Date: Tue, 20 Feb 2024 22:44:44 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: <linux-kernel@...r.kernel.org>
CC: "Gautham R. Shenoy" <gautham.shenoy@....com>, Richard Henderson
	<richard.henderson@...aro.org>, Ivan Kokshaysky <ink@...assic.park.msu.ru>,
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Subject: [RFC PATCH 01/14] thread_info: Add helpers to test and clear TIF_NOTIFY_IPI

From: "Gautham R. Shenoy" <gautham.shenoy@....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@...aro.org>
Cc: Ivan Kokshaysky <ink@...assic.park.msu.ru>
Cc: Matt Turner <mattst88@...il.com>
Cc: Russell King <linux@...linux.org.uk>
Cc: Guo Ren <guoren@...nel.org>
Cc: Michal Simek <monstr@...str.eu>
Cc: Dinh Nguyen <dinguyen@...nel.org>
Cc: Jonas Bonn <jonas@...thpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
Cc: Stafford Horne <shorne@...il.com>
Cc: "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>
Cc: Helge Deller <deller@....de>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
Cc: "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
Cc: Yoshinori Sato <ysato@...rs.sourceforge.jp>
Cc: Rich Felker <dalias@...c.org>
Cc: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Valentin Schneider <vschneid@...hat.com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Linus Walleij <linus.walleij@...aro.org>
Cc: Ard Biesheuvel <ardb@...nel.org>
Cc: Andrew Donnellan <ajd@...ux.ibm.com>
Cc: Nicholas Miehlbradt <nicholas@...ux.ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: Tony Battersby <tonyb@...ernetics.com>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: David Vernet <void@...ifault.com>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
Cc: linux-alpha@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org
Cc: linux-csky@...r.kernel.org
Cc: linux-openrisc@...r.kernel.org
Cc: linux-parisc@...r.kernel.org
Cc: linuxppc-dev@...ts.ozlabs.org
Cc: linux-sh@...r.kernel.org
Cc: sparclinux@...r.kernel.org
Cc: linux-pm@...r.kernel.org
Signed-off-by: Gautham R. Shenoy <gautham.shenoy@....com>
Co-developed-by: K Prateek Nayak <kprateek.nayak@....com>
Signed-off-by: K Prateek Nayak <kprateek.nayak@....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

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