Re: [PATCH v2 1/2] free_pcppages_bulk: do not hold lock when picking pages to free

[Aaron Lu]

On Thu, Feb 15, 2018 at 04:46:44AM -0800, Matthew Wilcox wrote:
> On Thu, Jan 25, 2018 at 03:21:44PM +0800, Aaron Lu wrote:
> > When freeing a batch of pages from Per-CPU-Pages(PCP) back to buddy,
> > the zone->lock is held and then pages are chosen from PCP's migratetype
> > list. While there is actually no need to do this 'choose part' under
> > lock since it's PCP pages, the only CPU that can touch them is us and
> > irq is also disabled.
> 
> I have no objection to this patch.  If you're looking for ideas for
> future improvement though, I wonder whether using a LIST_HEAD is the
> best way to store these pages temporarily.  If you batch them into a
> pagevec and then free the entire pagevec, the CPU should be a little
> faster scanning a short array than walking a linked list.

Thanks for the suggestion.

> It would also puts a hard boundary on how long zone->lock is held, as
> you'd drop it and go back for another batch after 15 pages.  That might
> be bad, of course.

Yes that's a concern.
As Mel reponded in another email, I think I'll just keep using list
here.

> 
> Another minor change I'd like to see is free_pcpages_bulk updating
> pcp->count itself; all of the callers do it currently.

Sounds good, I'll prepare a separate patch for this, thanks!