Re: Raw spinlocks and memory allocation

[Paul E. McKenney]

On Fri, Jul 31, 2020 at 01:38:34PM -0700, Andrew Morton wrote:
> On Thu, 30 Jul 2020 16:12:05 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> 
> > So, may we add a GFP_ flag that will cause kmalloc() and friends to return
> > NULL when they would otherwise need to acquire their non-raw spinlock?
> > This avoids adding any overhead to the slab-allocator fastpaths, but
> > allows callback invocation to reduce cache misses without having to
> > restructure some existing callers of call_rcu() and potential future
> > callers of kfree_rcu().
> 
> We have eight free gfp_t bits so that isn't a problem.

Whew!!!  ;-)

> Adding a test-n-branch to the kmalloc() fastpath may well be a concern.
> 
> Which of mm/sl?b.c are affected?

None of them, it turns out.  The initial patch will instead directly
invoke __get_free_page().  So we could just leave sl?b.c alone.

> A doesnt-need-to-really-work protopatch would help us understand the
> potential cost?

Makes sense!  My guess is that Uladzislau Rezki (CCed) will be sending
one along by the middle of next week.

							Thanx, Paul