Date: Wed, 2 Jun 2004 23:53:06 -0700
From: Andrew Morton <akpm@osdl.org>
To: Dmitry Torokhov <dtor_core@ameritech.net>
Cc: linux-kernel@vger.kernel.org, greg@kroah.com, vojtech@suse.cz
Subject: Re: [RFC] Changing SysRq - show registers handling
Message-Id: <20040602235306.1e6dd3fb.akpm@osdl.org>
In-Reply-To: <200406030134.04121.dtor_core@ameritech.net>
References: <200406030134.04121.dtor_core@ameritech.net>
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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).

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.
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