Re: [PATCH] s390: Remove GENERIC_LOCKBREAK Kconfig option

[Sven Schnelle <svens@linux.ibm.com>]

Heiko Carstens <hca@linux.ibm.com> writes:

> s390 selects GENERIC_LOCKBREAK if PREEMPT is enabled. Reason is a historic
> 18 years old commit [1] which fixed a compile error for PREEMPT enabled
> kernels. Back than only PREEMPT_NONE and PREEMPT_VOLUNTARY kernels were
> considered to be important for s390. PREEMPT should "just work".
>
> However, since recently PREEMPT is always enabled [2], which also causes
> GENERIC_LOCKBREAK to be always enabled. For some workloads this leads to
> massive performance degradation; e.g. a simple kernel compile on machines
> with many CPUs may take up to four times longer.
>
> To fix this just remove the GENERIC_LOCKBREAK from s390's Kconfig, since
> the compile error from 18 years ago does not exist anymore.
>
> [1] commit b6b40c532a36 ("[S390] Define GENERIC_LOCKBREAK.")
> [2] commit 7dadeaa6e851 ("sched: Further restrict the preemption modes")
>
> Cc: stable@vger.kernel.org
> Reported-by: Massimiliano Pellizzer <massimiliano.pellizzer@canonical.com>
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  arch/s390/Kconfig | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index ecbcbb781e40..9921a3772bf7 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -29,9 +29,6 @@ config GENERIC_BUG
>  config GENERIC_BUG_RELATIVE_POINTERS
>  	def_bool y
>  
> -config GENERIC_LOCKBREAK
> -	def_bool y if PREEMPTION
> -
>  config AUDIT_ARCH
>  	def_bool y

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>