On Tue 23-03-21 09:01:02, Peter Zijlstra wrote:

On Tue, Mar 23, 2021 at 08:50:53AM +0100, Michal Hocko wrote:

quoted

quoted

quoted

quoted

+static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
+{
+	unsigned long min_count;
+
+	min_count = h->resv_huge_pages + h->nr_huge_pages - h->free_huge_pages;
+	return max(count, min_count);

Just out of curiousity, is compiler allowed to inline this piece of code
and then cache the value? In other words do we need to make these
READ_ONCE or otherwise enforce the no-caching behavior?

I honestly do not know if the compiler is allowed to do that.  The
assembly code generated by my compiler does not cache the value, but
that does not guarantee anything.  I can add READ_ONCE to make the
function look something like:

static inline unsigned long min_hp_count(struct hstate *h, unsigned long count)
{
	unsigned long min_count;

	min_count = READ_ONCE(h->resv_huge_pages) + READ_ONCE(h->nr_huge_pages)
					- READ_ONCE(h->free_huge_pages);
	return max(count, min_count);
}

Maybe just forcing to never inline the function should be sufficient.
This is not a hot path to micro optimize for no function call. But there
are much more qualified people on the CC list on this matter who could
clarify. Peter?

I'm not sure I understand the code right.

We need to ensure the function is evaluated each time it is called
because it will be used after a lock is dropped and reacquired so
numbers could have changed. The point of wrapping this into a function
is to reduce the code duplication IIUC.

But inline or not doesn't
matter, LTO completely ruins that game. Just like if it was a static
function, then the compiler is free to inline it, even if the function
lacks an inline attribute.

OK

Basically, without READ_ONCE() the compiler is allowed to entirely elide
the load (and use a previous load), or to duplicate the load and do it
again later (reaching a different result).

Similarly, the compiler is allowed to byte-wise load the variable in any
random order and re-assemble.

If any of that is a problem, you have to use READ_ONCE().

Thanks for the confirmation!
-- 
Michal Hocko
SUSE Labs