Merge tag 'timers-nohz-2020-03-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull NOHZ update from Thomas Gleixner:
 "Remove TIF_NOHZ from three architectures

  These architectures use a static key to decide whether context
  tracking needs to be invoked and the TIF_NOHZ flag just causes a
  pointless slowpath execution for nothing"

* tag 'timers-nohz-2020-03-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  arm64: Remove TIF_NOHZ
  arm: Remove TIF_NOHZ
  x86: Remove TIF_NOHZ
  context-tracking: Introduce CONFIG_HAVE_TIF_NOHZ
  x86/entry: Remove _TIF_NOHZ from _TIF_WORK_SYSCALL_ENTRY

arch/Kconfig

@@ -540,11 +540,17 @@ config HAVE_CONTEXT_TRACKING
 	help
 	  Provide kernel/user boundaries probes necessary for subsystems
 	  that need it, such as userspace RCU extended quiescent state.
-	  Syscalls need to be wrapped inside user_exit()-user_enter() through
-	  the slow path using TIF_NOHZ flag. Exceptions handlers must be
-	  wrapped as well. Irqs are already protected inside
-	  rcu_irq_enter/rcu_irq_exit() but preemption or signal handling on
-	  irq exit still need to be protected.
+	  Syscalls need to be wrapped inside user_exit()-user_enter(), either
+	  optimized behind static key or through the slow path using TIF_NOHZ
+	  flag. Exceptions handlers must be wrapped as well. Irqs are already
+	  protected inside rcu_irq_enter/rcu_irq_exit() but preemption or signal
+	  handling on irq exit still need to be protected.
+
+config HAVE_TIF_NOHZ
+	bool
+	help
+	  Arch relies on TIF_NOHZ and syscall slow path to implement context
+	  tracking calls to user_enter()/user_exit().
 
 config HAVE_VIRT_CPU_ACCOUNTING
 	bool

arch/arm/include/asm/thread_info.h

@@ -141,7 +141,6 @@ extern int vfp_restore_user_hwstate(struct user_vfp *,
 #define TIF_SYSCALL_TRACEPOINT	6	/* syscall tracepoint instrumentation */
 #define TIF_SECCOMP		7	/* seccomp syscall filtering active */
 
-#define TIF_NOHZ		12	/* in adaptive nohz mode */
 #define TIF_USING_IWMMXT	17
 #define TIF_MEMDIE		18	/* is terminating due to OOM killer */
 #define TIF_RESTORE_SIGMASK	20

arch/arm64/include/asm/thread_info.h

@@ -63,7 +63,6 @@ void arch_release_task_struct(struct task_struct *tsk);
 #define TIF_FOREIGN_FPSTATE	3	/* CPU's FP state is not current's */
 #define TIF_UPROBE		4	/* uprobe breakpoint or singlestep */
 #define TIF_FSCHECK		5	/* Check FS is USER_DS on return */
-#define TIF_NOHZ		7
 #define TIF_SYSCALL_TRACE	8	/* syscall trace active */
 #define TIF_SYSCALL_AUDIT	9	/* syscall auditing */
 #define TIF_SYSCALL_TRACEPOINT	10	/* syscall tracepoint for ftrace */
@@ -83,7 +82,6 @@ void arch_release_task_struct(struct task_struct *tsk);
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
 #define _TIF_FOREIGN_FPSTATE	(1 << TIF_FOREIGN_FPSTATE)
-#define _TIF_NOHZ		(1 << TIF_NOHZ)
 #define _TIF_SYSCALL_TRACE	(1 << TIF_SYSCALL_TRACE)
 #define _TIF_SYSCALL_AUDIT	(1 << TIF_SYSCALL_AUDIT)
 #define _TIF_SYSCALL_TRACEPOINT	(1 << TIF_SYSCALL_TRACEPOINT)
@@ -100,7 +98,7 @@ void arch_release_task_struct(struct task_struct *tsk);
 
 #define _TIF_SYSCALL_WORK	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
 				 _TIF_SYSCALL_TRACEPOINT | _TIF_SECCOMP | \
-				 _TIF_NOHZ | _TIF_SYSCALL_EMU)
+				 _TIF_SYSCALL_EMU)
 
 #define INIT_THREAD_INFO(tsk)						\
 {									\

arch/mips/Kconfig

@@ -51,6 +51,7 @@ config MIPS
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_CBPF_JIT if !64BIT && !CPU_MICROMIPS
 	select HAVE_CONTEXT_TRACKING
+	select HAVE_TIF_NOHZ
 	select HAVE_COPY_THREAD_TLS
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_DEBUG_KMEMLEAK

arch/powerpc/Kconfig

@@ -182,6 +182,7 @@ config PPC
 	select HAVE_STACKPROTECTOR		if PPC64 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r13)
 	select HAVE_STACKPROTECTOR		if PPC32 && $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=r2)
 	select HAVE_CONTEXT_TRACKING		if PPC64
+	select HAVE_TIF_NOHZ			if PPC64
 	select HAVE_COPY_THREAD_TLS
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DEBUG_STACKOVERFLOW

arch/sparc/Kconfig

@@ -71,6 +71,7 @@ config SPARC64
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_CONTEXT_TRACKING
+	select HAVE_TIF_NOHZ
 	select HAVE_DEBUG_KMEMLEAK
 	select IOMMU_HELPER
 	select SPARSE_IRQ

arch/x86/include/asm/thread_info.h

@@ -92,7 +92,6 @@ struct thread_info {
 #define TIF_NOCPUID		15	/* CPUID is not accessible in userland */
 #define TIF_NOTSC		16	/* TSC is not accessible in userland */
 #define TIF_IA32		17	/* IA32 compatibility process */
-#define TIF_NOHZ		19	/* in adaptive nohz mode */
 #define TIF_MEMDIE		20	/* is terminating due to OOM killer */
 #define TIF_POLLING_NRFLAG	21	/* idle is polling for TIF_NEED_RESCHED */
 #define TIF_IO_BITMAP		22	/* uses I/O bitmap */
@@ -122,7 +121,6 @@ struct thread_info {
 #define _TIF_NOCPUID		(1 << TIF_NOCPUID)
 #define _TIF_NOTSC		(1 << TIF_NOTSC)
 #define _TIF_IA32		(1 << TIF_IA32)
-#define _TIF_NOHZ		(1 << TIF_NOHZ)
 #define _TIF_POLLING_NRFLAG	(1 << TIF_POLLING_NRFLAG)
 #define _TIF_IO_BITMAP		(1 << TIF_IO_BITMAP)
 #define _TIF_FORCED_TF		(1 << TIF_FORCED_TF)
@@ -133,14 +131,10 @@ struct thread_info {
 #define _TIF_X32		(1 << TIF_X32)
 #define _TIF_FSCHECK		(1 << TIF_FSCHECK)
 
-/*
- * work to do in syscall_trace_enter().  Also includes TIF_NOHZ for
- * enter_from_user_mode()
- */
+/* Work to do before invoking the actual syscall. */
 #define _TIF_WORK_SYSCALL_ENTRY	\
 	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_EMU | _TIF_SYSCALL_AUDIT |	\
-	 _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT |	\
-	 _TIF_NOHZ)
+	 _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT)
 
 /* flags to check in __switch_to() */
 #define _TIF_WORK_CTXSW_BASE					\

kernel/context_tracking.c

@@ -198,11 +198,13 @@ void __init context_tracking_cpu_set(int cpu)
 	if (initialized)
 		return;
 
+#ifdef CONFIG_HAVE_TIF_NOHZ
 	/*
 	 * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
 	 * This assumes that init is the only task at this early boot stage.
 	 */
 	set_tsk_thread_flag(&init_task, TIF_NOHZ);
+#endif
 	WARN_ON_ONCE(!tasklist_empty());
 
 	initialized = true;