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SubjectRe: mm, page_alloc: Swap likely to unlikely as code logic is different for next_zones_zonelist()
On Thu, Jan 05, 2017 at 08:01:02PM -0500, Steven Rostedt wrote:
> Commit 682a3385e773 "mm, page_alloc: inline the fast path of the
> zonelist iterator" changed how next_zones_zonelist() is called, by
> adding a static inline function to do the fast path. This function adds:
>
> if (likely(!nodes && zonelist_zone_idx(z) <= highest_zoneidx))
> return z;
> return __next_zones_zonelist(z, highest_zoneidx, nodes);
>
> Where __next_zones_zonelist() is only called when nodes is not NULL or
> zonelist_zone_idx(z) is less than highest_zoneidx.
>
> The original next_zone_zonelist() was converted to
> __next_zones_zonelist() but it still maintained:
>
> if (likely(nodes == NULL))
>
> Which is now actually a very unlikely, as it is only called with nodes
> equal to NULL when zonelist_zone_idx(z) is greater than highest_zoneidx.
>
> Before this commit, this if had this statistic:
>
> correct incorrect % Function File Line
> ------- --------- - -------- ---- ----
> 837895 446078 34 next_zones_zonelist mmzone.c 63
>
> After this commit, it has:
>
> correct incorrect % Function File Line
> ------- --------- - -------- ---- ----
> 10 173840 99 __next_zones_zonelist mmzone.c 63
>
> Thus, the if statement is now much more unlikely than it ever was as a
> likely.
>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Well spotted;

Acked-by: Mel Gorman <mgorman@techsingularity.net>

--
Mel Gorman
SUSE Labs

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