mm: Add unlikely for current_order test
Subsystems (2)
PATCH 1/1 mm: Add unlikely for current_order test
From: Zhang Yanfei <[email protected]>
Since we have an unlikely for the "current_order >= pageblock_order / 2" test above, adding an unlikely for this "current_order >= pageblock_order" test seems more appropriate.
--- mm/page_alloc.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)
mm/page_alloc.c +1 -1
@@ -1071,7 +1071,7 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)rmv_page_order (page);/* Take ownership for orders >= pageblock_order */ -if (current_order >= pageblock_order && +if (unlikely (current_order >= pageblock_order) && !is_migrate_cma (migratetype))change_pageblock_range (page, current_order, start_migratetype);
Signature · 7 lines
1.7.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
Quoted 6 lines · Zhang Yanfei
From: Zhang Yanfei <[email protected]>
Since we have an unlikely for the "
current_order>=pageblock_order/ 2" test above, adding an unlikely for this "current_order>=pageblock_order" test seems more appropriate.
I don't understand the justification at all, current_order being unlikely greater than or equal to pageblock_order / 2 doesn't imply at all that it's unlikely that current_order is greater than or equal to pageblock_order.
Quoted 10 lines
Signed-off-by Zhang Yanfei <[email protected]>--- mm/page_alloc.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)mm/page_alloc.c +0
-0
mm/page_alloc.c __rmqueue_fallback(struct zone *zone, int order, int start_migratetype) · line 1077
Quoted 8 lines
mm/page_alloc.c +1
-1@@ -1071,7 +1071,7 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)rmv_page_order (page);/* Take ownership for orders >= pageblock_order */ -if (current_order >= pageblock_order && +if (unlikely (current_order >= pageblock_order) && !is_migrate_cma (migratetype))change_pageblock_range (page, current_order, start_migratetype);
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Hi David,
Quoted 9 lines · David Rientjes
Quoted 6 lines · Zhang Yanfei
From: Zhang Yanfei <[email protected]>
Since we have an unlikely for the "
current_order>=pageblock_order/ 2" test above, adding an unlikely for this "current_order>=pageblock_order" test seems more appropriate.
Quoted 5 lines
I don't understand the justification at all,
current_orderbeing unlikely greater than or equal topageblock_order/ 2 doesn't imply at all that it's unlikely thatcurrent_orderis greater than or equal topageblock_order.
hmmm... I am confused. Since current_order is >= pageblock_order / 2 is unlikely, why current_order is >= pageblock_order isn't unlikely. Or there are other tips?
Actually, I am also a little confused about why current_order should be unlikely greater than or equal to pageblock_order / 2. When borrowing pages with other migrate_type, we always search from MAX_ORDER-1, which is greater or equal to pageblock_order.
Signature · 8 lines
Thanks. Zhang Yanfei -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
Quoted 6 lines · Zhang Yanfei
Quoted 5 lines
I don't understand the justification at all,
current_orderbeing unlikely greater than or equal topageblock_order/ 2 doesn't imply at all that it's unlikely thatcurrent_orderis greater than or equal topageblock_order.
Quoted 9 lines
hmmm... I am confused. Since
current_orderis >=pageblock_order/ 2 is unlikely, whycurrent_orderis >=pageblock_orderisn't unlikely. Or there are other tips?Actually, I am also a little confused about why
current_ordershould be unlikely greater than or equal topageblock_order/ 2. When borrowing pages with othermigrate_type, we always search from MAX_ORDER-1, which is greater or equal topageblock_order.
Look at what is being done in the function: current_order loops down from MAX_ORDER-1 to the order passed. It is not at all "unlikely" that current_order is greater than pageblock_order, or pageblock_order / 2.
MAX_ORDER is typically 11 and pageblock_order is typically 9 on x86. Integer division truncates, so pageblock_order / 2 is 4. For the first eight iterations, it's guaranteed that current_order >= pageblock_order / 2 if it even gets that far!
So just remove the unlikely() entirely, it's completely bogus.
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Quoted 22 lines · David Rientjes
Quoted 6 lines · Zhang Yanfei
Quoted 5 lines
I don't understand the justification at all,
current_orderbeing unlikely greater than or equal topageblock_order/ 2 doesn't imply at all that it's unlikely thatcurrent_orderis greater than or equal topageblock_order.Quoted 9 lines
hmmm... I am confused. Since
current_orderis >=pageblock_order/ 2 is unlikely, whycurrent_orderis >=pageblock_orderisn't unlikely. Or there are other tips?Actually, I am also a little confused about why
current_ordershould be unlikely greater than or equal topageblock_order/ 2. When borrowing pages with othermigrate_type, we always search from MAX_ORDER-1, which is greater or equal topageblock_order.Look at what is being done in the function:
current_orderloops down from MAX_ORDER-1 to the order passed. It is not at all "unlikely" thatcurrent_orderis greater thanpageblock_order, orpageblock_order/ 2.
Quoted 6 lines
MAX_ORDERis typically 11 andpageblock_orderis typically 9 on x86. Integer division truncates, sopageblock_order/ 2 is 4. For the first eight iterations, it's guaranteed thatcurrent_order>=pageblock_order/ 2 if it even gets that far!So just remove the
unlikely()entirely, it's completely bogus.
I see. Thanks!
I will send another patch.
Signature · 8 lines
Thanks. Zhang Yanfei -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"[email protected]"> [email protected] </a>