linux-kernel - Re: [patch V2 20/29] x86/dumpstack/64: Speedup in_exception

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Message-ID: <20190405215520.wsevtopb63oetkfd@treble>
Date:   Fri, 5 Apr 2019 16:55:20 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [patch V2 20/29] x86/dumpstack/64: Speedup in_exception_stack()

On Fri, Apr 05, 2019 at 05:07:18PM +0200, Thomas Gleixner wrote:
> The current implementation of in_exception_stack() iterates over the
> exception stacks array. Most of the time this is an useless exercise, but
> even for the actual use cases (perf and ftrace) it takes at least 2
> iterations to get to the NMI stack.
> 
> As the exception stacks and the guard pages are page aligned the loop can
> be avoided completely.
> 
> Add a initial check whether the stack pointer is inside the full exception
> stack area and leave early if not.
> 
> Create a lookup table which describes the stack area. The table index is
> the page offset from the beginning of the exception stacks. So for any
> given stack pointer the page offset is computed and a lookup in the
> description table is performed. If it is inside a guard page, return. If
> not, use the descriptor to fill in the info structure.
> 
> The table is filled at compile time and for the !KASAN case the interesting
> page descriptors exactly fit into a single cache line. Just the last guard
> page descriptor is in the next cacheline, but that should not be accessed
> in the regular case.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> V2: Simplify the macro maze

This is indeed a little better.  It's friday, have an ack.

Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

I don't like the clever ISTACK_* macro naming however...

-- 
Josh