From: Dmitry Torokhov <dtor_core@ameritech.net>
To: Andrew Morton <akpm@osdl.org>
Subject: Re: [RFC] Changing SysRq - show registers handling
Date: Thu, 3 Jun 2004 02:08:19 -0500
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On Thursday 03 June 2004 01:53 am, Andrew Morton wrote:
> Dmitry Torokhov <dtor_core@ameritech.net> 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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