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Date: Tue, 23 Mar 2021 18:52:34 +0000
From: Chuck Lever III <chuck.lever@...cle.com>
To: Mel Gorman <mgorman@...hsingularity.net>
CC: Jesper Dangaard Brouer <brouer@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>,
Christoph Hellwig <hch@...radead.org>,
Alexander Duyck <alexander.duyck@...il.com>,
Matthew Wilcox <willy@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
Linux-Net <netdev@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
Linux NFS Mailing List <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH 0/3 v5] Introduce a bulk order-0 page allocator
> On Mar 23, 2021, at 10:45 AM, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Tue, Mar 23, 2021 at 12:08:51PM +0100, Jesper Dangaard Brouer wrote:
>>>> <SNIP>
>>>> My results show that, because svc_alloc_arg() ends up calling
>>>> __alloc_pages_bulk() twice in this case, it ends up being
>>>> twice as expensive as the list case, on average, for the same
>>>> workload.
>>>>
>>>
>>> Ok, so in this case the caller knows that holes are always at the
>>> start. If the API returns an index that is a valid index and populated,
>>> it can check the next index and if it is valid then the whole array
>>> must be populated.
>>>
>>> <SNIP>
>>
>> I do know that I suggested moving prep_new_page() out of the
>> IRQ-disabled loop, but maybe was a bad idea, for several reasons.
>>
>> All prep_new_page does is to write into struct page, unless some
>> debugging stuff (like kasan) is enabled. This cache-line is hot as
>> LRU-list update just wrote into this cache-line. As the bulk size goes
>> up, as Matthew pointed out, this cache-line might be pushed into
>> L2-cache, and then need to be accessed again when prep_new_page() is
>> called.
>>
>> Another observation is that moving prep_new_page() into loop reduced
>> function size with 253 bytes (which affect I-cache).
>>
>> ./scripts/bloat-o-meter mm/page_alloc.o-prep_new_page-outside mm/page_alloc.o-prep_new_page-inside
>> add/remove: 18/18 grow/shrink: 0/1 up/down: 144/-397 (-253)
>> Function old new delta
>> __alloc_pages_bulk 1965 1712 -253
>> Total: Before=60799, After=60546, chg -0.42%
>>
>> Maybe it is better to keep prep_new_page() inside the loop. This also
>> allows list vs array variant to share the call. And it should simplify
>> the array variant code.
>>
>
> I agree. I did not like the level of complexity it incurred for arrays
> or the fact it required that a list to be empty when alloc_pages_bulk()
> is called. I thought the concern for calling prep_new_page() with IRQs
> disabled was a little overblown but did not feel strongly enough to push
> back on it hard given that we've had problems with IRQs being disabled
> for long periods before. At worst, at some point in the future we'll have
> to cap the number of pages that can be requested or enable/disable IRQs
> every X pages.
>
> New candidate
>
> git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-bulk-rebase-v6r4
I have rebased the SUNRPC patches to v6r4. Testing has shown a
minor functional regression, which I'm chasing down. But
performance is in the same ballpark. FYI
> Interface is still the same so a rebase should be trivial. Diff between
> v6r2 and v6r4 is as follows. I like the diffstat if nothing else :P
>
>
> mm/page_alloc.c | 54 +++++++++++++-----------------------------------------
> 1 file changed, 13 insertions(+), 41 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 547a84f11310..be1e33a4df39 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4999,25 +4999,20 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> struct alloc_context ac;
> gfp_t alloc_gfp;
> unsigned int alloc_flags;
> - int nr_populated = 0, prep_index = 0;
> - bool hole = false;
> + int nr_populated = 0;
>
> if (WARN_ON_ONCE(nr_pages <= 0))
> return 0;
>
> - if (WARN_ON_ONCE(page_list && !list_empty(page_list)))
> - return 0;
> -
> - /* Skip populated array elements. */
> - if (page_array) {
> - while (nr_populated < nr_pages && page_array[nr_populated])
> - nr_populated++;
> - if (nr_populated == nr_pages)
> - return nr_populated;
> - prep_index = nr_populated;
> - }
> + /*
> + * Skip populated array elements to determine if any pages need
> + * to be allocated before disabling IRQs.
> + */
> + while (page_array && page_array[nr_populated] && nr_populated < nr_pages)
> + nr_populated++;
>
> - if (nr_pages == 1)
> + /* Use the single page allocator for one page. */
> + if (nr_pages - nr_populated == 1)
> goto failed;
>
> /* May set ALLOC_NOFRAGMENT, fragmentation will return 1 page. */
> @@ -5056,22 +5051,17 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> if (!zone)
> goto failed;
>
> -retry_hole:
> /* Attempt the batch allocation */
> local_irq_save(flags);
> pcp = &this_cpu_ptr(zone->pageset)->pcp;
> pcp_list = &pcp->lists[ac.migratetype];
>
> while (nr_populated < nr_pages) {
> - /*
> - * Stop allocating if the next index has a populated
> - * page or the page will be prepared a second time when
> - * IRQs are enabled.
> - */
> +
> + /* Skip existing pages */
> if (page_array && page_array[nr_populated]) {
> - hole = true;
> nr_populated++;
> - break;
> + continue;
> }
>
> page = __rmqueue_pcplist(zone, ac.migratetype, alloc_flags,
> @@ -5092,6 +5082,7 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> __count_zid_vm_events(PGALLOC, zone_idx(zone), 1);
> zone_statistics(ac.preferred_zoneref->zone, zone);
>
> + prep_new_page(page, 0, gfp, 0);
> if (page_list)
> list_add(&page->lru, page_list);
> else
> @@ -5101,25 +5092,6 @@ int __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>
> local_irq_restore(flags);
>
> - /* Prep pages with IRQs enabled. */
> - if (page_list) {
> - list_for_each_entry(page, page_list, lru)
> - prep_new_page(page, 0, gfp, 0);
> - } else {
> - while (prep_index < nr_populated)
> - prep_new_page(page_array[prep_index++], 0, gfp, 0);
> -
> - /*
> - * If the array is sparse, check whether the array is
> - * now fully populated. Continue allocations if
> - * necessary.
> - */
> - while (nr_populated < nr_pages && page_array[nr_populated])
> - nr_populated++;
> - if (hole && nr_populated < nr_pages)
> - goto retry_hole;
> - }
> -
> return nr_populated;
>
> failed_irq:
>
> --
> Mel Gorman
> SUSE Labs
--
Chuck Lever
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