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Commit 05736e4a authored by Thomas Gleixner's avatar Thomas Gleixner
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cpu/hotplug: Provide knobs to control SMT 



Provide a command line and a sysfs knob to control SMT.

The command line options are:

 'nosmt':	Enumerate secondary threads, but do not online them
 		
 'nosmt=force': Ignore secondary threads completely during enumeration
 		via MP table and ACPI/MADT.

The sysfs control file has the following states (read/write):

 'on':		 SMT is enabled. Secondary threads can be freely onlined
 'off':		 SMT is disabled. Secondary threads, even if enumerated
 		 cannot be onlined
 'forceoff':	 SMT is permanentely disabled. Writes to the control
 		 file are rejected.
 'notsupported': SMT is not supported by the CPU

The command line option 'nosmt' sets the sysfs control to 'off'. This
can be changed to 'on' to reenable SMT during runtime.

The command line option 'nosmt=force' sets the sysfs control to
'forceoff'. This cannot be changed during runtime.

When SMT is 'on' and the control file is changed to 'off' then all online
secondary threads are offlined and attempts to online a secondary thread
later on are rejected.

When SMT is 'off' and the control file is changed to 'on' then secondary
threads can be onlined again. The 'off' -> 'on' transition does not
automatically online the secondary threads.

When the control file is set to 'forceoff', the behaviour is the same as
setting it to 'off', but the operation is irreversible and later writes to
the control file are rejected.

When the control status is 'notsupported' then writes to the control file
are rejected.

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: default avatarIngo Molnar <mingo@kernel.org>
parent cc1fe215

Documentation/ABI/testing/sysfs-devices-system-cpu

+20 −0
Original line number Diff line number Diff line
@@ -487,3 +487,23 @@ Description: Information about CPU vulnerabilities 
		"Not affected"	  CPU is not affected by the vulnerability

		"Vulnerable"	  CPU is affected and no mitigation in effect

		"Mitigation: $M"  CPU is affected and mitigation $M is in effect



What:		/sys/devices/system/cpu/smt

		/sys/devices/system/cpu/smt/active

		/sys/devices/system/cpu/smt/control

Date:		June 2018

Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>

Description:	Control Symetric Multi Threading (SMT)



		active:  Tells whether SMT is active (enabled and siblings online)



		control: Read/write interface to control SMT. Possible

			 values:



			 "on"		SMT is enabled

			 "off"		SMT is disabled

			 "forceoff"	SMT is force disabled. Cannot be changed.

			 "notsupported" SMT is not supported by the CPU



			 If control status is "forceoff" or "notsupported" writes

			 are rejected.

Documentation/admin-guide/kernel-parameters.txt

+8 −0
Original line number Diff line number Diff line
@@ -2687,6 +2687,14 @@ 
	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).

			Equivalent to smt=1.



			[KNL,x86] Disable symmetric multithreading (SMT).

			nosmt=force: Force disable SMT, similar to disabling

				     it in the BIOS except that some of the

				     resource partitioning effects which are

				     caused by having SMT enabled in the BIOS

				     cannot be undone. Depending on the CPU

				     type this might have a performance impact.



	nospectre_v2	[X86] Disable all mitigations for the Spectre variant 2

			(indirect branch prediction) vulnerability. System may

			allow data leaks with this option, which is equivalent

arch/Kconfig

+3 −0
Original line number Diff line number Diff line
@@ -13,6 +13,9 @@ config KEXEC_CORE 
config HAVE_IMA_KEXEC

	bool



config HOTPLUG_SMT

	bool



config OPROFILE

	tristate "OProfile system profiling"

	depends on PROFILING

arch/x86/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -187,6 +187,7 @@ config X86 
	select HAVE_SYSCALL_TRACEPOINTS

	select HAVE_UNSTABLE_SCHED_CLOCK

	select HAVE_USER_RETURN_NOTIFIER

	select HOTPLUG_SMT			if SMP

	select IRQ_FORCED_THREADING

	select NEED_SG_DMA_LENGTH

	select PCI_LOCKLESS_CONFIG

include/linux/cpu.h

+13 −0
Original line number Diff line number Diff line
@@ -168,4 +168,17 @@ void cpuhp_report_idle_dead(void); 
static inline void cpuhp_report_idle_dead(void) { }

#endif /* #ifdef CONFIG_HOTPLUG_CPU */



enum cpuhp_smt_control {

	CPU_SMT_ENABLED,

	CPU_SMT_DISABLED,

	CPU_SMT_FORCE_DISABLED,

	CPU_SMT_NOT_SUPPORTED,

};



#if defined(CONFIG_SMP) && defined(CONFIG_HOTPLUG_SMT)

extern enum cpuhp_smt_control cpu_smt_control;

#else

# define cpu_smt_control		(CPU_SMT_ENABLED)

#endif



#endif /* _LINUX_CPU_H_ */