Re: [PATCH v7] mm/page_alloc.c: memory_hotplug: free pages as higher order

[Arun KS]

On 2019-01-09 21:39, Alexander Duyck wrote:
> On Wed, 2019-01-09 at 11:51 +0530, Arun KS wrote:
> > On 2019-01-09 03:47, Alexander Duyck wrote:
> > > On Fri, 2019-01-04 at 10:31 +0530, Arun KS wrote:
> > > > When freeing pages are done with higher order, time spent on
> > > > coalescing
> > > > pages by buddy allocator can be reduced.  With section size of 256MB,
> > > > hot
> > > > add latency of a single section shows improvement from 50-60 ms to
> > > > less
> > > > than 1 ms, hence improving the hot add latency by 60 times.  Modify
> > > > external providers of online callback to align with the change.
> > > >
> > > > Signed-off-by: Arun KS <arunks@xxxxxxxxxxxxxx>
> > > > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > > > Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> > >
> > > Sorry, ended up encountering a couple more things that have me a bit
> > > confused.
> > >
> > > [...]
> > >
> > > > diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> > > > index 5301fef..211f3fe 100644
> > > > --- a/drivers/hv/hv_balloon.c
> > > > +++ b/drivers/hv/hv_balloon.c
> > > > @@ -771,7 +771,7 @@ static void hv_mem_hot_add(unsigned long start,
> > > > unsigned long size,
> > > >  	}
> > > >  }
> > > >
> > > > -static void hv_online_page(struct page *pg)
> > > > +static int hv_online_page(struct page *pg, unsigned int order)
> > > >  {
> > > >  	struct hv_hotadd_state *has;
> > > >  	unsigned long flags;
> > > > @@ -783,10 +783,12 @@ static void hv_online_page(struct page *pg)
> > > >  		if ((pfn < has->start_pfn) || (pfn >= has->end_pfn))
> > > >  			continue;
> > > >
> > > > -		hv_page_online_one(has, pg);
> > > > +		hv_bring_pgs_online(has, pfn, (1UL << order));
> > > >  		break;
> > > >  	}
> > > >  	spin_unlock_irqrestore(&dm_device.ha_lock, flags);
> > > > +
> > > > +	return 0;
> > > >  }
> > > >
> > > >  static int pfn_covered(unsigned long start_pfn, unsigned long
> > > > pfn_cnt)
> > >
> > > So the question I have is why was a return value added to these
> > > functions? They were previously void types and now they are int. What
> > > is the return value expected other than 0?
> >
> > Earlier with returning a void there was now way for an arch code to
> > denying onlining of this particular page. By using an int as return
> > type, we can implement this. In one of the boards I was using, there 
> > are
> > some pages which should not be onlined because they are used for other
> > purposes(like secure trust zone or hypervisor).
>
> So where is the code using that? I don't see any functions in the
> kernel that are returning anything other than 0. Maybe you should hold
> off on changing the return type and make that a separate patch to be
> enabled when you add the new functions that can return non-zero values.
>
> That way if someone wants to backport this they are just getting the
> bits needed to enable the improved hot-plug times without adding the
> extra overhead for changing the return type.

The implementation was in our downstream code. I thought this might be 
useful for someone else in similar situations.
Considering the above mentioned reasons, I ll remove changing the return 
type.

Regards,
Arun