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Message-Id: <20240827044333.88596-9-john.ogness@linutronix.de>
Date: Tue, 27 Aug 2024 06:49:24 +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 v4 08/17] 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.
As a side-effect, it also allows consolidating the printing
code for legacy printing and the kthread printer.
Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
kernel/printk/internal.h | 4 +-
kernel/printk/nbcon.c | 95 +++++++++++++++++++++++-----------------
kernel/printk/printk.c | 5 ++-
3 files changed, 59 insertions(+), 45 deletions(-)
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 14f7fc71e20d..a96d4114a1db 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -90,7 +90,7 @@ 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_kthreads_wake(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 16e7d992b054..623f3a29bfc6 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1043,9 +1043,10 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
}
/*
- * 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.
@@ -1058,12 +1059,25 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
* 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);
+ struct console *con = ctxt->console;
+ unsigned long flags;
+ bool ret = false;
+
+ if (!use_atomic) {
+ con->device_lock(con, &flags);
+
+ /*
+ * Ensure this stays on the CPU to make handover and
+ * takeover possible.
+ */
+ cant_migrate();
+ }
if (!nbcon_context_try_acquire(ctxt))
- return false;
+ goto out;
/*
* nbcon_emit_next_record() returns false when the console was
@@ -1073,12 +1087,16 @@ 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))
- return false;
+ if (!nbcon_emit_next_record(wctxt, use_atomic))
+ goto out;
nbcon_context_release(ctxt);
- return ctxt->backlog;
+ ret = ctxt->backlog;
+out:
+ if (!use_atomic)
+ con->device_unlock(con, flags);
+ return ret;
}
/**
@@ -1163,30 +1181,8 @@ static int nbcon_kthread_func(void *__console)
con_flags = console_srcu_read_flags(con);
- if (console_is_usable(con, con_flags, false)) {
- unsigned long lock_flags;
-
- con->device_lock(con, &lock_flags);
-
- /*
- * Ensure this stays on the CPU to make handover and
- * takeover possible.
- */
- cant_migrate();
-
- if (nbcon_context_try_acquire(ctxt)) {
- /*
- * If the emit fails, this context is no
- * longer the owner.
- */
- if (nbcon_emit_next_record(&wctxt, false)) {
- nbcon_context_release(ctxt);
- backlog = ctxt->backlog;
- }
- }
-
- con->device_unlock(con, lock_flags);
- }
+ if (console_is_usable(con, con_flags, false))
+ backlog = nbcon_emit_one(&wctxt, false);
console_srcu_read_unlock(cookie);
@@ -1359,6 +1355,13 @@ enum nbcon_prio nbcon_get_default_prio(void)
* 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
@@ -1374,26 +1377,36 @@ enum nbcon_prio nbcon_get_default_prio(void)
* 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();
+ }
- 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 {
+ /* Non-atomic does not perform legacy spinning handovers. */
+ *handover = false;
+ }
return progress;
}
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f27c76c3b5cf..55d75db00042 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3077,12 +3077,13 @@ 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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