Subject: Re: [RFC PATCH v3 1/5] irq: add tracepoint to softirq_raise

From: Koki Sanagi <[email-masked]>
Date: Thu, 22 Jul 2010 17:41:37 +0900

(2010/07/21 20:14), Neil Horman wrote:
> On Wed, Jul 21, 2010 at 03:57:05PM +0900, Koki Sanagi wrote:
>> (2010/07/20 20:04), Neil Horman wrote:
>>> On Tue, Jul 20, 2010 at 09:45:31AM +0900, Koki Sanagi wrote:
>>>> From: Lai Jiangshan <[email-masked]>
>>>>
>>>> Add a tracepoint for tracing when softirq action is raised.
>>>>
>>>> It and the existed tracepoints complete softirq's tracepoints:
>>>> softirq_raise, softirq_entry and softirq_exit.
>>>>
>>>> And when this tracepoint is used in combination with
>>>> the softirq_entry tracepoint we can determine
>>>> the softirq raise latency.
>>>>
>>>> Signed-off-by: Lai Jiangshan <[email-masked]>
>>>> Acked-by: Mathieu Desnoyers <[email-masked]>
>>>> Acked-by: Frederic Weisbecker <[email-masked]>
>>>>
>>>> [ factorize softirq events with DECLARE_EVENT_CLASS ]
>>>> Signed-off-by: Koki Sanagi <[email-masked]>
>>>> ---
>>>>  include/linux/interrupt.h  |    8 +++++-
>>>>  include/trace/events/irq.h |   57 ++++++++++++++++++++++++++-----------------
>>>>  kernel/softirq.c           |    4 +-
>>>>  3 files changed, 43 insertions(+), 26 deletions(-)
>>>>
>>>> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
>>>> index c233113..1cb5726 100644
>>>> --- a/include/linux/interrupt.h
>>>> +++ b/include/linux/interrupt.h
>>>> @@ -18,6 +18,7 @@
>>>>  #include <asm/atomic.h>
>>>>  #include <asm/ptrace.h>
>>>>  #include <asm/system.h>
>>>> +#include <trace/events/irq.h>
>>>>  
>>>>  /*
>>>>   * These correspond to the IORESOURCE_IRQ_* defines in
>>>> @@ -402,7 +403,12 @@ asmlinkage void do_softirq(void);
>>>>  asmlinkage void __do_softirq(void);
>>>>  extern void open_softirq(int nr, void (*action)(struct softirq_action *));
>>>>  extern void softirq_init(void);
>>>> -#define __raise_softirq_irqoff(nr) do { or_softirq_pending(1UL << (nr)); } while (0)
>>>> +static inline void __raise_softirq_irqoff(unsigned int nr)
>>>> +{
>>>> +	trace_softirq_raise(nr);
>>>> +	or_softirq_pending(1UL << nr);
>>>> +}
>>>> +
>>> We already have tracepoints in irq_enter and irq_exit.  If the goal here is to
>>> detect latency during packet processing, cant the delta in time between those
>>> two points be used to determine interrupt handling latency?
>>
>> Certainly, the time between irq_entry and irq_exit is not directly related to
>> latency during packet processing. But it's indirectly related it.
>> Because softirq_entry isn't passed until irq exits and softirq_entry time is
>> related to packet processing latency. So I show it as a reference.
>>
> Its not directly related no, but look at it, the amount of processing between
> irq_exit and softirq_entry is minimal.  The information you are trying to
> extract by computing the delta from irq_entry to softirq_entry is almost exactly
> the same as that from irq_entry to irq_exit.  For that matter, since you're
> trying to guage lantency for packet processing, I expect you could get the same
> delta by measuring irq_entry to napi_poll tracepoint time, and save the hassle
> of needing to filter on softirq processing that doesn't relate to packet
> processing.

Yeah, to determine interrput latency, we need either one irq_exit or
softirq_entry, not both.
And I think softirq_entry should be left because there is a possibility that
softirq isn't triggered immidiately after irq_exit.
softirq_exit isn't needed because it is not related to packet processing.
softirq_raise is needed because it connects irq_entry and softirq_entry but
there is no need to show it. Currently, my idea is like the following.

irq_entry(+0.000000msec,irq=77:eth3)
    |
softirq_entry(+0.003562msec)
    |
    |---netif_receive_skb(+0.006279msec,len=100)
    |            |
    |   skb_copy_datagram_iovec(+0.038778msec, 2285:sshd)
    |
napi_poll_exit(+0.017160msec, eth3)

> 
>>>
>>>
>>>>  extern void raise_softirq_irqoff(unsigned int nr);
>>>>  extern void raise_softirq(unsigned int nr);
>>>>  extern void wakeup_softirqd(void);
>>>> diff --git a/include/trace/events/irq.h b/include/trace/events/irq.h
>>>> index 0e4cfb6..717744c 100644
>>>> --- a/include/trace/events/irq.h
>>>> +++ b/include/trace/events/irq.h
>>>> @@ -5,7 +5,9 @@
>>>>  #define _TRACE_IRQ_H
>>>>  
>>>>  #include <linux/tracepoint.h>
>>>> -#include <linux/interrupt.h>
>>>> +
>>>> +struct irqaction;
>>>> +struct softirq_action;
>>>>  
>>>>  #define softirq_name(sirq) { sirq##_SOFTIRQ, #sirq }
>>>>  #define show_softirq_name(val)				\
>>>> @@ -84,56 +86,65 @@ TRACE_EVENT(irq_handler_exit,
>>>>  
>>>>  DECLARE_EVENT_CLASS(softirq,
>>>>  
>>>> -	TP_PROTO(struct softirq_action *h, struct softirq_action *vec),
>>>> +	TP_PROTO(unsigned int nr),
>>>>  
>>>> -	TP_ARGS(h, vec),
>>>> +	TP_ARGS(nr),
>>>>  
>>>>  	TP_STRUCT__entry(
>>>> -		__field(	int,	vec			)
>>>> +		__field(	unsigned int,	vec	)
>>>>  	),
>>>>  
>>>>  	TP_fast_assign(
>>>> -		__entry->vec = (int)(h - vec);
>>>> +		__entry->vec	= nr;
>>>>  	),
>>>>  
>>>>  	TP_printk("vec=%d [action=%s]", __entry->vec,
>>>> -		  show_softirq_name(__entry->vec))
>>>> +		show_softirq_name(__entry->vec))
>>>> +);
>>>> +
>>>> +/**
>>>> + * softirq_raise - called immediately when a softirq is raised
>>>> + * @nr: softirq vector number
>>>> + *
>>>> + * Tracepoint for tracing when softirq action is raised.
>>>> + * Also, when used in combination with the softirq_entry tracepoint
>>>> + * we can determine the softirq raise latency.
>>>> + */
>>>> +DEFINE_EVENT(softirq, softirq_raise,
>>>> +
>>>> +	TP_PROTO(unsigned int nr),
>>>> +
>>>> +	TP_ARGS(nr)
>>>>  );
>>>>  
>>>>  /**
>>>>   * softirq_entry - called immediately before the softirq handler
>>>> - * @h: pointer to struct softirq_action
>>>> - * @vec: pointer to first struct softirq_action in softirq_vec array
>>>> + * @nr: softirq vector number
>>>>   *
>>>> - * The @h parameter, contains a pointer to the struct softirq_action
>>>> - * which has a pointer to the action handler that is called. By subtracting
>>>> - * the @vec pointer from the @h pointer, we can determine the softirq
>>>> - * number. Also, when used in combination with the softirq_exit tracepoint
>>>> + * Tracepoint for tracing when softirq action starts.
>>>> + * Also, when used in combination with the softirq_exit tracepoint
>>>>   * we can determine the softirq latency.
>>>>   */
>>>>  DEFINE_EVENT(softirq, softirq_entry,
>>>>  
>>>> -	TP_PROTO(struct softirq_action *h, struct softirq_action *vec),
>>>> +	TP_PROTO(unsigned int nr),
>>>>  
>>>> -	TP_ARGS(h, vec)
>>>> +	TP_ARGS(nr)
>>>>  );
>>>>  
>>>>  /**
>>>>   * softirq_exit - called immediately after the softirq handler returns
>>>> - * @h: pointer to struct softirq_action
>>>> - * @vec: pointer to first struct softirq_action in softirq_vec array
>>>> + * @nr: softirq vector number
>>>>   *
>>>> - * The @h parameter contains a pointer to the struct softirq_action
>>>> - * that has handled the softirq. By subtracting the @vec pointer from
>>>> - * the @h pointer, we can determine the softirq number. Also, when used in
>>>> - * combination with the softirq_entry tracepoint we can determine the softirq
>>>> - * latency.
>>>> + * Tracepoint for tracing when softirq action ends.
>>>> + * Also, when used in combination with the softirq_entry tracepoint
>>>> + * we can determine the softirq latency.
>>>>   */
>>>>  DEFINE_EVENT(softirq, softirq_exit,
>>>>  
>>>> -	TP_PROTO(struct softirq_action *h, struct softirq_action *vec),
>>>> +	TP_PROTO(unsigned int nr),
>>>>  
>>>> -	TP_ARGS(h, vec)
>>>> +	TP_ARGS(nr)
>>>>  );
>>>>  
>>>>  #endif /*  _TRACE_IRQ_H */
>>>> diff --git a/kernel/softirq.c b/kernel/softirq.c
>>>> index 825e112..6790599 100644
>>>> --- a/kernel/softirq.c
>>>> +++ b/kernel/softirq.c
>>>> @@ -215,9 +215,9 @@ restart:
>>>>  			int prev_count = preempt_count();
>>>>  			kstat_incr_softirqs_this_cpu(h - softirq_vec);
>>>>  
>>>> -			trace_softirq_entry(h, softirq_vec);
>>>> +			trace_softirq_entry(h - softirq_vec);
>>>>  			h->action(h);
>>>> -			trace_softirq_exit(h, softirq_vec);
>>>> +			trace_softirq_exit(h - softirq_vec);
>>>
>>> You're loosing information here by reducing the numbers of parameters in this
>>> tracepoint.  How many other tracepoint scripts rely on having both pointers
>>> handy?  Why not just do the pointer math inside your tracehook instead?
>>
>> In __raise_softirq_irqoff macro there is no method to refer softirq_vec, so it
>> can't use softirq DECLARE_EVENT_CLASS as is.
>> Currently,  there is no script using softirq_entry or softirq_exit.
>>
> That shouldn't matter, just pass in NULL for softirq_vec in
> __raise_softirq_irqoff as the second argument to the trace function.  You may
> need to fix up the class definition so that the assignment or printk doesn't try
> to dereference that pointer when its NULL, but thats easy enough, and it avoids
> breaking any other perf scripts floating out there.
> Neil
> 
>> Thanks,
>> Koki Sanagi.
>>
>>>
>>>>  			if (unlikely(prev_count != preempt_count())) {
>>>>  				printk(KERN_ERR "huh, entered softirq %td %s %p"
>>>>  				       "with preempt_count %08x,"
>>>>
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>>>
>>>
>>
>>
>>
> 
>