linux-kernel - Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception

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Date:   Tue, 2 Apr 2019 17:48:56 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack()

On Tue, 2 Apr 2019, Josh Poimboeuf wrote:
> On Tue, Apr 02, 2019 at 12:19:46PM +0200, Thomas Gleixner wrote:
> > +/*
> > + * Array of exception stack page descriptors. If the stack is larger than
> > + * PAGE_SIZE, all pages covering a particular stack will have the same
> > + * info.
> > + */
> > +static const struct estack_pages estack_pages[ESTACK_PAGES] ____cacheline_aligned = {
> > +	[CONDRANGE(DF)]		= ESTACK_PAGE(DOUBLEFAULT_IST, DF),
> > +	[CONDRANGE(NMI)]	= ESTACK_PAGE(NMI_IST, NMI),
> > +	[PAGERANGE(DB)]		= ESTACK_PAGE(DEBUG_IST, DB),
> > +	[CONDRANGE(MCE)]	= ESTACK_PAGE(MCE_IST, MCE),
> 
> It would be nice if the *_IST macro naming aligned with the struct
> cea_exception_stacks field naming.  Then you could just do, e.g.
> ESTACKPAGE(DF).

Yes, lemme fix that up.

> Also it's a bit unfortunate that some of the stack size knowledge is
> hard-coded here, i.e #DB always being > 1 page and non-#DB being
> sometimes 1 page.

The problem is that there is no way to make this macro maze conditional on
sizeof(). But my macro foo is rusty.

> > +	begin = (unsigned long)__this_cpu_read(cea_exception_stacks);
> > +	end = begin + sizeof(struct cea_exception_stacks);
> > +	/* Bail if @stack is outside the exception stack area. */
> > +	if (stk <= begin || stk >= end)
> > +		return false;
> 
> This check is the most important piece.  Exception stack dumps are quite
> rare, so this ensures an early exit in most cases regardless of whether
> there's a loop below.
> 
> > +
> > +	/* Calc page offset from start of exception stacks */
> > +	k = (stk - begin) >> PAGE_SHIFT;
> > +	/* Lookup the page descriptor */
> > +	ep = &estack_pages[k];
> > +	/* Guard page? */
> > +	if (unlikely(!ep->size))
> > +		return false;
> > +
> > +	begin += (unsigned long)ep->offs;
> > +	end = begin + (unsigned long)ep->size;
> > +	regs = (struct pt_regs *)end - 1;
> > +
> > +	info->type	= ep->type;
> > +	info->begin	= (unsigned long *)begin;
> > +	info->end	= (unsigned long *)end;
> > +	info->next_sp	= (unsigned long *)regs->sp;
> > +	return true;
> 
> With the above "(stk <= begin || stk >= end)" check, removing the loop
> becomes not all that important since exception stack dumps are quite
> rare and not performance sensitive.  With all the macros this code
> becomes a little more obtuse, so I'm not sure whether removal of the
> loop is a net positive.

What about perf? It's NMI context and probably starts from there. Peter?

Thanks,

	tglx