Re: [PATCH v3] memory-hotplug: add automatic onlining policy for the newly added memory

[Daniel Kiper]

On Mon, Jan 11, 2016 at 04:03:35PM +0100, Vitaly Kuznetsov wrote:
> Daniel Kiper <daniel.kiper@xxxxxxxxxx> writes:
>
> [skip]
>
> >> > > And we want to have it working out of the box.
> >> > > So, I think that we should find proper solution. I suppose that we can schedule
> >> > > a task here which auto online attached blocks. Hmmm... Not nice but should work.
> >> > > Or maybe you have better idea how to fix this issue.
> >> >
> >> > I'd like to avoid additional delays and memory allocations between
> >> > adding new memory and onlining it (and this is the main purpose of the
> >> > patch). Maybe we can have a tristate online parameter ('online_now',
> >> > 'online_delay', 'keep_offlined') and handle it
> >> > accordingly. Alternatively I can suggest we have the onlining in Xen
> >> > balloon driver code, memhp_auto_online is exported so we can call
> >> > online_pages() after we release the ballon_mutex.
> >>
> >> This is not nice too. I prefer the same code path for every case.
> >> Give me some time. I will think how to solve that issue.
> >
> > It looks that we can safely call mutex_unlock() just before add_memory_resource()
> > call and retake lock immediately after add_memory_resource(). add_memory_resource()
> > itself does not play with balloon stuff and even if online_pages() does then it
> > take balloon_mutex in right place. Additionally, only one balloon task can run,
> > so, I think that we are on safe side. Am I right?
>
> I think you are as balloon_mutex is internal to xen driver and there is
> only one balloon_process() running at the time. I just smoke-tested the
> following:
>
> commit 0fce4746a0090d533e9302cc42b3d3c0645d756d
> Author: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Date:   Mon Jan 11 14:22:11 2016 +0100
>
>     xen_balloon: make hotplug auto online work
>
>     Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 890c3b5..08bbf35 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -338,7 +338,10 @@ static enum bp_state reserve_additional_memory(void)
>  	}
>  #endif
>
> -	rc = add_memory_resource(nid, resource, false);
> +	mutex_unlock(&balloon_mutex);
> +	rc = add_memory_resource(nid, resource, memhp_auto_online);
> +	mutex_lock(&balloon_mutex);
> +
>  	if (rc) {
>  		pr_warn("Cannot add additional memory (%i)\n", rc);
>  		goto err;
> @@ -565,8 +568,10 @@ static void balloon_process(struct work_struct *work)
>  		if (credit > 0) {
>  			if (balloon_is_inflated())
>  				state = increase_reservation(credit);
> -			else
> +			else {
> +				printk("balloon_process: adding memory (credit: %ld)!\n", credit);
>  				state = reserve_additional_memory();
> +			}
>  		}
>
>  		if (credit < 0)
>
> And it seems to work (unrelated rant: 'xl mem-set' after 'xl max-mem'

Great! Thanks!

Let's go further. Please add bool online argument to reserve_additional_memory() and
then call add_memory_resource() with it. Then call reserve_additional_memory() with
memhp_auto_online from balloon_process() and with false from add_ballooned_pages(). Voila!

Please do not forget to add comment for mutex_unlock() and mutex_lock()
around add_memory_resource() (why it is needed and why it works correctly).

> doesn't work with "libxl: error: libxl.c:4809:libxl_set_memory_target:
> memory_dynamic_max must be less than or equal to memory_static_max". At

Ignore that. It must be fixed and it is on my TODO list. However, I am busy
with more important stuff right now.

Daniel