Re: [PATCH v4] ftrace: add a tracepoint for __raise_softirq_irqoff()

Ingo Molnar
2009-07-03T09:45:20Z

* Lai Jiangshan <laijs@cn.fujitsu.com> wrote:

> Ingo Molnar wrote:
> > * Zhaolei <zhaolei@cn.fujitsu.com> wrote:
> >
> >> * From: "Xiao Guangrong" <xiaoguangrong@cn.fujitsu.com>
> >>> Steven Rostedt wrote:
> >>>> On Thu, 14 May 2009, Mathieu Desnoyers wrote:
> >>>>>> From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.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).
> >
> > 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
> >
>
> Hi, Ingo
>
> Could you give me the .config file that this bug occurs?

I havent reproduced this - it was reported in a recent thread on
lkml. I dont have an URI for it - does anyone on the Cc: remember
the details?

Ingo