s390: reintroduce diag 44 calls for cpu_relax()
authorHeiko Carstens <heiko.carstens@de.ibm.com>
Wed, 28 Jan 2015 06:43:56 +0000 (07:43 +0100)
committerMartin Schwidefsky <schwidefsky@de.ibm.com>
Thu, 29 Jan 2015 08:19:16 +0000 (09:19 +0100)
commit4d92f50249eb3ed1c066276e214e8cc7be81e96d
treed5d772fb65e20be03a5cdb5f323a66308a6d52e8
parentbdea1f1bb273383312f0eca56241794b06ed4205
s390: reintroduce diag 44 calls for cpu_relax()

Christian Borntraeger reported that the now missing diag 44 calls (voluntary
time slice end) does cause a performance regression for stop_machine() calls
if a machine has more virtual cpus than the host has physical cpus.

This patch mainly reverts 57f2ffe14fd125c2 ("s390: remove diag 44 calls from
cpu_relax()") with the exception that we still do not issue diag 44 calls if
running with smt enabled. Due to group scheduling algorithms when running in
LPAR this would lead to significant latencies.
However, when running in LPAR we do not have more virtual than physical cpus.

Reported-and-tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
arch/s390/include/asm/processor.h
arch/s390/kernel/processor.c