Re: [PATCH v7 2/5] arm: arm64: page_alloc: reduce unnecessary binary search in memblock_next_valid_pfn()

[Matthew Wilcox]

On Thu, Apr 05, 2018 at 08:44:12PM +0800, Jia He wrote:
> 
> 
> On 4/5/2018 7:34 PM, Matthew Wilcox Wrote:
> > On Thu, Apr 05, 2018 at 01:04:35AM -0700, Jia He wrote:
> > > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
> > > where possible") optimized the loop in memmap_init_zone(). But there is
> > > still some room for improvement. E.g. if pfn and pfn+1 are in the same
> > > memblock region, we can simply pfn++ instead of doing the binary search
> > > in memblock_next_valid_pfn.
> > Sure, but I bet if we are >end_pfn, we're almost certainly going to the
> > start_pfn of the next block, so why not test that as well?
> > 
> > > +	/* fast path, return pfn+1 if next pfn is in the same region */
> > > +	if (early_region_idx != -1) {
> > > +		start_pfn = PFN_DOWN(regions[early_region_idx].base);
> > > +		end_pfn = PFN_DOWN(regions[early_region_idx].base +
> > > +				regions[early_region_idx].size);
> > > +
> > > +		if (pfn >= start_pfn && pfn < end_pfn)
> > > +			return pfn;
> > 		early_region_idx++;
> > 		start_pfn = PFN_DOWN(regions[early_region_idx].base);
> > 		if (pfn >= end_pfn && pfn <= start_pfn)
> > 			return start_pfn;
> Thanks, thus the binary search in next step can be discarded?

I don't know all the circumstances in which this is called.  Maybe a linear
search with memo is more appropriate than a binary search.