linux-kernel - Re: mm, page_alloc: Swap likely to unlikely as code logic is different for next_zones

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Message-ID: <20170106094112.6aae3e5f@gandalf.local.home>
Date:   Fri, 6 Jan 2017 09:41:12 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dirk Hohndel <dirkhh@...are.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: mm, page_alloc: Swap likely to unlikely as code logic is
 different for next_zones_zonelist()

On Fri, 6 Jan 2017 09:00:23 +0100
Vlastimil Babka <vbabka@...e.cz> wrote:

> 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.

I didn't have any cpusets configured on either system.

But the number is exaggerated, as it too was affected by the "constant"
optimization by gcc. But when I fixed that, it still gives me about
50%. Probably best just to remove the likely() in that case.

> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>  
> 
> Anyway this makes sense.
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

-- Steve