Date:	Thu, 4 Dec 2014 08:13:21 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v2] tinification: Introduce CONFIG_SRCU to make SRCU
 optional

On Wed, Dec 03, 2014 at 11:03:52PM -0500, Pranith Kumar wrote:
> (CC list trimmed since this is just for initial feedback)
> 
> SRCU is not necessary to be compiled by default in all cases. For tinification
> efforts not compiling SRCU unless necessary is desirable.
> 
> The current patch tries to make compiling SRCU optional by introducing a new
> Kconfig option CONFIG_SRCU which is selected when any of the components making
> use of SRCU are selected.
> 
> The current tinyconfig on x86 enables SRCU by default since x86 enables
> PERF_EVENTS which in-turn require SRCU support. If we disable
> PERF_EVENTS, we can disable compiling SRCU for tiny configs on x86 systems.

To the best of my knowledge, "select" still doesn't recursively enable
things that the selected symbol selects, so I think you need to "select
SRCU" from X86, right next to the "select PERF_EVENTS".

Also, can you try your patch on top of the current patch series to
compile without perf (tiny/disable-perf), and see if you can
successfully compile without SRCU on x86?

> I tested this using randconfig and building about 15 kernels. This is definitely
> not complete and could use more testing. Sticking it in next might help in that
> regards.
> 
> Comments and suggestions are welcome. Please let me know if I should split up
> this patch.

I don't think this needs splitting; it's a very small and obvious patch
combined with a few selects.

The code looks good to me.  Assuming it compiles on x86, with tinyconfig
and with allyesconfig minus SRCU (and whatever requires it), this seems
reasonable.

Thanks for working on this!

- Josh Triplett

> v2:
>  - handle cpufreq, devfreq and notifier
>  - build tested tinyconfig on powerpc
> 
>  arch/arm/kvm/Kconfig     |  1 +
>  arch/arm64/kvm/Kconfig   |  1 +
>  arch/ia64/kvm/Kconfig    |  1 +
>  arch/mips/kvm/Kconfig    |  1 +
>  arch/powerpc/kvm/Kconfig |  1 +
>  arch/s390/kvm/Kconfig    |  1 +
>  arch/tile/kvm/Kconfig    |  1 +
>  arch/x86/kvm/Kconfig     |  1 +
>  drivers/clk/Kconfig      |  1 +
>  drivers/cpufreq/Kconfig  |  1 +
>  drivers/devfreq/Kconfig  |  1 +
>  drivers/md/Kconfig       |  1 +
>  drivers/net/Kconfig      |  1 +
>  fs/btrfs/Kconfig         |  1 +
>  fs/notify/Kconfig        |  1 +
>  init/Kconfig             | 10 ++++++++++
>  kernel/notifier.c        |  3 +++
>  kernel/rcu/Makefile      |  3 ++-
>  lib/Kconfig.debug        |  1 +
>  mm/Kconfig               |  1 +
>  security/tomoyo/Kconfig  |  1 +
>  21 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/kvm/Kconfig b/arch/arm/kvm/Kconfig
> index 466bd29..3afee5f 100644
> --- a/arch/arm/kvm/Kconfig
> +++ b/arch/arm/kvm/Kconfig
> @@ -23,6 +23,7 @@ config KVM
>  	select HAVE_KVM_CPU_RELAX_INTERCEPT
>  	select KVM_MMIO
>  	select KVM_ARM_HOST
> +	select SRCU
>  	depends on ARM_VIRT_EXT && ARM_LPAE
>  	---help---
>  	  Support hosting virtualized guest machines. You will also
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 8ba85e9..b334084 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -26,6 +26,7 @@ config KVM
>  	select KVM_ARM_HOST
>  	select KVM_ARM_VGIC
>  	select KVM_ARM_TIMER
> +	select SRCU
>  	---help---
>  	  Support hosting virtualized guest machines.
>  
> diff --git a/arch/ia64/kvm/Kconfig b/arch/ia64/kvm/Kconfig
> index 3d50ea9..f7d380b 100644
> --- a/arch/ia64/kvm/Kconfig
> +++ b/arch/ia64/kvm/Kconfig
> @@ -29,6 +29,7 @@ config KVM
>  	select HAVE_KVM_IRQ_ROUTING
>  	select KVM_APIC_ARCHITECTURE
>  	select KVM_MMIO
> +	select SRCU
>  	---help---
>  	  Support hosting fully virtualized guest machines using hardware
>  	  virtualization extensions.  You will need a fairly recent
> diff --git a/arch/mips/kvm/Kconfig b/arch/mips/kvm/Kconfig
> index 30e334e..2ae1282 100644
> --- a/arch/mips/kvm/Kconfig
> +++ b/arch/mips/kvm/Kconfig
> @@ -20,6 +20,7 @@ config KVM
>  	select PREEMPT_NOTIFIERS
>  	select ANON_INODES
>  	select KVM_MMIO
> +	select SRCU
>  	---help---
>  	  Support for hosting Guest kernels.
>  	  Currently supported on MIPS32 processors.
> diff --git a/arch/powerpc/kvm/Kconfig b/arch/powerpc/kvm/Kconfig
> index 602eb51..af18e0f 100644
> --- a/arch/powerpc/kvm/Kconfig
> +++ b/arch/powerpc/kvm/Kconfig
> @@ -21,6 +21,7 @@ config KVM
>  	select PREEMPT_NOTIFIERS
>  	select ANON_INODES
>  	select HAVE_KVM_EVENTFD
> +	select SRCU
>  
>  config KVM_BOOK3S_HANDLER
>  	bool
> diff --git a/arch/s390/kvm/Kconfig b/arch/s390/kvm/Kconfig
> index 646db9c..5fce52c 100644
> --- a/arch/s390/kvm/Kconfig
> +++ b/arch/s390/kvm/Kconfig
> @@ -28,6 +28,7 @@ config KVM
>  	select HAVE_KVM_IRQCHIP
>  	select HAVE_KVM_IRQFD
>  	select HAVE_KVM_IRQ_ROUTING
> +	select SRCU
>  	---help---
>  	  Support hosting paravirtualized guest machines using the SIE
>  	  virtualization capability on the mainframe. This should work
> diff --git a/arch/tile/kvm/Kconfig b/arch/tile/kvm/Kconfig
> index 2298cb1..1e968f7 100644
> --- a/arch/tile/kvm/Kconfig
> +++ b/arch/tile/kvm/Kconfig
> @@ -21,6 +21,7 @@ config KVM
>  	depends on HAVE_KVM && MODULES
>  	select PREEMPT_NOTIFIERS
>  	select ANON_INODES
> +	select SRCU
>  	---help---
>  	  Support hosting paravirtualized guest machines.
>  
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index f9d16ff..7dc7ba5 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -40,6 +40,7 @@ config KVM
>  	select HAVE_KVM_MSI
>  	select HAVE_KVM_CPU_RELAX_INTERCEPT
>  	select KVM_VFIO
> +	select SRCU
>  	---help---
>  	  Support hosting fully virtualized guest machines using hardware
>  	  virtualization extensions.  You will need a fairly recent
> diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
> index 455fd17..53e64f1 100644
> --- a/drivers/clk/Kconfig
> +++ b/drivers/clk/Kconfig
> @@ -13,6 +13,7 @@ config COMMON_CLK
>  	bool
>  	select HAVE_CLK_PREPARE
>  	select CLKDEV_LOOKUP
> +	select SRCU
>  	---help---
>  	  The common clock framework is a single definition of struct
>  	  clk, useful across many platforms, as well as an
> diff --git a/drivers/cpufreq/Kconfig b/drivers/cpufreq/Kconfig
> index 3489f8f..ebaa0cc 100644
> --- a/drivers/cpufreq/Kconfig
> +++ b/drivers/cpufreq/Kconfig
> @@ -2,6 +2,7 @@ menu "CPU Frequency scaling"
>  
>  config CPU_FREQ
>  	bool "CPU Frequency scaling"
> +	select SRCU
>  	help
>  	  CPU Frequency scaling allows you to change the clock speed of 
>  	  CPUs on the fly. This is a nice method to save power, because 
> diff --git a/drivers/devfreq/Kconfig b/drivers/devfreq/Kconfig
> index faf4e70..3891f67 100644
> --- a/drivers/devfreq/Kconfig
> +++ b/drivers/devfreq/Kconfig
> @@ -1,5 +1,6 @@
>  menuconfig PM_DEVFREQ
>  	bool "Generic Dynamic Voltage and Frequency Scaling (DVFS) support"
> +	select SRCU
>  	help
>  	  A device may have a list of frequencies and voltages available.
>  	  devfreq, a generic DVFS framework can be registered for a device
> diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
> index 5bdedf6..c355a22 100644
> --- a/drivers/md/Kconfig
> +++ b/drivers/md/Kconfig
> @@ -5,6 +5,7 @@
>  menuconfig MD
>  	bool "Multiple devices driver support (RAID and LVM)"
>  	depends on BLOCK
> +	select SRCU
>  	help
>  	  Support multiple physical spindles through a single logical device.
>  	  Required for RAID and logical volume management.
> diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
> index 4706386..221e623 100644
> --- a/drivers/net/Kconfig
> +++ b/drivers/net/Kconfig
> @@ -176,6 +176,7 @@ config NETCONSOLE_DYNAMIC
>  
>  config NETPOLL
>  	def_bool NETCONSOLE
> +	select SRCU
>  
>  config NET_POLL_CONTROLLER
>  	def_bool NETPOLL
> diff --git a/fs/btrfs/Kconfig b/fs/btrfs/Kconfig
> index a66768e..80e9c18 100644
> --- a/fs/btrfs/Kconfig
> +++ b/fs/btrfs/Kconfig
> @@ -8,6 +8,7 @@ config BTRFS_FS
>  	select LZO_DECOMPRESS
>  	select RAID6_PQ
>  	select XOR_BLOCKS
> +	select SRCU
>  
>  	help
>  	  Btrfs is a general purpose copy-on-write filesystem with extents,
> diff --git a/fs/notify/Kconfig b/fs/notify/Kconfig
> index 22c629e..2a24249 100644
> --- a/fs/notify/Kconfig
> +++ b/fs/notify/Kconfig
> @@ -1,5 +1,6 @@
>  config FSNOTIFY
>  	def_bool n
> +	select SRCU
>  
>  source "fs/notify/dnotify/Kconfig"
>  source "fs/notify/inotify/Kconfig"
> diff --git a/init/Kconfig b/init/Kconfig
> index 3ee28ae..80ac0a4 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -501,6 +501,14 @@ config TINY_RCU
>  
>  endchoice
>  
> +config SRCU
> +	bool "Sleepable form of RCU"
> +	def_bool n
> +	help
> +	  This option selects the sleepable version of RCU. This version
> +	  permits arbitrary sleeping or blocking within RCU read-side critical
> +	  sections.
> +
>  config PREEMPT_RCU
>  	def_bool TREE_PREEMPT_RCU
>  	help
> @@ -510,6 +518,7 @@ config PREEMPT_RCU
>  config TASKS_RCU
>  	bool "Task_based RCU implementation using voluntary context switch"
>  	default n
> +	select SRCU
>  	help
>  	  This option enables a task-based RCU implementation that uses
>  	  only voluntary context switch (not preemption!), idle, and
> @@ -1586,6 +1595,7 @@ config PERF_EVENTS
>  	depends on HAVE_PERF_EVENTS
>  	select ANON_INODES
>  	select IRQ_WORK
> +	select SRCU
>  	help
>  	  Enable kernel support for various performance events provided
>  	  by software and hardware.
> diff --git a/kernel/notifier.c b/kernel/notifier.c
> index 4803da6..f52858a 100644
> --- a/kernel/notifier.c
> +++ b/kernel/notifier.c
> @@ -402,6 +402,7 @@ int raw_notifier_call_chain(struct raw_notifier_head *nh,
>  }
>  EXPORT_SYMBOL_GPL(raw_notifier_call_chain);
>  
> +#if CONFIG_SRCU
>  /*
>   *	SRCU notifier chain routines.    Registration and unregistration
>   *	use a mutex, and call_chain is synchronized by SRCU (no locks).
> @@ -528,6 +529,8 @@ void srcu_init_notifier_head(struct srcu_notifier_head *nh)
>  }
>  EXPORT_SYMBOL_GPL(srcu_init_notifier_head);
>  
> +#endif /* CONFIG_SRCU */
> +
>  static ATOMIC_NOTIFIER_HEAD(die_chain);
>  
>  int notrace notify_die(enum die_val val, const char *str,
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 807ccfb..a484080 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -1,4 +1,5 @@
> -obj-y += update.o srcu.o
> +obj-y += update.o
> +obj-$(CONFIG_SRCU) += srcu.o
>  obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
>  obj-$(CONFIG_TREE_RCU) += tree.o
>  obj-$(CONFIG_TREE_PREEMPT_RCU) += tree.o
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 4e35a5d..797511a 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1199,6 +1199,7 @@ config RCU_TORTURE_TEST
>  	tristate "torture tests for RCU"
>  	depends on DEBUG_KERNEL
>  	select TORTURE_TEST
> +	select SRCU
>  	default n
>  	help
>  	  This option provides a kernel module that runs torture tests
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 1d1ae6b..4395b12 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -325,6 +325,7 @@ config VIRT_TO_BUS
>  
>  config MMU_NOTIFIER
>  	bool
> +	select SRCU
>  
>  config KSM
>  	bool "Enable KSM for page merging"
> diff --git a/security/tomoyo/Kconfig b/security/tomoyo/Kconfig
> index 8eb779b9d..604e718 100644
> --- a/security/tomoyo/Kconfig
> +++ b/security/tomoyo/Kconfig
> @@ -5,6 +5,7 @@ config SECURITY_TOMOYO
>  	select SECURITYFS
>  	select SECURITY_PATH
>  	select SECURITY_NETWORK
> +	select SRCU
>  	default n
>  	help
>  	  This selects TOMOYO Linux, pathname-based access control.
> -- 
> 1.9.1
> 
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