netdev - Re: question about softirqs

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Date:	Tue, 12 May 2009 09:18:19 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>, linuxppc-dev@...abs.org,
	paulus@...ba.org, netdev@...r.kernel.org
Subject: Re: question about softirqs

Ingo Molnar wrote:
> * Chris Friesen <cfriesen@...tel.com> wrote:

>>I think I see a possible problem with this. Suppose I have a 
>>SCHED_FIFO task spinning on recvmsg() with MSG_DONTWAIT set. Under 
>>the scenario above, schedule() would re-run the spinning task 
>>rather than ksoftirqd, thus preventing any incoming packets from 
>>being sent up the stack until we get a real hardware 
>>interrupt--which could be a whole jiffy if interrupt mitigation is 
>>enabled in the net device.

>>DaveM pointed out that if we're doing transmits we're likely to 
>>hit local_bh_enable(), which would process the softirq work.  
>>However, I think we may still have a problem in the above rx-only 
>>scenario--or is it too contrived to matter?

> This could occur, and the problem is really that task priorities do 
> not extend across softirq work processing.
> 
> This could occur in ordinary SCHED_OTHER tasks as well, if the 
> softirq is bounced to ksoftirqd - which it only should be if there's 
> serious softirq overload - or, as you describe it above, if the 
> softirq is raised in process context:

One of the reasons I brought up this issue is that there is a lot of
documentation out there that says "softirqs will be processed on return
from a syscall".  The fact that it actually depends on the scheduler
parameters of the task issuing the syscall isn't ever mentioned.

In fact, "Documentation/DocBook/kernel-hacking.tmpl" in the kernel
source still has the following:

    Whenever a system call is about to return to userspace, or a
    hardware interrupt handler exits, any 'software interrupts'
    which are marked pending (usually by hardware interrupts) are
    run (<filename>kernel/softirq.c</filename>).

If anyone is looking at changing this code, it might be good to ensure
that at least the kernel docs are updated.

Chris
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