On 01/06/2017 02:01 AM, 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))
Your profiling stats posted online suggested even this one was wrong:
function.file.line correct incorrect percent
next_zones_zonelist.mmzone.h.965 216840653 25415261543 %0.85
Was it bogus then, or is there other explanation? I have speculated that
this might be due to multiple cpusets configured in the system.
> 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) <[email protected]>
Anyway this makes sense.
Acked-by: Vlastimil Babka <[email protected]>
> ---
> diff --git a/mm/mmzone.c b/mm/mmzone.c
> index 5652be858e5e..a51c0a67ea3d 100644
> --- a/mm/mmzone.c
> +++ b/mm/mmzone.c
> @@ -60,7 +60,7 @@ struct zoneref *__next_zones_zonelist(struct zoneref *z,
> * Find the next suitable zone to use for the allocation.
> * Only filter based on nodemask if it's set
> */
> - if (likely(nodes == NULL))
> + if (unlikely(nodes == NULL))
> while (zonelist_zone_idx(z) > highest_zoneidx)
> z++;
> else
>
