linux-kernel - Re: [RFC PATCH 0/2] net: threadable napi poll loop

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Date:	Thu, 12 May 2016 14:05:18 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Paolo Abeni <pabeni@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Eric Dumazet <edumazet@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jiri@...lanox.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Alexander Duyck <aduyck@...antis.com>,
	Tom Herbert <tom@...bertland.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/2] net: threadable napi poll loop

On Thu, 2016-05-12 at 22:58 +0200, Paolo Abeni wrote:
> On Thu, 2016-05-12 at 13:49 -0700, Eric Dumazet wrote:
> > On Thu, 2016-05-12 at 22:07 +0200, Paolo Abeni wrote:
> > 
> > > > > static inline bool ksoftirqd_running(void)
> > > > > {
> > > > > 	return __this_cpu_read(ksoftirqd)->state == TASK_RUNNING;
> > > 
> > > here something like:
> > > 
> > > 	struct task_struct *tsk = __this_cpu_read(ksoftirqd);
> > >         return tsk && (tsk->state == TASK_RUNNING);
> > > 
> > > is needed since __this_cpu_read(ksoftirqd) can be NULL on boot.
> > 
> > Indeed I've seen this but only when backporting to an older linux kernel
> > this morning.
> > 
> > Have you got this with current linux kernel ?
> 
> Yes, on net-next updated to 
> 
> commit c66b2581123cd1527b6a084f39e9271cb02673b7
> Author: Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
> Date:   Sat May 7 14:09:01 2016 -0700
> 
>     sh_eth: reuse sh_eth_chip_reset()
> 

Yeah, I was unsure if the same test in wakeup_softirqd() was still
relevant today.

static void wakeup_softirqd(void)
{
        /* Interrupts are disabled: no need to stop preemption */
        struct task_struct *tsk = __this_cpu_read(ksoftirqd);

        if (tsk && tsk->state != TASK_RUNNING)
                wake_up_process(tsk);
}


I guess we could avoid the NULL test if all these per_cpu var where
pointing to a dummy task_struct at boot time, before they are properly
allocated.