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

[Pekka Enberg <penberg@kernel.org>]

On Mon, Jan 3, 2011 at 3:45 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> 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@mit.edu> 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@gmail.com> 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@gmail.com> 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.

Thanks! AFAICT, that number is high enough to justify removing the
unlikely() annotations, no?

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