Re: [PATCH v2 6/9] mm: vmalloc: Offload free_vmap_area_lock lock

Uladzislau Rezki
2023-09-11T21:10:14Z

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@gmail.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