Re: calibrate_migration_costs takes ages on s390

[Heiko Carstens <heiko.carstens@de.ibm.com>]

> > We did a bit of testing, -rc2-git3 + the patch below was still ok.
> > 
> >  [PATCH] s390: earlier initialization of cpu_possible_map
> >  9733e2407ad2237867cb13c04e7d619397fa3090
> 
> I need to double check, but -git5 + that patch was reported to be slow.

I did a quick git bisect search. This is one is the hurting one:

Author: Ingo Molnar <mingo@elte.hu>  2006-02-07 21:58:54
Committer: Linus Torvalds <torvalds@g5.osdl.org>  2006-02-08 01:12:33
Parent: 8519fb30e438f8088b71a94a7d5a660a814d3872 ([PATCH] mm: compound release fix)
Child:  0d4c3e7a8c65892c7d6a748fdbb4499e988880db ([PATCH] unshare system call -v5: Documentation file)

    [PATCH] Fix spinlock debugging delays to not time out too early
    
    The spinlock-debug wait-loop was using loops_per_jiffy to detect too long
    spinlock waits - but on fast CPUs this led to a way too fast timeout and false
    messages.
    
    The fix is to include a __delay(1) call in the loop, to correctly approximate
    the intended delay timeout of 1 second.  The code assumes that every
    architecture implements __delay(1) to last around 1/(loops_per_jiffy*HZ)
    seconds.
    
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    Cc: Andi Kleen <ak@muc.de>
    Signed-off-by: Andrew Morton <akpm@osdl.org>
    Signed-off-by: Linus Torvalds <torvalds@osdl.org>

I guess we're once again suffering from being a virtualized platform: the
formerly used call to cpu_relax() informed the underlying hypervisor that
we want to give up the current cpu while __delay() keeps it.
Unless we're scheduled away involuntarily.
The "Detect Soft Lockups" option doesn't make too much sense too on our
platform, since we get a lot of false positives.
Quick fix: turn off the options CONFIG_DEBUG_SPINLOCK and
CONFIG_DETECT_SOFTLOCKUP.

Heiko
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