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

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Date:   Fri, 14 Sep 2018 10:32:35 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     boris.ostrovsky@...cle.com, paul.durrant@...rix.com,
        wei.liu2@...rix.com, konrad.wilk@...cle.com, roger.pau@...rix.com,
        srinivas.eeda@...cle.com
Subject: Re: [PATCH 1/6] xenbus: prepare data structures and parameter for
 xenwatch multithreading

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@...cle.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.

> +
>  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.


Juergen