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

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Message-ID: <alpine.LFD.2.00.0811130749440.3468@nehalem.linux-foundation.org>
Date:	Thu, 13 Nov 2008 08:00:08 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] x86: optimise page fault path a little



On Thu, 13 Nov 2008, Nick Piggin wrote:
> 
> 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?

I think splitting it up is good, but I hate how your split-up ends up also 
splitting the locking (ie now you do a "down_read()" and "up_read()" in 
different functions).

I also think that to some degree you made it less readable, particularly 
this area:

	+       if (write) {
	+               if (unlikely(!(vma->vm_flags & VM_WRITE))) {
	+                       bad_area_accerr(regs, error_code, address);
	+                       return;
	+               }
	+       } else if (unlikely(error_code & PF_PROT)) {
	+               bad_area_accerr(regs, error_code, address);
	+               return;
	+       } else if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)))) {
	+               bad_area_accerr(regs, error_code, address);
	+               return;

makes me go "whaa?" and I wonder if it wouldn't be nicer to have one 
complex conditional hidden in an inline function, and then just have

	if (unlikely(access_error(write, error_code, vma))) {
		bad_area_access_error(regs, error_code, address);
		return;
	}

where the point is that we don't want to duplicate the error case three 
times, and that "accerr" is bad naming.

IOW, I do think that the patch looks like a step in the right direction, 
but cleanliness should be a primary concern.

		Linus
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