Re: [RFC] Changing SysRq - show registers handling

From: Dmitry Torokhov
Date: Thu Jun 03 2004 - 02:09:21 EST


On Thursday 03 June 2004 01:53 am, Andrew Morton wrote:
> Dmitry Torokhov <dtor_core@xxxxxxxxxxxxx> wrote:
> >
> > Currently SysRq "show registers" command dumps registers and the call
> > trace from keyboard interrupt context when SysRq-P. For that struct pt_regs *
> > has to be dragged throughout entire input and USB systems. Other than passing
> > this pointer to SysRq handler these systems has no interest in it, it is
> > completely foreign piece of data for them and I would like to get rid of it.
> >
> > I am suggesting slightly changing semantics of SysRq-P handling - instread
> > of dumping registers and call trace immediately it will simply post a request
> > for this information to be dumped. When next HW interrupt arrives and is
> > handled, before running softirqs then current stack trace will be printed.
> > This approach adds small overhead to the HW interrupt handling routine as the
> > condition has to be checked with every interrupt but I expect it to be
> > negligible as it is only check and conditional jump that is almost never
> > taken. The code should be hot in cache so branch prediction should work just
> > fine.
>
> Makes sense I guess.
>
> There have been other times when I've needed access to the registers from
> within hard IRQ. But I forget the reason.
>
> It would be more general, although a little slower to do:
>
> DEFINE_PER_CPU(global_irq_regs);
>
> do_IRQ(...)
> {
> ...
> struct pt_regs **cpu_regs_slot = __get_cpu_var(global_irq_regs);
> struct pt_regs *save = *cpu_regs_slot;
> *cpu_regs_slot = &regs;
> ...
> *cpu_regs_slot = save;
> }
>
> And to teach the sysrq code to grab *__get_cpu_var(global_irq_regs).

Ok, so by making it a tad slower you can keep the old semantics - printing
registers right when keyboard interrupt is processed instead of waiting till
the next one arrives.

Hmm... the path is pretty hot, I am not sure what is best.

>
> Note that global_irq_regs is only valid if in_interrupt(). The sysrq
> handler can be called from process context via /proc/sysrq-trigger and
> should bale out if !in_interrupt().
>
> +static inline void sysrq_show_registes(struct pt_regs *pt_regs)
>
> typo.
>

Yep, thanks for noticing.

--
Dmitry
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