tick-sched.c    |    2 +-
>  3 files changed, 22 insertions(+), 1 deletion(-)
> 
> --- a/include/linux/bottom_half.h
> +++ b/include/linux/bottom_half.h
> @@ -32,4 +32,10 @@ static inline void local_bh_enable(void)
>  	__local_bh_enable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);
>  }
>  
> +#ifdef CONFIG_PREEMPT_RT
> +extern bool local_bh_blocked(void);
> +#else
> +static inline bool local_bh_blocked(void) { return false; }
> +#endif
> +
>  #endif /* _LINUX_BH_H */
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -138,6 +138,21 @@ static DEFINE_PER_CPU(struct softirq_ctr
>  	.lock	= INIT_LOCAL_LOCK(softirq_ctrl.lock),
>  };
>  
> +/**
> + * local_bh_blocked() - Check for idle whether BH processing is blocked
> + *
> + * Returns false if the per CPU softirq::cnt is 0 otherwise true.
> + *
> + * This is invoked from the idle task to guard against false positive
> + * softirq pending warnings, which would happen when the task which holds
> + * softirq_ctrl::lock was the only running task on the CPU and blocks on
> + * some other lock.
> + */
> +bool local_bh_blocked(void)
> +{
> +	return this_cpu_read(softirq_ctrl.cnt) != 0;

__this_cpu_read()

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>