Re: Should we be using unlikely() around tests of GFP_ZERO?

[Steven Rostedt]

On Mon, 2011-01-03 at 09:40 +0200, Pekka Enberg wrote:
> Hi,
> 
> On Mon, Jan 3, 2011 at 8:48 AM, Theodore Ts'o <tytso@xxxxxxx> wrote:
> >> Given the patches being busily submitted by trivial patch submitters to
> >> make use kmem_cache_zalloc(), et. al, I believe we should remove the
> >> unlikely() tests around the (gfp_flags & __GFP_ZERO) tests, such as:
> >>
> >> -       if (unlikely((flags & __GFP_ZERO) && objp))
> >> +       if ((flags & __GFP_ZERO) && objp)
> >>                memset(objp, 0, obj_size(cachep));
> >>
> >> Agreed?  If so, I'll send a patch...
> 
> On Mon, Jan 3, 2011 at 5:46 AM, Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> > I support it.
> 
> I guess the rationale here is that if you're going to take the hit of
> memset() you can take the hit of unlikely() as well. We're optimizing
> for hot call-sites that allocate a small amount of memory and
> initialize everything themselves. That said, I don't think the
> unlikely() annotation matters much either way and am for removing it
> unless people object to that.
> 
> On Mon, Jan 3, 2011 at 5:46 AM, Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> > Recently Steven tried to gather the information.
> > http://thread.gmane.org/gmane.linux.kernel/1072767
> > Maybe he might have a number for that.
> 
> That would be interesting, sure.

Note, you could do it yourself too. Just enable:

  Kernel Hacking -> Tracers -> Branch Profiling
    (Trace likely/unlikely profiler)

   CONFIG_PROFILE_ANNOTATED_BRANCHES

Then search /debug/tracing/trace_stats/branch_annotated.

(hmm, the help in Kconfig is wrong, I need to fix that)


Anyway, here's my box. I just started it an hour ago, and have not been
doing too much on it yet. But here's what I got (using SLUB)


 correct incorrect  %        Function                  File              Line
 ------- ---------  -        --------                  ----              ----
 6890998  2784830  28        slab_alloc                slub.c            1719

That's incorrect 28% of the time.

-- Steve


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