Re: [PATCH v6 1/3] xenbus/backend: Add memory pressure handler callback

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Dec 11, 2019 at 04:26:57AM +0000, SeongJae Park wrote:
> Granting pages consumes backend system memory.  In systems configured
> with insufficient spare memory for those pages, it can cause a memory
> pressure situation.  However, finding the optimal amount of the spare
                                                              ^ s/the//
> memory is challenging for large systems having dynamic resource
> utilization patterns.  Also, such a static configuration might lack
> flexibility.
> 
> To mitigate such problems, this commit adds a memory reclaim callback to
> 'xenbus_driver'.  If a memory pressure is detected, 'xenbus' requests
                       ^ s/a//
> every backend driver to volunarily release its memory.
> 
> Note that it would be able to improve the callback facility for more
                        ^ possible
> sophisticated handlings of general pressures.  For example, it would be
                ^ handling of resource starvation.
> possible to monitor the memory consumption of each device and issue the
> release requests to only devices which causing the pressure.  Also, the
> callback could be extended to handle not only memory, but general
> resources.  Nevertheless, this version of the implementation defers such
> sophisticated goals as a future work.
> 
> Reviewed-by: Juergen Gross <jgross@suse.com>
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>  drivers/xen/xenbus/xenbus_probe_backend.c | 32 +++++++++++++++++++++++
>  include/xen/xenbus.h                      |  1 +
>  2 files changed, 33 insertions(+)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> index b0bed4faf44c..aedbe2198de5 100644
> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> @@ -248,6 +248,35 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>  	return NOTIFY_DONE;
>  }
>  
> +static int xenbus_backend_reclaim(struct device *dev, void *data)

No need for the xenbus_ prefix since it's a static function, ie:
backend_reclaim_memory should be fine IMO.

> +{
> +	struct xenbus_driver *drv;

I've asked for this variable to be constified in v5, is it not
possible to make it const?

> +
> +	if (!dev->driver)
> +		return 0;
> +	drv = to_xenbus_driver(dev->driver);
> +	if (drv && drv->reclaim)
> +		drv->reclaim(to_xenbus_device(dev));
> +	return 0;
> +}
> +
> +/*
> + * Returns 0 always because we are using shrinker to only detect memory
> + * pressure.
> + */
> +static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
> +				struct shrink_control *sc)
> +{
> +	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
> +			xenbus_backend_reclaim);
> +	return 0;
> +}
> +
> +static struct shrinker xenbus_backend_shrinker = {

I would drop the xenbus prefix, and I think it's not possible to
constify this due to register_shrinker expecting a non-const
parameter?

> +	.count_objects = xenbus_backend_shrink_count,
> +	.seeks = DEFAULT_SEEKS,
> +};
> +
>  static int __init xenbus_probe_backend_init(void)
>  {
>  	static struct notifier_block xenstore_notifier = {
> @@ -264,6 +293,9 @@ static int __init xenbus_probe_backend_init(void)
>  
>  	register_xenstore_notifier(&xenstore_notifier);
>  
> +	if (register_shrinker(&xenbus_backend_shrinker))
> +		pr_warn("shrinker registration failed\n");

Can you add a xenbus prefix to the error message? Or else it's hard to
know which subsystem is complaining when you see such message on the
log. ie: "xenbus: shrinker ..."

> +
>  	return 0;
>  }
>  subsys_initcall(xenbus_probe_backend_init);
> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> index 869c816d5f8c..196260017666 100644
> --- a/include/xen/xenbus.h
> +++ b/include/xen/xenbus.h
> @@ -104,6 +104,7 @@ struct xenbus_driver {
>  	struct device_driver driver;
>  	int (*read_otherend_details)(struct xenbus_device *dev);
>  	int (*is_ready)(struct xenbus_device *dev);
> +	void (*reclaim)(struct xenbus_device *dev);

reclaim_memory (if Juergen agrees).

Thanks, Roger.