linux-kernel - [RFC PATCH 40/86] context_tracking: add ct_state

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Message-Id: <20231107215742.363031-41-ankur.a.arora@oracle.com>
Date:   Tue,  7 Nov 2023 13:57:26 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, peterz@...radead.org,
        torvalds@...ux-foundation.org, paulmck@...nel.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com,
        Ankur Arora <ankur.a.arora@...cle.com>
Subject: [RFC PATCH 40/86] context_tracking: add ct_state_cpu()

While making up its mind about whether to reschedule a target
runqueue eagerly or lazily, resched_curr() needs to know if the
target is executing in the kernel or in userspace.

Add ct_state_cpu().

Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>

---
Using context-tracking for this seems like overkill. Is there a better
way to achieve this? One problem with depending on user_enter() is that
it happens much too late for our purposes. From the scheduler's
point-of-view the exit state has effectively transitioned once the
task exits the exit_to_user_loop() so we will see stale state
while the task is done with exit_to_user_loop() but has not yet
executed user_enter().

---
 include/linux/context_tracking_state.h | 21 +++++++++++++++++++++
 kernel/Kconfig.preempt                 |  1 +
 2 files changed, 22 insertions(+)

diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
index bbff5f7f8803..6a8f1c7ba105 100644
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -53,6 +53,13 @@ static __always_inline int __ct_state(void)
 {
 	return raw_atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_STATE_MASK;
 }
+
+static __always_inline int __ct_state_cpu(int cpu)
+{
+	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
+
+	return atomic_read(&ct->state) & CT_STATE_MASK;
+}
 #endif
 
 #ifdef CONFIG_CONTEXT_TRACKING_IDLE
@@ -139,6 +146,20 @@ static __always_inline int ct_state(void)
 	return ret;
 }
 
+static __always_inline int ct_state_cpu(int cpu)
+{
+	int ret;
+
+	if (!context_tracking_enabled_cpu(cpu))
+		return CONTEXT_DISABLED;
+
+	preempt_disable();
+	ret = __ct_state_cpu(cpu);
+	preempt_enable();
+
+	return ret;
+}
+
 #else
 static __always_inline bool context_tracking_enabled(void) { return false; }
 static __always_inline bool context_tracking_enabled_cpu(int cpu) { return false; }
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 715e7aebb9d8..aa87b5cd3ecc 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -80,6 +80,7 @@ config PREEMPT_COUNT
 config PREEMPTION
        bool
        select PREEMPT_COUNT
+       select CONTEXT_TRACKING_USER
 
 config SCHED_CORE
 	bool "Core Scheduling for SMT"
-- 
2.31.1