Re: calibrate_migration_costs takes ages on s390

[Heiko Carstens]

> I did a quick git bisect search. This is one is the hurting one:
> 
> Author: Ingo Molnar <mingo@xxxxxxx>  2006-02-07 21:58:54
> Committer: Linus Torvalds <torvalds@xxxxxxxxxxx>  2006-02-08 01:12:33
> Parent: 8519fb30e438f8088b71a94a7d5a660a814d3872 ([PATCH] mm: compound release fix)
> Child:  0d4c3e7a8c65892c7d6a748fdbb4499e988880db ([PATCH] unshare system call -v5: Documentation file)
>
>     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.
> 
> 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.

Wrong analysis. Our __delay() implementation is broken. This doesn't help for
the CONFIG_DETECT_SOFTLOCKUP case, but at least CONFIG_DEBUG_SPINLOCK works
again with this.

Andrew, could you pick this one up, or should I send it separately?

[PATCH] s390: fix __delay implementation

From: Heiko Carstens <heiko.carstens@xxxxxxxxxx>

Fix __delay implementation. Called with an argument "1" or "0" it
would loop nearly forever (since (1/2)-1 = 0xffffffff).

Signed-off-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
---

 arch/s390/lib/delay.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/lib/delay.c b/arch/s390/lib/delay.c
index e96c35b..71f0a2f 100644
--- a/arch/s390/lib/delay.c
+++ b/arch/s390/lib/delay.c
@@ -30,7 +30,7 @@ void __delay(unsigned long loops)
          */
         __asm__ __volatile__(
                 "0: brct %0,0b"
-                : /* no outputs */ : "r" (loops/2) );
+                : /* no outputs */ : "r" ((loops/2) + 1));
 }
 
 /*
-
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