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

[Dongli Zhang]

Hi Paul,

On 09/14/2018 04:11 PM, Paul Durrant wrote:
>> -----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?

I would prefer to leave this feature disabled until it is stable, used and
tested by more people, or when maintainers think it is time to enable it by default.

> 
>> 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?

As more people suggest, I would enable this on all domains used as pv backend.

I will use terms like 'per-frontend-domain' or 'frontend-id'. Seems this does
not change the existing code a lot.

Dongli Zhang



> 
>>
>> 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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