Re: [PATCH 1/2] mm: page_alloc: speed up fallbacks in rmqueue_bulk()

[Brendan Jackman]

On Tue Apr 8, 2025 at 6:50 PM UTC, Johannes Weiner wrote:
>> >	/*
>> >	 * Find the largest available free page in the other list. This roughly
>> >	 * approximates finding the pageblock with the most free pages, which
>> >	 * would be too costly to do exactly.
>> >	 */
>> >	for (current_order = MAX_PAGE_ORDER; current_order >= min_order;
>> >				--current_order) {
>> 
>> IIUC we could go one step further here and also avoid repeating this
>> iteration? Maybe something for a separate patch though?
>
> That might be worth a test, but agree this should be a separate patch.
>
> AFAICS, in the most common configurations MAX_PAGE_ORDER is only one
> step above pageblock_order or even the same. It might not be worth the
> complication.

Oh yeah, makes sense.

>>         /*
>> -        * Try the different freelists, native then foreign.
>> +        * First try the freelists of the requested migratetype, then try
>> +        * fallbacks. Roughly, each fallback stage poses more of a fragmentation
>> +        * risk.
>
> How about "then try fallback modes with increasing levels of
> fragmentation risk."

Yep, nice thanks.

>>          * The fallback logic is expensive and rmqueue_bulk() calls in
>>          * a loop with the zone->lock held, meaning the freelists are
>> @@ -2332,7 +2329,7 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
>>         case RMQUEUE_CLAIM:
>>                 page = __rmqueue_claim(zone, order, migratetype, alloc_flags);
>>                 if (page) {
>> -                       /* Replenished native freelist, back to normal mode */
>> +                       /* Replenished requested migratetype's freelist, back to normal mode */
>>                         *mode = RMQUEUE_NORMAL;
>
> This line is kind of long now. How about:
>
> 			/* Replenished preferred freelist, back to normal mode */

Yep, sounds good - it's still 81 characters, the rest of this file
sticks to 80 for comments, I guess I'll leave it to Andrew to decide if
that is an issue?

> But yeah, I like your proposed changes. Would you care to send a
> proper patch?

Sure, pasting below. Andrew, could you fold this in? Also, I haven't
done this style of patch sending before, please let me know if I'm doing
something to make your life difficult.

> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Aside from the commen stuff fixed by the patch below:

Reviewed-by: Brendan Jackman <jackmanb@xxxxxxxxxx>

---

>From 8ff20dbb52770d082e182482d2b47e521de028d1 Mon Sep 17 00:00:00 2001                                                                                                                                                                                                                    
From: Brendan Jackman <jackmanb@xxxxxxxxxx>
Date: Wed, 9 Apr 2025 17:22:14 +000
Subject: [PATCH] page_alloc: speed up fallbacks in rmqueue_bulk() - comment updates

Tidy up some terminology and redistribute commentary.                                                                                                                                                                                                                                                                                            
Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
---
 mm/page_alloc.c | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dfb2b3f508af4..220bd0bcc38c3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2183,21 +2183,13 @@ try_to_claim_block(struct zone *zone, struct page *page,
 }
  
 /*
- * Try finding a free buddy page on the fallback list.
- *
- * This will attempt to claim a whole pageblock for the requested type
- * to ensure grouping of such requests in the future.
- *
- * If a whole block cannot be claimed, steal an individual page, regressing to
- * __rmqueue_smallest() logic to at least break up as little contiguity as
- * possible.
+ * Try to allocate from some fallback migratetype by claiming the entire block,
+ * i.e. converting it to the allocation's start migratetype.
  *
  * The use of signed ints for order and current_order is a deliberate 
  * deviation from the rest of this file, to make the for loop
  * condition simpler.
  */
-
-/* Try to claim a whole foreign block, take a page, expand the remainder */
 static __always_inline struct page *
 __rmqueue_claim(struct zone *zone, int order, int start_migratetype,
                                                unsigned int alloc_flags)
@@ -2247,7 +2239,10 @@ __rmqueue_claim(struct zone *zone, int order, int start_migratetype,
        return NULL;
 }
  
-/* Try to steal a single page from a foreign block */
+/*
+ * Try to steal a single page from some fallback migratetype. Leave the rest of
+ * the block as its current migratetype, potentially causing fragmentation.
+ */
 static __always_inline struct page *
 __rmqueue_steal(struct zone *zone, int order, int start_migratetype)
 {
@@ -2307,7 +2302,8 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
        }
  
        /*
-        * Try the different freelists, native then foreign.
+        * First try the freelists of the requested migratetype, then try
+        * fallbacks modes with increasing levels of fragmentation risk.
         *
         * The fallback logic is expensive and rmqueue_bulk() calls in
         * a loop with the zone->lock held, meaning the freelists are
@@ -2332,7 +2328,7 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
        case RMQUEUE_CLAIM:
                page = __rmqueue_claim(zone, order, migratetype, alloc_flags);
                if (page) {
-                       /* Replenished native freelist, back to normal mode */
+                       /* Replenished preferred freelist, back to normal mode. */
                        *mode = RMQUEUE_NORMAL;
                        return page;
                }

base-commit: aa42382db4e2a4ed1f4ba97ffc50e2ce45accb0c
-- 
2.49.0.504.g3bcea36a83-goog