Re: [PATCH v4 03/11] mm/gup: Applies counting method to monitor gup_pgd_range

Leonardo Bras
2019-10-01T19:40:59Z

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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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