linux-kernel - [RFC PATCH 04/14] x86/thread_info: Introduce TIF_NOTIFY

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Date: Tue, 20 Feb 2024 22:44:47 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: <linux-kernel@...r.kernel.org>
CC: "Gautham R. Shenoy" <gautham.shenoy@....com>, Thomas Gleixner
	<tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
	<bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin"
	<hpa@...or.com>, "Rafael J. Wysocki" <rafael@...nel.org>, Daniel Lezcano
	<daniel.lezcano@...aro.org>, 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>, Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>, Tim Chen <tim.c.chen@...ux.intel.com>, "David
 Vernet" <void@...ifault.com>, <x86@...nel.org>, <linux-pm@...r.kernel.org>,
	"K Prateek Nayak" <kprateek.nayak@....com>
Subject: [RFC PATCH 04/14] x86/thread_info: Introduce TIF_NOTIFY_IPI flag

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

Add support for TIF_NOTIFY_IPI on x86. 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.

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

Link: https://github.com/antonblanchard/ipistorm [1]
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: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arnd Bergmann <arnd@...db.de>
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-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>
---
 arch/x86/include/asm/thread_info.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index d63b02940747..90cdce971397 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -83,6 +83,7 @@ struct thread_info {
 #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
 #define TIF_SINGLESTEP		4	/* reenable singlestep on user return*/
 #define TIF_SSBD		5	/* Speculative store bypass disable */
+#define TIF_NOTIFY_IPI		6	/* Pending IPI on TIF_POLLLING idle CPU */
 #define TIF_SPEC_IB		9	/* Indirect branch speculation mitigation */
 #define TIF_SPEC_L1D_FLUSH	10	/* Flush L1D on mm switches (processes) */
 #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
@@ -106,6 +107,7 @@ struct thread_info {
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
 #define _TIF_SINGLESTEP		(1 << TIF_SINGLESTEP)
 #define _TIF_SSBD		(1 << TIF_SSBD)
+#define _TIF_NOTIFY_IPI		(1 << TIF_NOTIFY_IPI)
 #define _TIF_SPEC_IB		(1 << TIF_SPEC_IB)
 #define _TIF_SPEC_L1D_FLUSH	(1 << TIF_SPEC_L1D_FLUSH)
 #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
-- 
2.34.1