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

[Daniel Kiper]

On Mon, Jan 11, 2016 at 05:22:58PM +0100, Daniel Kiper wrote:
> 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).

I forgot about one thing. Please update help for XEN_BALLOON_MEMORY_HOTPLUG
config option accordingly.

Daniel