On Tue, 2019-10-01 at 12:04 -0700, John Hubbard wrote:
> On 10/1/19 10:56 AM, Leonardo Bras wrote:
> > On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote:
> > > On 9/27/19 4:40 PM, Leonardo Bras wrote:
> ...
> > > > diff --git a/mm/gup.c b/mm/gup.c
> > > > index 98f13ab37bac..7105c829cf44 100644
> > > > --- a/mm/gup.c
> > > > +++ b/mm/gup.c
> > > > @@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long
> > > > start, unsigned long end)
> > > > int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> > > > struct page **pages)
> > > > {
> > > > + struct mm_struct *mm;
> > >
> > > I don't think that this local variable adds any value, so let's not use
> > > it.
> > > Similar point in a few other patches too.
> >
> > It avoids 1 deference of current->mm, it's a little performance gain.
> >
>
> No, it isn't. :)
>
> Longer answer: at this level (by which I mean, "wrote the C code, haven't
> looked
> at the generated asm yet, and haven't done a direct perf test yet"), none of
> us
> C programmers are entitled to imagine that we can second guess both the
> compiler
> and the CPU well enough to claim that declaring a local pointer variable on
> the
> stack will even *affect* performance, much less know which way it will go!
> I did this based on how costly can be 'current', and I could notice reduction in assembly size most of the time. (powerpc) But I get what you mean, maybe the (possible) performance gain don't worth the extra work. > The compiler at -O2 will *absolutely* optimize away any local variables that > it doesn't need. > > And that leads to how kernel programmers routinely decide about that kind of > variable: "does the variable's added clarity compensate for the extra visual > noise and for the need to manage the variable?" That's a good way to decide it. :) > > Here, and in most (all?) other points in the patchset where you've added an > mm local variable, the answer is no. > Well, IMHO it's cleaner that way. But I get that other people may disagree. > > ... start_lockless_pgtbl_walk(mm); > > > Minor: I'd like to rename this register_lockless_pgtable_walker(). > > > > > > > local_irq_disable(); > > > > gup_pgd_range(addr, end, gup_flags, pages, &nr); > > > > local_irq_enable(); > > > > + end_lockless_pgtbl_walk(mm); > > > > > > ...and deregister_lockless_pgtable_walker(). > > > > > > > I have no problem changing the name, but I don't register/deregister > > are good terms for this. > > > > I would rather use start/finish, begin/end, and so on. Register sounds > > like something more complicated than what we are trying to achieve > > here. > > > > OK, well, I don't want to bikeshed on naming more than I usually do, and > what you have is reasonable, so I'll leave that alone. :) > > thanks, Thank for the feedback,
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