Subject: Re: [PATCH 0/9 v6] Introduce a bulk order-0 page allocator with two in-tree users

From: Uladzislau Rezki <[email-masked]>
Date: Thu, 25 Mar 2021 15:46:11 +0100

On Thu, Mar 25, 2021 at 02:26:24PM +0000, Mel Gorman wrote:
> On Thu, Mar 25, 2021 at 03:06:57PM +0100, Uladzislau Rezki wrote:
> > > On Thu, Mar 25, 2021 at 12:50:01PM +0000, Matthew Wilcox wrote:
> > > > On Thu, Mar 25, 2021 at 11:42:19AM +0000, Mel Gorman wrote:
> > > > > This series introduces a bulk order-0 page allocator with sunrpc and
> > > > > the network page pool being the first users. The implementation is not
> > > > > efficient as semantics needed to be ironed out first. If no other semantic
> > > > > changes are needed, it can be made more efficient.  Despite that, this
> > > > > is a performance-related for users that require multiple pages for an
> > > > > operation without multiple round-trips to the page allocator. Quoting
> > > > > the last patch for the high-speed networking use-case
> > > > > 
> > > > >             Kernel          XDP stats       CPU     pps           Delta
> > > > >             Baseline        XDP-RX CPU      total   3,771,046       n/a
> > > > >             List            XDP-RX CPU      total   3,940,242    +4.49%
> > > > >             Array           XDP-RX CPU      total   4,249,224   +12.68%
> > > > > 
> > > > > >From the SUNRPC traces of svc_alloc_arg()
> > > > > 
> > > > > 	Single page: 25.007 us per call over 532,571 calls
> > > > > 	Bulk list:    6.258 us per call over 517,034 calls
> > > > > 	Bulk array:   4.590 us per call over 517,442 calls
> > > > > 
> > > > > Both potential users in this series are corner cases (NFS and high-speed
> > > > > networks) so it is unlikely that most users will see any benefit in the
> > > > > short term. Other potential other users are batch allocations for page
> > > > > cache readahead, fault around and SLUB allocations when high-order pages
> > > > > are unavailable. It's unknown how much benefit would be seen by converting
> > > > > multiple page allocation calls to a single batch or what difference it may
> > > > > make to headline performance.
> > > > 
> > > > We have a third user, vmalloc(), with a 16% perf improvement.  I know the
> > > > email says 21% but that includes the 5% improvement from switching to
> > > > kvmalloc() to allocate area->pages.
> > > > 
> > > > https://lore.kernel.org/linux-mm/[email-masked]/
> > > > 
> > > 
> > > That's fairly promising. Assuming the bulk allocator gets merged, it would
> > > make sense to add vmalloc on top. That's for bringing it to my attention
> > > because it's far more relevant than my imaginary potential use cases.
> > > 
> > For the vmalloc we should be able to allocating on a specific NUMA node,
> > at least the current interface takes it into account. As far as i see
> > the current interface allocate on a current node:
> > 
> > static inline unsigned long
> > alloc_pages_bulk_array(gfp_t gfp, unsigned long nr_pages, struct page **page_array)
> > {
> >     return __alloc_pages_bulk(gfp, numa_mem_id(), NULL, nr_pages, NULL, page_array);
> > }
> > 
> > Or am i missing something?
> > 
> 
> No, you're not missing anything. Options would be to add a helper similar
> alloc_pages_node or to directly call __alloc_pages_bulk specifying a node
> and using GFP_THISNODE. prepare_alloc_pages() should pick the correct
> zonelist containing only the required node.
> 
IMHO, a helper something like *_node() would be reasonable. I see that many
functions in "mm" have its own variants which explicitly add "_node()" prefix
to signal to users that it is a NUMA aware calls.

As for __alloc_pages_bulk(), i got it.

Thanks!

--
Vlad Rezki