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

[Dongli Zhang <dongli.zhang@oracle.com>]

Hi Juergen and Paul,

On 09/14/2018 04:32 PM, Juergen Gross wrote:
> On 14/09/18 09:34, Dongli Zhang wrote:
>> 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.
>>
>> 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. 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.
>>
>> 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);
> 
> Add a Kconfig item to set the default when building the kernel? We can
> start with default "off", but later we might want to enable this feature
> as the default.

Is there any weakness/downside configuring the param via early_param? Or is
there any strength via Kconfig?

I would prefer to not fix the configuration when building the kernel. The
administrator/user will not be able to choose the preferred option.

I would prefer to configure the option via kernel param and set default as 'off'
(false). In a common cloud environment, the administrator is only required to
enable the param in dom0/driver domain kernel.

For common domU (not using as backend), this feature is useless. I assume there
are always more domUs than dom0 (and driver domain), especially in cloud
environment.

> 
>> +
>>  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) {
> 
> Wouldn't it be better to drop the test for xen_initial_domain() and do
> this initialization only when a caller (backend) is requesting the
> multithread mode? This would avoid wasting memory in case it isn't going
> top be used and - more important - would support driver domains.

This would save us <4KB.

Suppose the default 'param_xen_mtwatch' is 'off'. The administrator knows the
domain is used as pv backend when he/she enables the option manually.

In such scenario, is there any benefit to make it on-demand to save <4KB memory?

Thank you very much!

Dongli Zhang

> 
> 
> Juergen
> 
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