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

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Date:   Thu, 25 Mar 2021 14:26:24 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-Net <netdev@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-NFS <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH 0/9 v6] Introduce a bulk order-0 page allocator with two
 in-tree users

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/20210323133948.GA10046@pc638.lan/
> > > 
> > 
> > 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.

> --
> Vlad Rezki

-- 
Mel Gorman
SUSE Labs