linux-kernel - Re: [rfc] x86: optimise page fault path a little

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Date:	Thu, 13 Nov 2008 08:41:09 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [rfc] x86: optimise page fault path a little


* Nick Piggin <npiggin@...e.de> wrote:

> Hi,
> 
> I was just looking around the page fault code for any obvious 
> performance improvements. I noticed do_page_fault is rather big, 
> uses a lot of stack, and generates some branch mispredicts.
> 
> It's only about 1.1% on the profile of the workload I'm looking at, 
> so my improvement is pretty close to in the noise, but I wonder if 
> micro optimisations like the following would be welcome?

it's definitely welcome!

> This patch adds branch hints and moves error condition code out of 
> line. It shrinks do_page_fault from 2410 bytes to 603 bytes, and 
> from 352 to 64 bytes of stack. Total text size does grow by about 
> 500 bytes due to the additional functions added.

Some small cleanliness nits:

> +/* TODO: match order of arguments */

please fix TODO ;)

> +static noinline void no_context(struct pt_regs *regs,
> +			unsigned long error_code, unsigned long address)
> +{
> +	struct task_struct *tsk = current;
> +#ifdef CONFIG_X86_64
> +	unsigned long flags;
> +#endif

we should just do this unconditionally on 32-bit too.

> +	/*
> +	 * Oops. The kernel tried to access some bad page. We'll have to
> +	 * terminate things with extreme prejudice.
> +	 */
> +#ifdef CONFIG_X86_32
> +	bust_spinlocks(1);
> +#else
> +	flags = oops_begin();
> +#endif

32-bit should use oops_begin() too. Solves the previous comment as 
well.

> +#ifdef CONFIG_X86_32
> +	die("Oops", regs, error_code);
> +	bust_spinlocks(0);
> +	do_exit(SIGKILL);
> +#else
> +	if (__die("Oops", regs, error_code))
> +		regs = NULL;
> +	/* Executive summary in case the body of the oops scrolled away */
> +	printk(KERN_EMERG "CR2: %016lx\n", address);
> +	oops_end(flags, regs, SIGKILL);
> +#endif

this difference seems unnecessary too - 32-bit should use oops_end() 
too.

> +/* TODO: fixup for oom handling */

please fix todo ;-)

this flow could be cleaned up further:

[...]
> +		bad_area(regs, error_code, address);
> +		return;
[...]
> +		bad_area(regs, error_code, address);
> +		return;
[...]
> +			bad_area(regs, error_code, address);
> +			return;
[...]
> +		bad_area(regs, error_code, address);
> +		return;

Any reason why that pattern shouldnt be changed to an appropriate goto 
bad_area? (probably the same goes for the nosemaphore error paths too)

	Ingo
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