Subject: Re: [PATCH v4 11/13] task_isolation: net: don't flush backlog on CPUs running isolated tasks

From: Frederic Weisbecker <[email-masked]>
Date: Thu, 1 Oct 2020 16:47:31 +0200

On Wed, Jul 22, 2020 at 02:58:24PM +0000, Alex Belits wrote:
> From: Yuri Norov <[email-masked]>
> 
> If CPU runs isolated task, there's no any backlog on it, and
> so we don't need to flush it.

What guarantees that we have no backlog on it?

> Currently flush_all_backlogs()
> enqueues corresponding work on all CPUs including ones that run
> isolated tasks. It leads to breaking task isolation for nothing.
> 
> In this patch, backlog flushing is enqueued only on non-isolated CPUs.
> 
> Signed-off-by: Yuri Norov <[email-masked]>
> [[email-masked]: use safe task_isolation_on_cpu() implementation]
> Signed-off-by: Alex Belits <[email-masked]>
> ---
>  net/core/dev.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 90b59fc50dc9..83a282f7453d 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -74,6 +74,7 @@
>  #include <linux/cpu.h>
>  #include <linux/types.h>
>  #include <linux/kernel.h>
> +#include <linux/isolation.h>
>  #include <linux/hash.h>
>  #include <linux/slab.h>
>  #include <linux/sched.h>
> @@ -5624,9 +5625,13 @@ static void flush_all_backlogs(void)
>  
>  	get_online_cpus();
>  
> -	for_each_online_cpu(cpu)
> +	smp_rmb();

What is it ordering?

> +	for_each_online_cpu(cpu) {
> +		if (task_isolation_on_cpu(cpu))
> +			continue;
>  		queue_work_on(cpu, system_highpri_wq,
>  			      per_cpu_ptr(&flush_works, cpu));
> +	}
>  
>  	for_each_online_cpu(cpu)
>  		flush_work(per_cpu_ptr(&flush_works, cpu));

Thanks.