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

From: Linus Torvalds
Date: Fri Sep 14 2012 - 18:02:03 EST

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.

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