linux-kernel - Re: [RFC v1 5/5] maple_tree: replace preallocation with slub percpu array prefill

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Date:   Tue, 8 Aug 2023 15:01:19 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [RFC v1 5/5] maple_tree: replace preallocation with slub percpu
 array prefill

* Liam R. Howlett <Liam.Howlett@...cle.com> [230808 10:37]:
> * Vlastimil Babka <vbabka@...e.cz> [230808 05:53]:
> > With the percpu array we can try not doing the preallocations in maple
> > tree, and instead make sure the percpu array is prefilled, and using
> > GFP_ATOMIC in places that relied on the preallocation (in case we miss
> > or fail trylock on the array), i.e. mas_store_prealloc(). For now simply
> > add __GFP_NOFAIL there as well.
> > 
> > First I tried to change mas_node_count_gfp() to not preallocate anything
> > anywhere, but that lead to warns and panics, even though the other
> > caller mas_node_count() uses GFP_NOWAIT | __GFP_NOWARN so it has no
> > guarantees... So I changed just mas_preallocate(). I let it still to
> > truly preallocate a single node, but maybe it's not necessary?
> 
> Ah, yes.  I added a check to make sure we didn't allocate more nodes
> when using preallocations.  This check is what you are hitting when you
> don't allocate anything.  This is tracked in mas_flags by
> setting/clearing MA_STATE_PREALLOC.  Good news, that check works!

Adding the attached patch to your series prior to the below allows for
the removal of the extra preallocation.

> 
> > ---
> >  lib/maple_tree.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > index 7a8e7c467d7c..5a209d88c318 100644
> > --- a/lib/maple_tree.c
> > +++ b/lib/maple_tree.c
> > @@ -5534,7 +5534,12 @@ void mas_store_prealloc(struct ma_state *mas, void *entry)
> >  
> >  	mas_wr_store_setup(&wr_mas);
> >  	trace_ma_write(__func__, mas, 0, entry);
> > +
> > +retry:
> >  	mas_wr_store_entry(&wr_mas);
> > +	if (unlikely(mas_nomem(mas, GFP_ATOMIC | __GFP_NOFAIL)))
> > +		goto retry;
> > +
> >  	MAS_WR_BUG_ON(&wr_mas, mas_is_err(mas));
> >  	mas_destroy(mas);
> >  }
> > @@ -5550,9 +5555,10 @@ EXPORT_SYMBOL_GPL(mas_store_prealloc);
> >  int mas_preallocate(struct ma_state *mas, gfp_t gfp)
> >  {
> >  	int ret;
> > +	int count = 1 + mas_mt_height(mas) * 3;
> >  
> > -	mas_node_count_gfp(mas, 1 + mas_mt_height(mas) * 3, gfp);
> > -	mas->mas_flags |= MA_STATE_PREALLOC;
> > +	mas_node_count_gfp(mas, 1, gfp);
> > +	kmem_cache_prefill_percpu_array(maple_node_cache, count, gfp);
> >  	if (likely(!mas_is_err(mas)))
> >  		return 0;
> >  
> > -- 
> > 2.41.0
> > 

View attachment "0001-maple_tree-Remove-MA_STATE_PREALLOC.patch" of type "text/x-diff" (2743 bytes)