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

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Message-ID: <87h7vig17z.fsf@nanos.tec.linutronix.de>
Date:   Wed, 10 Jun 2020 21:38:56 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Qian Cai <cai@....pw>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [patch V9 10/39] x86/entry: Provide helpers for execute on irqstack

Qian Cai <cai@....pw> writes:
> On Tue, Jun 09, 2020 at 10:50:50PM +0200, Thomas Gleixner wrote:
>> Thomas Gleixner <tglx@...utronix.de> 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