linux-kernel - [RFC PATCH 34/86] thread_info: accessors for TIF_NEED

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Message-Id: <20231107215742.363031-35-ankur.a.arora@oracle.com>
Date:   Tue,  7 Nov 2023 13:57:20 -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,
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        David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com,
        Ankur Arora <ankur.a.arora@...cle.com>
Subject: [RFC PATCH 34/86] thread_info: accessors for TIF_NEED_RESCHED*

Add tif_resched() which will be used as an accessor for TIF_NEED_RESCHED
and TIF_NEED_RESCHED_LAZY. The intent is to force the caller to make an
explicit choice of how eagerly they want a reschedule.

This interface will be used almost entirely from core kernel code, so
forcing a choice shouldn't be too onerous.

Originally-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>

---

1) Adding an enum for an interface that doesn't do all that much, seems
   to be overkill. This could have been an int/bool etc, but that seemed
   much less clear and thus more error prone.

2) Also there's no fallback path for architectures that don't define
   define TIF_NEED_RESCHD_LAZY. That's because arch support is easy
   to add (modulo ARCH_NO_PREEMPT, discussed in a different patch)
   so it will be simple to do that instead of thinking through what
   seemed like a slightly convoluted alternative model.

---
 include/linux/thread_info.h | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 9ea0b28068f4..4eb22b13bf64 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -59,6 +59,27 @@ enum syscall_work_bit {
 
 #include <asm/thread_info.h>
 
+#ifndef TIF_NEED_RESCHED_LAZY
+#error "Arch needs to define TIF_NEED_RESCHED_LAZY"
+#endif
+
+#define TIF_NEED_RESCHED_LAZY_OFFSET	(TIF_NEED_RESCHED_LAZY - TIF_NEED_RESCHED)
+
+typedef enum {
+	RESCHED_eager = 0,
+	RESCHED_lazy = TIF_NEED_RESCHED_LAZY_OFFSET,
+} resched_t;
+
+static inline int tif_resched(resched_t r)
+{
+	return TIF_NEED_RESCHED + r;
+}
+
+static inline int _tif_resched(resched_t r)
+{
+	return 1 << tif_resched(r);
+}
+
 #ifdef __KERNEL__
 
 #ifndef arch_set_restart_data
-- 
2.31.1