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Message-Id: <20231107230822.371443-8-ankur.a.arora@oracle.com>
Date: Tue, 7 Nov 2023 15:08:00 -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>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Petr Mladek <pmladek@...e.com>,
John Ogness <john.ogness@...utronix.de>,
Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [RFC PATCH 64/86] treewide: printk: remove cond_resched()
The printk code goes to great lengths to ensure that there are no
scheduling stalls which would cause softlockup/RCU splats and make
things worse.
With PREEMPT_COUNT=y and PREEMPTION=y, this should be a non-issue as
the scheduler can determine when this logic can be preempted.
So, remove cond_resched() and related code.
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Petr Mladek <pmladek@...e.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
include/linux/console.h | 2 +-
kernel/printk/printk.c | 65 +++++++++--------------------------------
2 files changed, 15 insertions(+), 52 deletions(-)
diff --git a/include/linux/console.h b/include/linux/console.h
index 7de11c763eb3..db418dab5674 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -347,7 +347,7 @@ extern int unregister_console(struct console *);
extern void console_lock(void);
extern int console_trylock(void);
extern void console_unlock(void);
-extern void console_conditional_schedule(void);
+static inline void console_conditional_schedule(void) { }
extern void console_unblank(void);
extern void console_flush_on_panic(enum con_flush_mode mode);
extern struct tty_driver *console_device(int *);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0b3af1529778..2708d9f499a3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -375,9 +375,6 @@ static int preferred_console = -1;
int console_set_on_cmdline;
EXPORT_SYMBOL(console_set_on_cmdline);
-/* Flag: console code may call schedule() */
-static int console_may_schedule;
-
enum con_msg_format_flags {
MSG_FORMAT_DEFAULT = 0,
MSG_FORMAT_SYSLOG = (1 << 0),
@@ -2651,7 +2648,6 @@ void console_lock(void)
down_console_sem();
console_locked = 1;
- console_may_schedule = 1;
}
EXPORT_SYMBOL(console_lock);
@@ -2671,7 +2667,6 @@ int console_trylock(void)
if (down_trylock_console_sem())
return 0;
console_locked = 1;
- console_may_schedule = 0;
return 1;
}
EXPORT_SYMBOL(console_trylock);
@@ -2922,9 +2917,6 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
/*
* Print out all remaining records to all consoles.
*
- * @do_cond_resched is set by the caller. It can be true only in schedulable
- * context.
- *
* @next_seq is set to the sequence number after the last available record.
* The value is valid only when this function returns true. It means that all
* usable consoles are completely flushed.
@@ -2942,7 +2934,7 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
*
* Requires the console_lock.
*/
-static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handover)
+static bool console_flush_all(u64 *next_seq, bool *handover)
{
bool any_usable = false;
struct console *con;
@@ -2983,9 +2975,6 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
/* Allow panic_cpu to take over the consoles safely. */
if (other_cpu_in_panic())
goto abandon;
-
- if (do_cond_resched)
- cond_resched();
}
console_srcu_read_unlock(cookie);
} while (any_progress);
@@ -3011,28 +3000,26 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
*/
void console_unlock(void)
{
- bool do_cond_resched;
bool handover;
bool flushed;
u64 next_seq;
/*
- * Console drivers are called with interrupts disabled, so
- * @console_may_schedule should be cleared before; however, we may
+ * Console drivers are called with interrupts disabled, so in
+ * general we cannot schedule. There are also cases where we will
* end up dumping a lot of lines, for example, if called from
- * console registration path, and should invoke cond_resched()
- * between lines if allowable. Not doing so can cause a very long
- * scheduling stall on a slow console leading to RCU stall and
- * softlockup warnings which exacerbate the issue with more
- * messages practically incapacitating the system. Therefore, create
- * a local to use for the printing loop.
+ * console registration path.
+ *
+ * Not scheduling while working on a slow console could lead to
+ * RCU stalls and softlockup warnings which exacerbate the issue
+ * with more messages practically incapacitating the system.
+ *
+ * However, most of the console code is preemptible, so the scheduler
+ * should be able to preempt us and make forward progress.
*/
- do_cond_resched = console_may_schedule;
do {
- console_may_schedule = 0;
-
- flushed = console_flush_all(do_cond_resched, &next_seq, &handover);
+ flushed = console_flush_all(&next_seq, &handover);
if (!handover)
__console_unlock();
@@ -3055,22 +3042,6 @@ void console_unlock(void)
}
EXPORT_SYMBOL(console_unlock);
-/**
- * console_conditional_schedule - yield the CPU if required
- *
- * If the console code is currently allowed to sleep, and
- * if this CPU should yield the CPU to another task, do
- * so here.
- *
- * Must be called within console_lock();.
- */
-void __sched console_conditional_schedule(void)
-{
- if (console_may_schedule)
- cond_resched();
-}
-EXPORT_SYMBOL(console_conditional_schedule);
-
void console_unblank(void)
{
bool found_unblank = false;
@@ -3118,7 +3089,6 @@ void console_unblank(void)
console_lock();
console_locked = 1;
- console_may_schedule = 0;
cookie = console_srcu_read_lock();
for_each_console_srcu(c) {
@@ -3154,13 +3124,6 @@ void console_flush_on_panic(enum con_flush_mode mode)
* - semaphores are not NMI-safe
*/
- /*
- * If another context is holding the console lock,
- * @console_may_schedule might be set. Clear it so that
- * this context does not call cond_resched() while flushing.
- */
- console_may_schedule = 0;
-
if (mode == CONSOLE_REPLAY_ALL) {
struct console *c;
int cookie;
@@ -3179,7 +3142,7 @@ void console_flush_on_panic(enum con_flush_mode mode)
console_srcu_read_unlock(cookie);
}
- console_flush_all(false, &next_seq, &handover);
+ console_flush_all(&next_seq, &handover);
}
/*
@@ -3364,7 +3327,7 @@ static void console_init_seq(struct console *newcon, bool bootcon_registered)
* Flush all consoles and set the console to start at
* the next unprinted sequence number.
*/
- if (!console_flush_all(true, &newcon->seq, &handover)) {
+ if (!console_flush_all(&newcon->seq, &handover)) {
/*
* Flushing failed. Just choose the lowest
* sequence of the enabled boot consoles.
--
2.31.1
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