Messages in this thread | | | Subject | Re: mm, page_alloc: Swap likely to unlikely as code logic is different for next_zones_zonelist() | From | Vlastimil Babka <> | Date | Fri, 6 Jan 2017 09:00:23 +0100 |
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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) <rostedt@goodmis.org>
Anyway this makes sense.
Acked-by: Vlastimil Babka <vbabka@suse.cz>
> --- > 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 >
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