linux-kernel - Re: [PATCH v2 6/9] mm: vmalloc: Offload free_vmap_area

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Date:   Mon, 11 Sep 2023 19:10:43 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v2 6/9] mm: vmalloc: Offload free_vmap_area_lock lock

On Mon, Sep 11, 2023 at 11:25:01AM +0800, Baoquan He wrote:
> On 08/29/23 at 10:11am, Uladzislau Rezki (Sony) wrote:
> > Concurrent access to a global vmap space is a bottle-neck.
> > We can simulate a high contention by running a vmalloc test
> > suite.
> > 
> > To address it, introduce an effective vmap node logic. Each
> > node behaves as independent entity. When a node is accessed
> > it serves a request directly(if possible) also it can fetch
> > a new block from a global heap to its internals if no space
> > or low capacity is left.
> > 
> > This technique reduces a pressure on the global vmap lock.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > ---
> >  mm/vmalloc.c | 316 +++++++++++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 279 insertions(+), 37 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 5a8a9c1370b6..4fd4915c532d 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -779,6 +779,7 @@ struct rb_list {
> >  
> >  struct vmap_node {
> >  	/* Bookkeeping data of this node. */
> > +	struct rb_list free;
> >  	struct rb_list busy;
> >  	struct rb_list lazy;
> >  
> > @@ -786,6 +787,13 @@ struct vmap_node {
> >  	 * Ready-to-free areas.
> >  	 */
> >  	struct list_head purge_list;
> > +	struct work_struct purge_work;
> > +	unsigned long nr_purged;
> > +
> > +	/*
> > +	 * Control that only one user can pre-fetch this node.
> > +	 */
> > +	atomic_t fill_in_progress;
> >  };
> >  
> >  static struct vmap_node *nodes, snode;
> > @@ -804,6 +812,32 @@ addr_to_node(unsigned long addr)
> >  	return &nodes[addr_to_node_id(addr)];
> >  }
> >  
> > +static inline struct vmap_node *
> > +id_to_node(int id)
> > +{
> > +	return &nodes[id % nr_nodes];
> > +}
> > +
> > +static inline int
> > +this_node_id(void)
> > +{
> > +	return raw_smp_processor_id() % nr_nodes;
> > +}
> > +
> > +static inline unsigned long
> > +encode_vn_id(int node_id)
> > +{
> > +	/* Can store U8_MAX [0:254] nodes. */
> > +	return (node_id + 1) << BITS_PER_BYTE;
> > +}
> > +
> > +static inline int
> > +decode_vn_id(unsigned long val)
> > +{
> > +	/* Can store U8_MAX [0:254] nodes. */
> > +	return (val >> BITS_PER_BYTE) - 1;
> > +}
> 
> This patch looks good to me. However, should we split out the encoding
> vn_id into va->flags optimization into another patch? It looks like an
> independent optimization which can be described better with specific
> log. At least, in the pdf file pasted or patch log, it's not obvious
> that:
> 1) node's free tree could contains any address range;
> 2) nodes' busy tree only contains address range belonging to this node;
>    - could contain crossing node range, corner case.
> 3) nodes' purge tree could contain any address range;
>    - decided by encoded vn_id in va->flags.
>    - decided by address via addr_to_node(va->va_start).
> 
> Personal opinion, feel it will make reviewing easier.
> 
Sure, if it is easier to review, then i will split these two parts.
All three statements are correct and valid. The pdf file only covers
v1, so it is not up to date.

Anyway i will update a cover letter in v3 with more details.

--
Uladzislau Rezki