Re: [Xen-devel] [PATCH 1/6] xenbus: prepare data structures and parameter for xenwatch multithreading

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Dongli Zhang [mailto:dongli.zhang@xxxxxxxxxx]
> Sent: 14 September 2018 08:34
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Cc: boris.ostrovsky@xxxxxxxxxx; jgross@xxxxxxxx; Paul Durrant
> <Paul.Durrant@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>;
> konrad.wilk@xxxxxxxxxx; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
> srinivas.eeda@xxxxxxxxxx
> Subject: [PATCH 1/6] xenbus: prepare data structures and parameter for
> xenwatch multithreading
> 
> This is the 1st patch of a (6-patch) patch set.
> 
> This patch set of six patches introduces xenwatch multithreading (or
> multithreaded xenwatch, abbreviated as 'mtwatch') to dom0 kernel. In
> addition to the existing single xenwatch thread, each domU has its own
> kernel thread ([xen-mtwatch-<domid>]) to process its xenwatch event.
> 

^ You need to put comments like this in a cover letter. Each patch needs to 
stand on its own merit and the comments should only relate to the context of 
that patch or a 'subsequent patch'.

> A kernel parameter 'xen_mtwatch' is introduced to control whether the
> feature is enabled or not during dom0 kernel boot. The feature is disabled
> by default if 'xen_mtwatch' is not set in grub. 

Why is it disabled by default? Concerns about resource consumption?

> In addition, this patch
> also introduces the data structures to maintain the status of each per-
> domU
> xenwatch thread. The status of each xenwatch thread (except the default
> one) is maintained by a mtwatch domain.
> 
> The feature is available only on dom0.

Whilst I can see it is intended for a backend domain, why limit it to dom0? 
What about driver domains?

> 
> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  3 ++
>  drivers/xen/xenbus/xenbus_xs.c                  | 31 ++++++++++++
>  include/xen/xenbus.h                            | 65
> +++++++++++++++++++++++++
>  3 files changed, 99 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt
> b/Documentation/admin-guide/kernel-parameters.txt
> index 64a3bf5..fc295ef 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4992,6 +4992,9 @@
>                               the unplug protocol
>                       never -- do not unplug even if version check succeeds
> 
> +     xen_mtwatch     [KNL,XEN]
> +                     Enables the multithreaded xenwatch (mtwatch).
> +
>       xen_nopvspin    [X86,XEN]
>                       Disables the ticketlock slowpath using Xen PV
>                       optimizations.
> diff --git a/drivers/xen/xenbus/xenbus_xs.c
> b/drivers/xen/xenbus/xenbus_xs.c
> index 49a3874..3f137d2 100644
> --- a/drivers/xen/xenbus/xenbus_xs.c
> +++ b/drivers/xen/xenbus/xenbus_xs.c
> @@ -95,6 +95,19 @@ static pid_t xenwatch_pid;
>  static DEFINE_MUTEX(xenwatch_mutex);
>  static DECLARE_WAIT_QUEUE_HEAD(watch_events_waitq);
> 
> +bool xen_mtwatch;
> +EXPORT_SYMBOL_GPL(xen_mtwatch);
> +
> +struct mtwatch_info *mtwatch_info;
> +
> +static bool param_xen_mtwatch;
> +static __init int xen_parse_mtwatch(char *arg)
> +{
> +     param_xen_mtwatch = true;
> +     return 0;
> +}
> +early_param("xen_mtwatch", xen_parse_mtwatch);
> +
>  static void xs_suspend_enter(void)
>  {
>       spin_lock(&xs_state_lock);
> @@ -929,6 +942,24 @@ int xs_init(void)
>       if (err)
>               return err;
> 
> +     if (xen_initial_domain() && param_xen_mtwatch) {
> +             int i;
> +
> +             mtwatch_info = kmalloc(sizeof(*mtwatch_info), GFP_KERNEL);
> +
> +             for (i = 0; i < MTWATCH_HASH_SIZE; i++)
> +                     INIT_HLIST_HEAD(&mtwatch_info->domain_hash[i]);
> +             spin_lock_init(&mtwatch_info->domain_lock);
> +             INIT_LIST_HEAD(&mtwatch_info->domain_list);
> +
> +             spin_lock_init(&mtwatch_info->purge_lock);
> +             INIT_LIST_HEAD(&mtwatch_info->purge_list);
> +
> +             xen_mtwatch = true;
> +
> +             pr_info("xenwatch multithreading is enabled\n");
> +     }
> +
>       task = kthread_run(xenwatch_thread, NULL, "xenwatch");
>       if (IS_ERR(task))
>               return PTR_ERR(task);
> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> index 869c816..e807114 100644
> --- a/include/xen/xenbus.h
> +++ b/include/xen/xenbus.h
> @@ -62,6 +62,13 @@ struct xenbus_watch
>       /* Callback (executed in a process context with no locks held). */
>       void (*callback)(struct xenbus_watch *,
>                        const char *path, const char *token);
> +
> +     /* Callback to help calculate the domid the path belongs to */
> +     domid_t (*get_domid)(struct xenbus_watch *watch,
> +                          const char *path, const char *token);
> +
> +     /* The owner's domid if the watch is for a specific domain */
> +     domid_t owner_id;
>  };
> 
> 
> @@ -93,6 +100,7 @@ struct xenbus_device_id
>  struct xenbus_driver {
>       const char *name;       /* defaults to ids[0].devicetype */
>       const struct xenbus_device_id *ids;
> +     bool use_mtwatch;
>       int (*probe)(struct xenbus_device *dev,
>                    const struct xenbus_device_id *id);
>       void (*otherend_changed)(struct xenbus_device *dev,
> @@ -233,4 +241,61 @@ extern const struct file_operations xen_xenbus_fops;
>  extern struct xenstore_domain_interface *xen_store_interface;
>  extern int xen_store_evtchn;
> 
> +extern bool xen_mtwatch;
> +
> +#define MTWATCH_HASH_SIZE 256
> +#define MTWATCH_HASH(_id) ((int)(_id)&(MTWATCH_HASH_SIZE-1))
> +
> +struct mtwatch_info {
> +     /*
> +      * The mtwatch_domain is put on both a hash table and a list.
> +      * domain_list is used to optimize xenbus_watch un-registration.
> +      *
> +      * The mtwatch_domain is removed from domain_hash (with state set
> +      * to MTWATCH_DOMAIN_DOWN) when its refcnt is zero. However, it is
> +      * left on domain_list until all events belong to such
> +      * mtwatch_domain are processed in mtwatch_thread().
> +      *
> +      * While there may exist two mtwatch_domain with the same domid on
> +      * domain_list simultaneously, all mtwatch_domain on hash_hash
> +      * should have unique domid.
> +      */
> +     spinlock_t domain_lock;
> +     struct hlist_head domain_hash[MTWATCH_HASH_SIZE];
> +     struct list_head domain_list;
> +
> +     /*
> +      * When a per-domU 

'per-frontend-domain' to be more descriptive?

  Paul

> kthread is going to be destroyed, it is put
> +      * on the purge_list, and will be flushed by purge_work later.
> +      */
> +     struct work_struct purge_work;
> +     spinlock_t purge_lock;
> +     struct list_head purge_list;
> +};
> +
> +enum mtwatch_domain_state {
> +     MTWATCH_DOMAIN_UP = 1,
> +     MTWATCH_DOMAIN_DOWN = 2,
> +};
> +
> +struct mtwatch_domain {
> +     domid_t domid;
> +     struct task_struct *task;
> +     atomic_t refcnt;
> +
> +     pid_t pid;
> +     struct mutex domain_mutex;
> +     struct rcu_head rcu;
> +
> +     struct hlist_node hash_node;
> +     struct list_head list_node;
> +     struct list_head purge_node;
> +
> +     wait_queue_head_t events_wq;
> +
> +     spinlock_t events_lock;
> +     struct list_head events;
> +     enum mtwatch_domain_state state;
> +};
> +
>  #endif /* _XEN_XENBUS_H */
> --
> 2.7.4


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