Re: [patch V9 10/39] x86/entry: Provide helpers for execute on irqstack

[Thomas Gleixner]

Qian Cai <cai@xxxxxx> writes:
> On Tue, Jun 09, 2020 at 10:50:50PM +0200, Thomas Gleixner wrote:
>> Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
>> >> BAD (after oom02)
>> >> # cat /sys/kernel/debug/stackdepot/info
>> >> Unique stacks: 140476
>> >
>> > That's indeed odd. I try to reproduce and figure out what really breaks
>> > here.
>> 
>> I checked your config file and I think I know where this comes from. Can
>> you plase disable KASAN just for testing purposes and compare before
>> after again?
>
> It turns out I'll need a few days to be able to get ahold of those
> affected systems again. I'll be reporting back as soon as possible.

I figured it out. It has nothing to do with asm_call_on...(). It's also
unrelated to page_owner. It's purely a KASAN issue.

KASAN trims stack traces with the help of filter_irq_stacks() which
relies on __irqentry_text_start/end

The entry rework moved the interrupt entry points out of the irqentry
section, which breaks that filter function.

I made it work again. Here are the numbers which make that obvious:

Mainline:
Unique stacks: 23403
Depot index:   244
Depot offset:  4208

x86/entry:
Unique stacks: 38835
Depot index:   464
Depot offset:  7616

x86/entry + fix:
Unique stacks: 23607
Depot index:   247
Depot offset:  14224

So with the non-working trimming this generates more unique stacks and
because they are not trimmed they become larger and eat more storage
space. The resulting average per stack shows that:

Mainline:           171 bytes per stack
x86/entry:          195 bytes per stack
x86/entry + fix:    172 bytes per stack

I'll point you to a test branch shortly.

Thanks,

        tglx