Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to3.6-rc5 on AMD chipsets - bisected

[Borislav Petkov]

On Fri, Sep 14, 2012 at 03:01:44PM -0700, Linus Torvalds wrote:
> On Fri, Sep 14, 2012 at 2:56 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> >
> > Both things change semantics, not looking at the entire package is new
> > too.
> 
> Well, the "idle_buddy" thing on its own could be considered to be
> purely a caching thing.
> 
> Sure, it doesn't take tsk_cpus_allowed() into account while setting up
> the cache (since it's not dynamic enough), but *assuming* the common
> case is that people let threads be on any of the cores of a package,
> it should be possible to make the cache 100% equivalent with no
> semantic change. No?
> 
> The code doesn't even try to do that kind of "don't change semantics",
> though, and makes the idle-buddy thing entirely different.

Could it be that this machine has a non-power of two cores (6) and maybe
the idle buddy selection doesn't work as expected?

 (Although it shouldn't since the commit message speaks also of non-power of
  two cores (40) and it works fine there).

Anyway, here's the topology of the box:

$ for i in $(ls cpu?/topology/*); do echo -n $i": "; cat $i; done
cpu0/topology/core_id: 0
cpu0/topology/core_siblings: 3f
cpu0/topology/core_siblings_list: 0-5
cpu0/topology/physical_package_id: 0
cpu0/topology/thread_siblings: 01
cpu0/topology/thread_siblings_list: 0
cpu1/topology/core_id: 1
cpu1/topology/core_siblings: 3f
cpu1/topology/core_siblings_list: 0-5
cpu1/topology/physical_package_id: 0
cpu1/topology/thread_siblings: 02
cpu1/topology/thread_siblings_list: 1
cpu2/topology/core_id: 2
cpu2/topology/core_siblings: 3f
cpu2/topology/core_siblings_list: 0-5
cpu2/topology/physical_package_id: 0
cpu2/topology/thread_siblings: 04
cpu2/topology/thread_siblings_list: 2
cpu3/topology/core_id: 3
cpu3/topology/core_siblings: 3f
cpu3/topology/core_siblings_list: 0-5
cpu3/topology/physical_package_id: 0
cpu3/topology/thread_siblings: 08
cpu3/topology/thread_siblings_list: 3
cpu4/topology/core_id: 4
cpu4/topology/core_siblings: 3f
cpu4/topology/core_siblings_list: 0-5
cpu4/topology/physical_package_id: 0
cpu4/topology/thread_siblings: 10
cpu4/topology/thread_siblings_list: 4
cpu5/topology/core_id: 5
cpu5/topology/core_siblings: 3f
cpu5/topology/core_siblings_list: 0-5
cpu5/topology/physical_package_id: 0
cpu5/topology/thread_siblings: 20
cpu5/topology/thread_siblings_list: 5

-- 
Regards/Gruss,
    Boris.
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