Re: [PATCH] mm: Add unlikely for current_order test

From: Zhang Yanfei
Date: Mon Jun 17 2013 - 17:57:04 EST

On 06/18/2013 05:37 AM, David Rientjes wrote:
> On Mon, 17 Jun 2013, Zhang Yanfei wrote:
>>> 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.
>> 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.
> 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.

I see. Thanks!

I will send another patch.

Zhang Yanfei
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at