Re: [RFC PATCH] irqchip/gic-v3: Make gic_handle_irq() notrace

[Zenghui Yu]

Hi Marc,

On 2019/3/29 21:58, Marc Zyngier wrote:
> Hi Zenghui,
>
> On 29/03/2019 13:23, Zenghui Yu wrote:
> > Enable pseudo NMI together with function_graph tracer, will lead
> > the system to a hang. This is easy to reproduce,
> >
> >    1) Set "irqchip.gicv3_pseudo_nmi=1" on the kernel command line
> >    2) echo function_graph > /sys/kernel/debug/tracing/current_tracer
> >
> > This patch (RFC) set gic_handle_irq() as notrace and it seems works
> > fine now. But I have no idea about what the issue is exactly, and
> > you can regard this patch as a report then :)
> >
> > Can someone give a look at it and provide some explanations ?
> >
> > Thanks!
> >
> > Cc: Julien Thierry <julien.thierry@xxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > ---
> >   drivers/irqchip/irq-gic-v3.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> > index 15e55d3..8d0c25f 100644
> > --- a/drivers/irqchip/irq-gic-v3.c
> > +++ b/drivers/irqchip/irq-gic-v3.c
> > @@ -487,7 +487,7 @@ static inline void gic_handle_nmi(u32 irqnr, struct pt_regs *regs)
> >   		gic_deactivate_unhandled(irqnr);
> >   }
> >   
> > -static asmlinkage void __exception_irq_entry gic_handle_irq(struct pt_regs *regs)
> > +static asmlinkage notrace void __exception_irq_entry gic_handle_irq(struct pt_regs *regs)
> >   {
> >   	u32 irqnr;
> >   
>
>
> That's interesting. Do you have any out of tree patch that actually
> makes use of the pseudo-NMI feature? Without those patches, the
> behaviour should stay unchanged.

I am at commit 1a9df9e29c2afecf6e3089442d429b377279ca3c. No more
patches, and this is the most confusing. Just out of curiosity, I
wanted to run Julien's "Use NMI for perf interrupt" patch (posted
on the mailing list), so I have to enable NMI first.

That said, with
  1) Select Kernel Feature -> Support for NMI-like interrupts
  2) Set "irqchip.gicv3_pseudo_nmi=1" on the kernel command line
  3) No pseudo-NMIs have been generated at all
and this issue was hit.


thanks,

zenghui

> On the other hand, if you can generate pseudo-NMIs, you could end-up
> tracing gic_handle_irq whilst being inside the tracing code with
> interrupts being notionally disabled (and that could be pretty bad).
>
> So, patches or no patches?
>
> Thanks,
>
> 	M.