Re: [PATCH] maple_tree: Remove GFP_ZERO from kmem_cache_alloc() and kmem_cache_alloc_bulk()

[David Hildenbrand]

On 06.01.23 19:36, Liam Howlett wrote:
> * Mike Rapoport <rppt@xxxxxxxxxx> [230106 02:28]:
> > On Thu, Jan 05, 2023 at 04:05:34PM +0000, Liam Howlett wrote:
> > > Preallocations are common in the VMA code to avoid allocating under
> > > certain locking conditions.  The preallocations must also cover the
> > > worst-case scenario.  Removing the GFP_ZERO flag from the
> > > kmem_cache_alloc() (and bulk variant) calls will reduce the amount of
> > > time spent zeroing memory that may not be used.  Only zero out the
> > > necessary area to keep track of the allocations in the maple state.
> > > Zero the entire node prior to using it in the tree.
> > >
> > > This required internal changes to node counting on allocation, so the
> > > test code is also updated.
> > >
> > > This restores some micro-benchmark performance:
> > > up to +9% in mmtests mmap1 by my testing
> > > +10% to +20% in mmap, mmapaddr, mmapmany tests reported by Red Hat
> > >
> > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2149636
> > > Reported-by: Jirka Hladky <jhladky@xxxxxxxxxx>
> > > Suggested-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > Signed-off-by: Liam Howlett <Liam.Howlett@xxxxxxxxxx>
> > > ---
> > >   lib/maple_tree.c                 | 80 +++++++++++++++++---------------
> > >   tools/testing/radix-tree/maple.c | 18 +++----
> > >   2 files changed, 52 insertions(+), 46 deletions(-)
> > >
> > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > > index 26e2045d3cda..82a8121fe49b 100644
> > > --- a/lib/maple_tree.c
> > > +++ b/lib/maple_tree.c
> > > @@ -149,13 +149,12 @@ struct maple_subtree_state {
> > >   /* Functions */
> > >   static inline struct maple_node *mt_alloc_one(gfp_t gfp)
> > >   {
> > > -	return kmem_cache_alloc(maple_node_cache, gfp | __GFP_ZERO);
> > > +	return kmem_cache_alloc(maple_node_cache, gfp);
> > >   }
> > >   
> > >   static inline int mt_alloc_bulk(gfp_t gfp, size_t size, void **nodes)
> > >   {
> > > -	return kmem_cache_alloc_bulk(maple_node_cache, gfp | __GFP_ZERO, size,
> > > -				     nodes);
> > > +	return kmem_cache_alloc_bulk(maple_node_cache, gfp, size, nodes);
> > >   }
> > >   
> > >   static inline void mt_free_bulk(size_t size, void __rcu **nodes)
> > > @@ -1127,9 +1126,10 @@ static inline struct maple_node *mas_pop_node(struct ma_state *mas)
> > >   {
> > >   	struct maple_alloc *ret, *node = mas->alloc;
> > >   	unsigned long total = mas_allocated(mas);
> > > +	unsigned int req = mas_alloc_req(mas);
> > >   
> > >   	/* nothing or a request pending. */
> > > -	if (unlikely(!total))
> > > +	if (WARN_ON(!total))
> >
> > Hmm, isn't WARN_ON() here too much?
>
> I don't think so.  If we get to the point of asking for a node while we
> don't have any to give, then it's too late.  It means we (I) have
> calculated the necessary nodes incorrectly and we won't have enough
> memory to fit things into the tree.  It should never happen.

Either way, the suggestion is to use WARN_ON_ONCE() instead of WARN_ON().

... now documented in Documentation/process/coding-style.rst

--
Thanks,

David / dhildenb