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

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Message-ID: <210ea658c3bdd074febbe90b19e88615@codeaurora.org>
Date:   Thu, 10 Jan 2019 10:09:40 +0530
From:   Arun KS <arunks@...eaurora.org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     arunks.linux@...il.com, akpm@...ux-foundation.org,
        mhocko@...nel.org, vbabka@...e.cz, osalvador@...e.de,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        getarunks@...il.com
Subject: Re: [PATCH v7] mm/page_alloc.c: memory_hotplug: free pages as higher
 order

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@...eaurora.org>
>> > > Acked-by: Michal Hocko <mhocko@...e.com>
>> > > Reviewed-by: Oscar Salvador <osalvador@...e.de>
>> >
>> > 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