Re: [Xen-devel] [PATCH 14/22] xen/arm: p2m: Switch the p2m lock from spinlock to rwlock

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 20 Jul 2016, Julien Grall wrote:
> P2M reads do not require to be serialized. This will add contention
> when PV drivers are using multi-queue because parallel grant
> map/unmaps/copies will happen on DomU's p2m.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


>     I have not done benchark to verify the performance, however a rwlock
>     is always an improvement compare to a spinlock when most of the
>     access only read data.
> 
>     It might be possible to convert the rwlock to a per-cpu rwlock which
>     show some improvement on x86.
> ---
>  xen/arch/arm/p2m.c        | 12 ++++++------
>  xen/include/asm-arm/p2m.h |  3 ++-
>  2 files changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index d74c249..6136767 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -49,27 +49,27 @@ static bool_t p2m_mapping(lpae_t pte)
>  
>  static inline void p2m_write_lock(struct p2m_domain *p2m)
>  {
> -    spin_lock(&p2m->lock);
> +    write_lock(&p2m->lock);
>  }
>  
>  static inline void p2m_write_unlock(struct p2m_domain *p2m)
>  {
> -    spin_unlock(&p2m->lock);
> +    write_unlock(&p2m->lock);
>  }
>  
>  static inline void p2m_read_lock(struct p2m_domain *p2m)
>  {
> -    spin_lock(&p2m->lock);
> +    read_lock(&p2m->lock);
>  }
>  
>  static inline void p2m_read_unlock(struct p2m_domain *p2m)
>  {
> -    spin_unlock(&p2m->lock);
> +    read_unlock(&p2m->lock);
>  }
>  
>  static inline int p2m_is_locked(struct p2m_domain *p2m)
>  {
> -    return spin_is_locked(&p2m->lock);
> +    return rw_is_locked(&p2m->lock);
>  }
>  
>  void p2m_dump_info(struct domain *d)
> @@ -1389,7 +1389,7 @@ int p2m_init(struct domain *d)
>      struct p2m_domain *p2m = &d->arch.p2m;
>      int rc = 0;
>  
> -    spin_lock_init(&p2m->lock);
> +    rwlock_init(&p2m->lock);
>      INIT_PAGE_LIST_HEAD(&p2m->pages);
>  
>      p2m->vmid = INVALID_VMID;
> diff --git a/xen/include/asm-arm/p2m.h b/xen/include/asm-arm/p2m.h
> index 20a220ea..abda70c 100644
> --- a/xen/include/asm-arm/p2m.h
> +++ b/xen/include/asm-arm/p2m.h
> @@ -3,6 +3,7 @@
>  
>  #include <xen/mm.h>
>  #include <xen/radix-tree.h>
> +#include <xen/rwlock.h>
>  #include <public/vm_event.h> /* for vm_event_response_t */
>  #include <public/memory.h>
>  #include <xen/p2m-common.h>
> @@ -20,7 +21,7 @@ extern void memory_type_changed(struct domain *);
>  /* Per-p2m-table state */
>  struct p2m_domain {
>      /* Lock that protects updates to the p2m */
> -    spinlock_t lock;
> +    rwlock_t lock;
>  
>      /* Pages used to construct the p2m */
>      struct page_list_head pages;
> -- 
> 1.9.1
> 

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