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

[K Prateek Nayak]

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