Re: [RFC] Changing SysRq - show registers handling

[Andy Lutomirski <luto@myrealbox.com>]

Dmitry Torokhov wrote:

> Hi,
> 
> 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.

What about checking the flag on return from the input interrupts?  That way 
the overhead would be confined to code paths take the hit from passing an 
extra parameter.


> 
> The patch below implements proposed changes in SysRq handler and adds
> necessary changes to do_IRQ() on i386. If it is agreed upon I will adjust
> other arches.
> 
> Please let me know what you think.
>  

--Andy