Re: [PATCH 1/4] tracing: Use the perf recursion protection from trace event

Frederic Weisbecker
2009-11-22T16:37:15Z

On Sun, Nov 22, 2009 at 12:00:14PM +0100, Peter Zijlstra wrote:
> On Sun, 2009-11-22 at 05:21 +0100, Frederic Weisbecker wrote:
> > diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> > index 718fa93..aba8227 100644
> > --- a/kernel/perf_event.c
> > +++ b/kernel/perf_event.c
> > @@ -3880,34 +3880,42 @@ static void perf_swevent_ctx_event(struct perf_event_context *ctx,
> > }
> > }
> >
> > -static int *perf_swevent_recursion_context(struct perf_cpu_context *cpuctx)
> > +/*
> > + * Must be called with preemption disabled
> > + */
> > +int perf_swevent_get_recursion_context(int **recursion)
> > {
> > + struct perf_cpu_context *cpuctx = &__get_cpu_var(perf_cpu_context);
> > +
> > if (in_nmi())
> > - return &cpuctx->recursion[3];
> > + *recursion = &cpuctx->recursion[3];
> > + else if (in_irq())
> > + *recursion = &cpuctx->recursion[2];
> > + else if (in_softirq())
> > + *recursion = &cpuctx->recursion[1];
> > + else
> > + *recursion = &cpuctx->recursion[0];
> >
> > - if (in_irq())
> > - return &cpuctx->recursion[2];
> > + if (**recursion)
> > + return -1;
> >
> > - if (in_softirq())
> > - return &cpuctx->recursion[1];
> > + (**recursion)++;
> >
> > - return &cpuctx->recursion[0];
> > + return 0;
> > }
>
> You lost the barrier();

I thought that putting that into a function would already do
the trick but that is notwithstanding the fact it could be inlined
as you said in another message.

Ok, I'll add them back.

> > -static void do_perf_sw_event(enum perf_type_id type, u32 event_id,
> > - u64 nr, int nmi,
> > - struct perf_sample_data *data,
> > - struct pt_regs *regs)
> > +void perf_swevent_put_recursion_context(int *recursion)
> > {
> > - struct perf_cpu_context *cpuctx = &get_cpu_var(perf_cpu_context);
> > - int *recursion = perf_swevent_recursion_context(cpuctx);
> > - struct perf_event_context *ctx;
> > -
> > - if (*recursion)
> > - goto out;
> > + (*recursion)--;
> > +}
>
> And here as well.
>
> Furthermore, its much cleaner if you simply use
> get_cpu_var(perf_cpu_context) in get_recursion_context, and
> put_cpu_var() in put_recursion_context.
>
> That way you put the preempt_disable where it belongs, instead of:
>
> > +static void do_perf_sw_event(enum perf_type_id type, u32 event_id,
> > + u64 nr, int nmi,
> > + struct perf_sample_data *data,
> > + struct pt_regs *regs)
> > +{
> > + int *recursion;
> > +
> > + preempt_disable();
> > +
> > + if (perf_swevent_get_recursion_context(&recursion))
> > + goto out;
> > +
> > + __do_perf_sw_event(type, event_id, nr, nmi, data, regs);
> >
> > + perf_swevent_put_recursion_context(recursion);
> > out:
> > - put_cpu_var(perf_cpu_context);
> > + preempt_enable();
> > }
>
> mucking about like here, also it cleans up the code in that you don't
> have to drag that recursion variable around like so.
>

That's cleaner but adds an unnecessary overhead in the trace event
path. We already disable the interrupts there. That's why I preferred
to let the caller decide.