Re: [Patch v2 2/2] mm/page_alloc.c: define node_order with all zero

[John Hubbard]

On 3/27/20 5:26 PM, Wei Yang wrote:
> On Fri, Mar 27, 2020 at 03:37:57PM -0700, John Hubbard wrote:
> > On 3/27/20 3:01 PM, Wei Yang wrote:
> > > Since we always clear node_order before getting it, we can leverage
> > > compiler to do this instead of at run time.
> > >
> > > Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> > > ---
> > >    mm/page_alloc.c | 3 +--
> > >    1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index dfcf2682ed40..49dd1f25c000 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -5585,7 +5585,7 @@ static void build_thisnode_zonelists(pg_data_t *pgdat)
> > >    static void build_zonelists(pg_data_t *pgdat)
> > >    {
> > > -	static int node_order[MAX_NUMNODES];
> > > +	static int node_order[MAX_NUMNODES] = {0};
> >
> >
> > Looks wrong: now the single instance of node_order is initialized just once by
> > the compiler. And that means that only the first caller of this function
> > gets a zeroed node_order array...
>
> What a shame on me. You are right, I miss the static word.
>
> Well, then I am curious about why we want to define it as static. Each time we
> call this function, node_order would be set to 0 and find_next_best_node()
> would sort a proper value into it. I don't see the reason to reserve it in a
> global area and be used next time.

It's not just about preserving the value. Sometimes it's about stack space.
Here's the trade-offs for static variables within a function:

Advantages of static variables within a function (compared to non-static
variables, also within a function):
-----------------------------------

* Doesn't use any of the scarce kernel stack space
* Preserves values (not always necessarily and advantage)

Disadvantages:
-----------------------------------

* Removes basic thread safety: multiple threads can no longer independently
  call the function without getting interaction, and generally that means
  data corruption.

So here, I suspect that the original motivation was probably to conserve stack
space, and the author likely observed that there was no concurrency to worry
about: the function was only being called by one thread at a time.  Given those
constraints (which I haven't confirmed just yet, btw), a static function variable
fits well.

> My suggestion is to remove the static and define it {0} instead of memset
> every time. Is my understanding correct here?


Not completely:

a) First of all, "instead of memset every time" is a misconception, because
   there is still a memset happening every time with {0}. It's just that the
   compiler silently writes that code for you, and you don't see it on the
   screen. But it's still there.

b) Switching away from a static to an on-stack variable requires that you first
   verify that stack space is not an issue. Or, if you determine that this
   function needs the per-thread isolation that a non-static variable provides,
   then you can switch to either an on-stack variable, or a *alloc() function.



thanks,
--
John Hubbard
NVIDIA