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From: Chuck Lever III <chuck.lever@oracle.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Christoph Hellwig <hch@infradead.org>,
	Alexander Duyck <alexander.duyck@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux-Net <netdev@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
	Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH 0/3 v5] Introduce a bulk order-0 page allocator
Date: Tue, 23 Mar 2021 18:52:34 +0000	[thread overview]
Message-ID: <AD267230-8422-4AE4-A315-82C8DE00AA46@oracle.com> (raw)
In-Reply-To: <20210323144541.GL3697@techsingularity.net>



> On Mar 23, 2021, at 10:45 AM, Mel Gorman <mgorman@techsingularity.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





  reply	other threads:[~2021-03-23 18:54 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-22  9:18 [PATCH 0/3 v5] Introduce a bulk order-0 page allocator Mel Gorman
2021-03-22  9:18 ` [PATCH 1/3] mm/page_alloc: Rename alloced to allocated Mel Gorman
2021-03-22  9:18 ` [PATCH 2/3] mm/page_alloc: Add a bulk page allocator Mel Gorman
2021-03-23 16:00   ` Jesper Dangaard Brouer
2021-03-23 18:43     ` Mel Gorman
2021-03-22  9:18 ` [PATCH 3/3] mm/page_alloc: Add an array-based interface to the " Mel Gorman
2021-03-22 12:04 ` [PATCH 0/3 v5] Introduce a bulk order-0 " Jesper Dangaard Brouer
2021-03-22 16:44   ` Mel Gorman
2021-03-22 18:25 ` Chuck Lever III
2021-03-22 19:49   ` Mel Gorman
2021-03-22 20:32     ` Chuck Lever III
2021-03-22 20:58       ` Mel Gorman
2021-03-23 11:08         ` Jesper Dangaard Brouer
2021-03-23 14:45           ` Mel Gorman
2021-03-23 18:52             ` Chuck Lever III [this message]
2021-03-23 11:13       ` Matthew Wilcox
2021-03-23 10:44 ` Mel Gorman
2021-03-23 15:08   ` Jesper Dangaard Brouer
2021-03-23 16:29     ` Mel Gorman
2021-03-23 17:06     ` Jesper Dangaard Brouer

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