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SubjectRe: [Xen-devel] [PATCH 1/6] xenbus: prepare data structures and parameter for xenwatch multithreading
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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
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xenproject.org
> https://lists.xenproject.org/mailman/listinfo/xen-devel
>

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