netdev - Re: [PATCH] softirq: punt to ksoftirqd if __do

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Date:	Fri, 11 Apr 2014 20:02:25 -0400
From:	Rik van Riel <riel@...hat.com>
To:	David Miller <davem@...emloft.net>
CC:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	jbenc@...hat.com, peterz@...radead.org, tglx@...utronix.de,
	fweisbec@...il.com
Subject: Re: [PATCH] softirq: punt to ksoftirqd if __do_softirq recently looped

On 04/11/2014 04:33 PM, David Miller wrote:
> From: Rik van Riel <riel@...hat.com>
> Date: Thu, 10 Apr 2014 11:57:06 -0400
> 
>> @@ -330,7 +334,11 @@ void irq_enter(void)
>>  
>>  static inline void invoke_softirq(void)
>>  {
>> -	if (!force_irqthreads) {
>> +	/*
>> +	 * If force_irqthreads is set, or if we looped in __do_softirq this
>> +	 * jiffie, punt to ksoftirqd to prevent userland starvation.
>> +	 */
>> +	if (!force_irqthreads && this_cpu_read(softirq_looped) != jiffies) {
> 
> If we do this, which I'm not convinced of yet, I think we should use two
> jiffies as the cutoff.

I am not fully convinced, either.  This patch mostly just illustrates
the problem, and gives something that solves Jiri's immediate problem.

It is quite likely that there is a better way to solve the problem of:
1) softirq handling starving out userspace processing,
2) which could be solved by moving the userspace process elsewhere, and
3) shifting softirq processing to ksoftirqd

A working patch seems to be one of the better ways to start a
discussion, though.

If anybody has a nicer idea on how to solve the problem, I'd even be
willing to implement your idea, and give Jiri another patch to test :)

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