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    SubjectRE: [PATCH 1/6] xenbus: prepare data structures and parameter for xenwatch multithreading
    Date
    > -----Original Message-----
    > From: Dongli Zhang [mailto:dongli.zhang@oracle.com]
    > Sent: 14 September 2018 08:34
    > To: xen-devel@lists.xenproject.org; linux-kernel@vger.kernel.org
    > Cc: boris.ostrovsky@oracle.com; jgross@suse.com; Paul Durrant
    > <Paul.Durrant@citrix.com>; Wei Liu <wei.liu2@citrix.com>;
    > konrad.wilk@oracle.com; Roger Pau Monne <roger.pau@citrix.com>;
    > srinivas.eeda@oracle.com
    > 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@oracle.com>
    > ---
    > 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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