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

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Date:   Thu, 1 Oct 2020 16:47:31 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Alex Belits <abelits@...vell.com>
Cc:     "rostedt@...dmis.org" <rostedt@...dmis.org>,
        Prasun Kapoor <pkapoor@...vell.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "will@...nel.org" <will@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 11/13] task_isolation: net: don't flush backlog on
 CPUs running isolated tasks

On Wed, Jul 22, 2020 at 02:58:24PM +0000, Alex Belits wrote:
> From: Yuri Norov <ynorov@...vell.com>
> 
> 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 <ynorov@...vell.com>
> [abelits@...vell.com: use safe task_isolation_on_cpu() implementation]
> Signed-off-by: Alex Belits <abelits@...vell.com>
> ---
>  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.