[RFC PATCH 01/14] thread_info: Add helpers to test and clear TIF_NOTIFY_IPI

[K Prateek Nayak <kprateek.nayak@amd.com>]

From: "Gautham R. Shenoy" <gautham.shenoy@amd.com>

Introduce the notion of TIF_NOTIFY_IPI flag. When a processor in
TIF_POLLING mode needs to process an IPI, the sender sets NEED_RESCHED
bit in idle task's thread_info to pull the target out of idle and avoids
sending an interrupt to the idle CPU. When NEED_RESCHED is set, the
scheduler assumes that a new task has been queued on the idle CPU and
calls schedule_idle(), however, it is not necessary that an IPI on an
idle CPU will necessarily end up waking a task on the said CPU. To avoid
spurious calls to schedule_idle() assuming an IPI on an idle CPU will
always wake a task on the said CPU, TIF_NOTIFY_IPI will be used to pull
a TIF_POLLING CPU out of idle.

Since the IPI handlers are processed before the call to schedule_idle(),
schedule_idle() will be called only if one of the handlers have woken up
a new task on the CPU and has set NEED_RESCHED.

Add tif_notify_ipi() and current_clr_notify_ipi() helpers to test if
TIF_NOTIFY_IPI is set in the current task's thread_info, and to clear it
respectively. These interfaces will be used in subsequent patches as
TIF_NOTIFY_IPI notion is integrated in the scheduler and in the idle
path.

[ prateek: Split the changes into a separate patch, add commit log ]

Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Guo Ren <guoren@kernel.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Andrew Donnellan <ajd@linux.ibm.com>
Cc: Nicholas Miehlbradt <nicholas@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Tony Battersby <tonyb@cybernetics.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: David Vernet <void@manifault.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-alpha@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-csky@vger.kernel.org
Cc: linux-openrisc@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-sh@vger.kernel.org
Cc: sparclinux@vger.kernel.org
Cc: linux-pm@vger.kernel.org
Signed-off-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Co-developed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
---
 include/linux/thread_info.h | 43 +++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 9ea0b28068f4..1e10dd8c0227 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -195,6 +195,49 @@ static __always_inline bool tif_need_resched(void)
 
 #endif /* _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H */
 
+#ifdef TIF_NOTIFY_IPI
+
+#ifdef _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H
+
+static __always_inline bool tif_notify_ipi(void)
+{
+	return arch_test_bit(TIF_NOTIFY_IPI,
+			     (unsigned long *)(&current_thread_info()->flags));
+}
+
+static __always_inline void current_clr_notify_ipi(void)
+{
+	arch_clear_bit(TIF_NOTIFY_IPI,
+		       (unsigned long *)(&current_thread_info()->flags));
+}
+
+#else
+
+static __always_inline bool tif_notify_ipi(void)
+{
+	return test_bit(TIF_NOTIFY_IPI,
+			(unsigned long *)(&current_thread_info()->flags));
+}
+
+static __always_inline void current_clr_notify_ipi(void)
+{
+	clear_bit(TIF_NOTIFY_IPI,
+		  (unsigned long *)(&current_thread_info()->flags));
+}
+
+#endif /* _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H */
+
+#else /* !TIF_NOTIFY_IPI */
+
+static __always_inline bool tif_notify_ipi(void)
+{
+	return false;
+}
+
+static __always_inline void current_clr_notify_ipi(void) { }
+
+#endif /* TIF_NOTIFY_IPI */
+
 #ifndef CONFIG_HAVE_ARCH_WITHIN_STACK_FRAMES
 static inline int arch_within_stack_frames(const void * const stack,
 					   const void * const stackend,
-- 
2.34.1