Re: [PATCH] arm64: kernel: Use a separate stack for irq interrupts.

[AKASHI Takahiro]

Jungseok,

On 09/08/2015 01:34 AM, Jungseok Lee wrote:
> On Sep 8, 2015, at 1:06 AM, James Morse wrote:
> > On 07/09/15 16:48, Jungseok Lee wrote:
> > > On Sep 7, 2015, at 11:36 PM, James Morse wrote:
> > >
> > > Hi James,
> > >
> > > > Having to handle interrupts on top of an existing kernel stack means the
> > > > kernel stack must be large enough to accomodate both the maximum kernel
> > > > usage, and the maximum irq handler usage. Switching to a different stack
> > > > when processing irqs allows us to make the stack size smaller.
> > > >
> > > > Maximum kernel stack usage (running ltp and generating usb+ethernet
> > > > interrupts) was 7256 bytes. With this patch, the same workload gives
> > > > a maximum stack usage of 5816 bytes.
> > >
> > > I'd like to know how to measure the max stack depth.
> > > AFAIK, a stack tracer on ftrace does not work well. Did you dump a stack
> > > region and find or track down an untouched region?
> >
> > I enabled the 'Trace max stack' option under menuconfig 'Kernel Hacking' ->
> > 'Tracers', then looked in debugfs:/tracing/stack_max_size.
> >
> > What problems did you encounter?
> > (I may be missing something…)
>
> When I enabled the feature, all entries had *0* size except the last entry.
> It can be reproduced easily as looking in debugs:/tracing/stack_trace.

I'm afraid that you have not applied one of patches in my RFC:
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/355919.html

I have not looked into James' patch in details, but hope that it will help
fix one of issues that are annoying me: Stack tracer (actually save_stack_trace())
will miss a function (and its parent function in some case) that is being executed
when an interrupt is taken.


-Takahiro AKASHI

> You can track down my report and Akashi's changes with the following links:
> - http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/354126.html
> - https://lkml.org/lkml/2015/7/13/29
>
> Although it is impossible to measure an exact depth at this moment, the feature
> could be utilized to check improvement.
>
> Cc'ing Akashi for additional comments if needed.
>
> Best Regards
> Jungseok Lee
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