linux-kernel - Re: Re: [PATCH v4] ftrace: add a tracepoint for __raise_softirq

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Message-ID: <4A14E925.4010708@cn.fujitsu.com>
Date:	Thu, 21 May 2009 13:39:49 +0800
From:	Zhaolei <zhaolei@...fujitsu.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, fweisbec@...il.com,
	laijs@...fujitsu.com, Li Zefan <lizf@...fujitsu.com>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Subject: Re: Re: [PATCH v4] ftrace: add a tracepoint for	__raise_softirq_irqoff()

Ingo Molnar wrote:
> * Zhaolei <zhaolei@...fujitsu.com> wrote:
> 
>> * From: "Xiao Guangrong" <xiaoguangrong@...fujitsu.com>
>>> Steven Rostedt wrote:
>>>> On Thu, 14 May 2009, Mathieu Desnoyers wrote:
>>>>>> From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
>>>>>>
>>>>>> This patch is modified from Mathieu Desnoyers' patch. The original patch
>>>>>> can be found here: 
>>>>>> http://marc.info/?l=linux-kernel&m=123791201816245&w=2
>>>>>> This tracepoint can trace the time stamp when softirq action is raised. 
>>>>>>
>>>>>> Changelog for v1 -> v2: 
>>>>>> 1: Use TRACE_EVENT instead of DEFINE_TRACE
>>>>>> 2: Move the tracepoint from raise_softirq_irqoff() to
>>>>>>    __raise_softirq_irqoff()
>>>>>>
>>>>>> Changelog for v2 -> v3: 
>>>>>> Move the definition of __raise_softifq_irqoff() to .c file when
>>>>>> CONFIG_TRACEPOINTS is enabled, to avoid recursive includes
>>>>>>
>>>>>> Changelog for v3 -> v4: 
>>>>>> 1: Come back to v2, and use forward declarations to avoid
>>>>>>    recursive includes as Mathieu's suggestion
>>>>>> 2: Modifiy the tracepoint name
>>>>>> 3: Add comments for this tracepoint
>>>>>>
>>>>> This is a step in the right direction, but please see my email to Lai
>>>>> about the fact that this assumes correct and undocumented include
>>>>> dependencies in kernel/trace/events.c. Not explicitely stating the
>>>>> include dependencies is a build error waiting to happen.
>>>>>
>>>>> Including interrupt.h under a ifdef would allow keeping track of
>>>>> TRACE_EVENT specific build dependencies neatly on a per header basis.
>>>> This is all moot, the events.c file no longer exists and as not an issue.
>>>>
>>> As Steve's says, use ftrace in ftrace.h not in events.c now. 
>>> So, this mistake does not exist.
>>> Dose this patch has other error? I expect for your views.
>>>
>>> Thanks for your review, is great help to me. ;-) 
>> Hello,
>>
>> It seems Mathieu has no other comments on this patch now.
>> Ingo, what is your opinion on this patch?
> 
> There's a complication: this area of the softirq code needs fixes 
> (unrelated to tracing).

Hello, Ingo

Thanks for your reply.

Since it is unrelated to tracing, why not add this tracepoint first
and fix the softirq code later? The tracepoint won't add any burden
to the fix work.

Thanks
Zhaolei

>  
> This API:
> 
> inline void raise_softirq_irqoff(unsigned int nr)
> {
>         __raise_softirq_irqoff(nr);
> 
>         /*
>          * If we're in an interrupt or softirq, we're done
>          * (this also catches softirq-disabled code). We will
>          * actually run the softirq once we return from
>          * the irq or softirq.
>          *
>          * Otherwise we wake up ksoftirqd to make sure we
>          * schedule the softirq soon.
>          */
>         if (!in_interrupt())
>                 wakeup_softirqd();
> }
> 
> is broken with RT tasks (as recently reported to lkml), as when a 
> real-time task wakes up ksoftirqd (which has lower priority) it wont 
> execute and we starve softirq execution.
> 
> The proper solution would be to have a new API:
> 
> 	raise_softirq_check()
> 
> and to remove the wakeup_softirqd() hack from raise_softirq_irqoff() 
> - and put raise_softirq_check() to all places that use 
> raise_softirq*() from process context.
>  
> raise_softirq_check() would execute softirq handlers from process 
> context, if there's any pending ones. It has to be called outside of 
> bh critical sections - i.e. often a bit after the raise_softirq() 
> has been done.
> 
> __raise_softirq_irqoff() would be made private to kernel/softirq.c, 
> and we'd only have two public APIs to trigger softirqs: 
> raise_softirq() and raise_softirq_irqoff(). Both just set the 
> pending flag and dont do any wakeup.
> 
> As a side-effect of these fixes, the tracepoints will be sorted out 
> as well - there wont be any need to hack into 
> __raise_softirq_irqoff().
> 
> 	Ingo
> 


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