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Message-Id: <20240722171939.3349410-11-john.ogness@linutronix.de>
Date: Mon, 22 Jul 2024 19:25:30 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org
Subject: [PATCH printk v3 10/19] printk: nbcon: Use thread callback if in task context for legacy
When printing via console_lock, the write_atomic() callback is
used for nbcon consoles. However, if it is known that the
current context is a task context, the write_thread() callback
can be used instead.
Using write_thread() instead of write_atomic() helps to reduce
large disabled preemption regions when the device_lock does not
disable preemption.
This is mainly a preparatory change to allow avoiding
write_atomic() completely during normal operation if boot
consoles are registered.
Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
kernel/printk/internal.h | 4 +--
kernel/printk/nbcon.c | 54 +++++++++++++++++++++++++++++-----------
kernel/printk/printk.c | 5 ++--
3 files changed, 45 insertions(+), 18 deletions(-)
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 12605e0aff11..bb02788acc7c 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -91,7 +91,7 @@ void nbcon_free(struct console *con);
enum nbcon_prio nbcon_get_default_prio(void);
void nbcon_atomic_flush_pending(void);
bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
- int cookie);
+ int cookie, bool use_atomic);
bool nbcon_kthread_create(struct console *con);
void nbcon_kthread_stop(struct console *con);
void nbcon_wake_kthreads(void);
@@ -174,7 +174,7 @@ static inline void nbcon_free(struct console *con) { }
static inline enum nbcon_prio nbcon_get_default_prio(void) { return NBCON_PRIO_NONE; }
static inline void nbcon_atomic_flush_pending(void) { }
static inline bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
- int cookie) { return false; }
+ int cookie, bool use_atomic) { return false; }
static inline void nbcon_kthread_wake(struct console *con) { }
static inline bool console_is_usable(struct console *con, short flags,
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 69cecf97bf24..233ab8f90fef 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1294,9 +1294,10 @@ enum nbcon_prio nbcon_get_default_prio(void)
}
/*
- * nbcon_atomic_emit_one - Print one record for an nbcon console using the
- * write_atomic() callback
+ * nbcon_emit_one - Print one record for an nbcon console using the
+ * specified callback
* @wctxt: An initialized write context struct to use for this context
+ * @use_atomic: True if the write_atomic() callback is to be used
*
* Return: True, when a record has been printed and there are still
* pending records. The caller might want to continue flushing.
@@ -1309,7 +1310,7 @@ enum nbcon_prio nbcon_get_default_prio(void)
* This is an internal helper to handle the locking of the console before
* calling nbcon_emit_next_record().
*/
-static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
+static bool nbcon_emit_one(struct nbcon_write_context *wctxt, bool use_atomic)
{
struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
@@ -1324,7 +1325,7 @@ static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
* The higher priority printing context takes over responsibility
* to print the pending records.
*/
- if (!nbcon_emit_next_record(wctxt, true))
+ if (!nbcon_emit_next_record(wctxt, use_atomic))
return false;
nbcon_context_release(ctxt);
@@ -1341,6 +1342,13 @@ static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
* both the console_lock and the SRCU read lock. Otherwise it
* is set to false.
* @cookie: The cookie from the SRCU read lock.
+ * @use_atomic: Set true when called in an atomic or unknown context.
+ * It affects which nbcon callback will be used: write_atomic()
+ * or write_thread().
+ *
+ * When false, the write_thread() callback is used and would be
+ * called in a preemtible context unless disabled by the
+ * device_lock. The legacy handover is not allowed in this mode.
*
* Context: Any context except NMI.
* Return: True, when a record has been printed and there are still
@@ -1356,26 +1364,44 @@ static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
* Essentially it is the nbcon version of console_emit_next_record().
*/
bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
- int cookie)
+ int cookie, bool use_atomic)
{
struct nbcon_write_context wctxt = { };
struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
unsigned long flags;
bool progress;
- /* Use the same procedure as console_emit_next_record(). */
- printk_safe_enter_irqsave(flags);
- console_lock_spinning_enable();
- stop_critical_timings();
-
ctxt->console = con;
ctxt->prio = nbcon_get_default_prio();
- progress = nbcon_atomic_emit_one(&wctxt);
+ if (use_atomic) {
+ /*
+ * In an atomic or unknown context, use the same procedure as
+ * in console_emit_next_record(). It allows to handover.
+ */
+ printk_safe_enter_irqsave(flags);
+ console_lock_spinning_enable();
+ stop_critical_timings();
+ } else {
+ /*
+ * In task context, use the same procedure as in
+ * nbcon_kthread_func(). It might allow scheduling depending
+ * on the device_lock().
+ */
+ con->device_lock(con, &flags);
+ cant_migrate();
+ }
- start_critical_timings();
- *handover = console_lock_spinning_disable_and_check(cookie);
- printk_safe_exit_irqrestore(flags);
+ progress = nbcon_emit_one(&wctxt, use_atomic);
+
+ if (use_atomic) {
+ start_critical_timings();
+ *handover = console_lock_spinning_disable_and_check(cookie);
+ printk_safe_exit_irqrestore(flags);
+ } else {
+ con->device_unlock(con, flags);
+ *handover = false;
+ }
return progress;
}
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index cf0b35eaf047..71e946a8c5fa 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3024,12 +3024,13 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
u64 printk_seq;
bool progress;
- if (!console_is_usable(con, flags, true))
+ if (!console_is_usable(con, flags, !do_cond_resched))
continue;
any_usable = true;
if (flags & CON_NBCON) {
- progress = nbcon_legacy_emit_next_record(con, handover, cookie);
+ progress = nbcon_legacy_emit_next_record(con, handover, cookie,
+ !do_cond_resched);
printk_seq = nbcon_seq_read(con);
} else {
progress = console_emit_next_record(con, handover, cookie);
--
2.39.2
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