linux-kernel - [RFC PATCH 05/14] arm/thread_info: Introduce TIF_NOTIFY

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Date: Tue, 20 Feb 2024 22:44:48 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: <linux-kernel@...r.kernel.org>
CC: "Gautham R. Shenoy" <gautham.shenoy@....com>, K Prateek Nayak
	<kprateek.nayak@....com>, Russell King <linux@...linux.org.uk>, "Rafael J.
 Wysocki" <rafael@...nel.org>, Daniel Lezcano <daniel.lezcano@...aro.org>,
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, "Juri
 Lelli" <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, "Valentin
 Schneider" <vschneid@...hat.com>, Linus Walleij <linus.walleij@...aro.org>,
	Ard Biesheuvel <ardb@...nel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-pm@...r.kernel.org>
Subject: [RFC PATCH 05/14] arm/thread_info: Introduce TIF_NOTIFY_IPI flag

Add support for TIF_NOTIFY_IPI on ARM. With TIF_NOTIFY_IPI, a sender
sending an IPI to an idle CPU in TIF_POLLING mode will set the
TIF_NOTIFY_IPI flag in the target's idle tasks's thread_info to pull the
CPU out of idle, as opposed to setting TIF_NEED_RESCHED previously. This
avoids spurious calls to schedule_idle() in cases where an IPI does not
necessarily wake up a task on the idle CPU.

IPI throughput measured using a modified version of Anton Blanchard's
ipistorm benchmark [1], configured to measure time taken to perform a
fixed number of smp_call_function_single() (with wait set to 1),
improves significantly with TIF_NOTIFY_IPI on a dual socket Ampere Server
(2 x 64C) with the benchmark time reducing to less than half for
100000 IPIs between two CPUs. (Note: Only WFI idle mode was left enabled
during testing to reduce variance)

cmdline: insmod ipistorm.ko numipi=100000 single=1 offset=8 cpulist=8 wait=1

  ==================================================================
  Test          : ipistorm (modified)
  Units         : Normalized runtime
  Interpretation: Lower is better
  Statistic     : AMean
  ==================================================================
  kernel:				time [pct imp]
  tip:sched/core			1.00 [0.00]
  tip:sched/core + TIF_NOTIFY_IPI	0.41 [59.29]

tip:sched/core was at tag "sched-core-2024-01-08" at the time of
testing.

Cc: Russell King <linux@...linux.org.uk>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Ingo Molnar <mingo@...hat.com>
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: Linus Walleij <linus.walleij@...aro.org>
Cc: Ard Biesheuvel <ardb@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org
Cc: linux-pm@...r.kernel.org
Link: https://github.com/antonblanchard/ipistorm [1]
Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
 arch/arm/include/asm/thread_info.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
index 943ffcf069d2..324248d87c9e 100644
--- a/arch/arm/include/asm/thread_info.h
+++ b/arch/arm/include/asm/thread_info.h
@@ -136,6 +136,7 @@ extern int vfp_restore_user_hwstate(struct user_vfp *,
  * thread information flags:
  *  TIF_USEDFPU		- FPU was used by this task this quantum (SMP)
  *  TIF_POLLING_NRFLAG	- true if poll_idle() is polling TIF_NEED_RESCHED
+ *			  or TIF_NOTIFY_IPI
  *
  * Any bit in the range of 0..15 will cause do_work_pending() to be invoked.
  */
@@ -144,6 +145,7 @@ extern int vfp_restore_user_hwstate(struct user_vfp *,
 #define TIF_NOTIFY_RESUME	2	/* callback before returning to user */
 #define TIF_UPROBE		3	/* breakpointed or singlestepping */
 #define TIF_NOTIFY_SIGNAL	4	/* signal notifications exist */
+#define TIF_NOTIFY_IPI		5	/* pending IPI on TIF_POLLLING idle CPU */
 
 #define TIF_USING_IWMMXT	17
 #define TIF_MEMDIE		18	/* is terminating due to OOM killer */
@@ -164,6 +166,7 @@ extern int vfp_restore_user_hwstate(struct user_vfp *,
 #define _TIF_SECCOMP		(1 << TIF_SECCOMP)
 #define _TIF_NOTIFY_SIGNAL	(1 << TIF_NOTIFY_SIGNAL)
 #define _TIF_USING_IWMMXT	(1 << TIF_USING_IWMMXT)
+#define _TIF_NOTIFY_IPI		(1 << TIF_NOTIFY_IPI)
 
 /* Checks for any syscall work in entry-common.S */
 #define _TIF_SYSCALL_WORK (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
-- 
2.34.1