Re: [PATCH] mm, x86, pat: Improve scaling of pat_pagerange_is_ram()

[Ingo Molnar]

* Suresh Siddha <suresh.b.siddha@xxxxxxxxx> wrote:

> On Mon, 2012-05-14 at 15:26 -0500, John Dykstra wrote:
> > Function pat_pagerange_is_ram() scales poorly to large address ranges,
> > because it probes the resource tree for each page.  On a 2.6 GHz
> > Opteron, this function consumes 34 ms. for a 1 GB range.  It is called
> > twice during untrack_pfn_vma(), slowing process cleanup and handicapping
> > the OOM killer.
> > 
> > This replacement based on walk_system_ram_range() consumes less than 1
> > ms. under the same conditions.

Nice performance improvement!

> > 
> > Signed-off-by: John Dykstra <jdykstra@xxxxxxxx> on behalf of Cray Inc. 
> > Cc: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
> > ---
> >  arch/x86/mm/pat.c      |   55 ++++++++++++++++++++++++++++++-----------------
> >  include/linux/ioport.h |    2 +
> >  kernel/resource.c      |    2 +-
> >  3 files changed, 38 insertions(+), 21 deletions(-)
> > 
> > diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> > index f6ff57b..c119afb 100644
> > --- a/arch/x86/mm/pat.c
> > +++ b/arch/x86/mm/pat.c
> > @@ -160,29 +160,44 @@ static unsigned long pat_x_mtrr_type(u64 start, u64 end, unsigned long req_type)
> >  
> >  static int pat_pagerange_is_ram(resource_size_t start, resource_size_t end)
> >  {
> > -	int ram_page = 0, not_rampage = 0;
> > -	unsigned long page_nr;
> > +	struct resource res;
> > +	resource_size_t pg_end, after_ram;
> > +	int ram = 0, not_ram = 0;
> >  
> > -	for (page_nr = (start >> PAGE_SHIFT); page_nr < (end >> PAGE_SHIFT);
> > -	     ++page_nr) {
> > -		/*
> > -		 * For legacy reasons, physical address range in the legacy ISA
> > -		 * region is tracked as non-RAM. This will allow users of
> > -		 * /dev/mem to map portions of legacy ISA region, even when
> > -		 * some of those portions are listed(or not even listed) with
> > -		 * different e820 types(RAM/reserved/..)
> > -		 */
> > -		if (page_nr >= (ISA_END_ADDRESS >> PAGE_SHIFT) &&
> > -		    page_is_ram(page_nr))
> > -			ram_page = 1;
> > -		else
> > -			not_rampage = 1;
> > +	res.start = start & PHYSICAL_PAGE_MASK;
> >  
> > -		if (ram_page == not_rampage)
> > +	/*
> > +	 * For legacy reasons, physical address range in the legacy ISA
> > +	 * region is tracked as non-RAM. This will allow users of
> > +	 * /dev/mem to map portions of legacy ISA region, even when
> > +	 * some of those portions are listed(or not even listed) with
> > +	 * different e820 types(RAM/reserved/..)
> > +	 */
> > +	if (res.start < ISA_END_ADDRESS) {
> > +		not_ram = 1;
> > +		res.start = ISA_END_ADDRESS;
> > +	}
> > +
> > +	pg_end = (end + PAGE_SIZE - 1) & PHYSICAL_PAGE_MASK;
> > +	res.end = pg_end;
> > +	res.flags = IORESOURCE_MEM | IORESOURCE_BUSY;
> > +	after_ram = res.start;
> > +	while ((res.start < res.end) &&
> > +		(find_next_system_ram(&res, "System RAM") >= 0)) {
> > +		if (res.start > after_ram)
> > +			not_ram = 1;
> > +		if (res.end > res.start)
> > +			ram = 1;
> > +
> > +		if (ram && not_ram)
> >  			return -1;
> > +
> > +		after_ram = res.end + 1;
> > +		res.start = res.end + 1;
> > +		res.end = pg_end;
> >  	}
> 
> Instead of duplicating what kernel/resource.c:walk_system_ram_range() is
> already doing, can we just provide a callback that can be used with
> walk_system_ram_range() and see if the expected RAM pages is what the
> callback also sees.
> 
> That will greatly simplify the patch and avoid code duplication.

Agreed.

Thanks,

	Ingo
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