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-rw-r--r--debian/patches-rt/0030-printk-nbcon-Introduce-printing-kthreads.patch478
1 files changed, 478 insertions, 0 deletions
diff --git a/debian/patches-rt/0030-printk-nbcon-Introduce-printing-kthreads.patch b/debian/patches-rt/0030-printk-nbcon-Introduce-printing-kthreads.patch
new file mode 100644
index 0000000000..85792a75e2
--- /dev/null
+++ b/debian/patches-rt/0030-printk-nbcon-Introduce-printing-kthreads.patch
@@ -0,0 +1,478 @@
+From: Thomas Gleixner <tglx@linutronix.de>
+Date: Fri, 22 Sep 2023 14:12:21 +0000
+Subject: [PATCH 30/46] printk: nbcon: Introduce printing kthreads
+Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.9/older/patches-6.9-rt5.tar.xz
+
+Provide the main implementation for running a printer kthread
+per nbcon console that is takeover/handover aware.
+
+Co-developed-by: John Ogness <john.ogness@linutronix.de>
+Signed-off-by: John Ogness <john.ogness@linutronix.de>
+Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
+Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
+---
+ include/linux/console.h | 26 ++++++
+ kernel/printk/internal.h | 26 ++++++
+ kernel/printk/nbcon.c | 196 +++++++++++++++++++++++++++++++++++++++++++++--
+ kernel/printk/printk.c | 31 +++++++
+ 4 files changed, 271 insertions(+), 8 deletions(-)
+
+--- a/include/linux/console.h
++++ b/include/linux/console.h
+@@ -17,6 +17,7 @@
+ #include <linux/atomic.h>
+ #include <linux/bits.h>
+ #include <linux/rculist.h>
++#include <linux/rcuwait.h>
+ #include <linux/types.h>
+ #include <linux/vesa.h>
+
+@@ -324,6 +325,8 @@ struct nbcon_write_context {
+ * @nbcon_seq: Sequence number of the next record for nbcon to print
+ * @nbcon_driver_ctxt: Context available for driver non-printing operations
+ * @pbufs: Pointer to nbcon private buffer
++ * @kthread: Printer kthread for this console
++ * @rcuwait: RCU-safe wait object for @kthread waking
+ */
+ struct console {
+ char name[16];
+@@ -374,6 +377,27 @@ struct console {
+ void (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt);
+
+ /**
++ * @write_thread:
++ *
++ * NBCON callback to write out text in task context. (Optional)
++ *
++ * This callback is called with the console already acquired. Any
++ * additional driver synchronization should have been performed by
++ * device_lock().
++ *
++ * This callback is always called from task context but with migration
++ * disabled.
++ *
++ * The same criteria for console ownership verification and unsafe
++ * sections applies as with write_atomic(). The difference between
++ * this callback and write_atomic() is that this callback is used
++ * during normal operation and is always called from task context.
++ * This provides drivers with a relatively relaxed locking context
++ * for synchronizing output to the hardware.
++ */
++ void (*write_thread)(struct console *con, struct nbcon_write_context *wctxt);
++
++ /**
+ * @device_lock:
+ *
+ * NBCON callback to begin synchronization with driver code.
+@@ -420,6 +444,8 @@ struct console {
+ atomic_long_t __private nbcon_seq;
+ struct nbcon_context __private nbcon_driver_ctxt;
+ struct printk_buffers *pbufs;
++ struct task_struct *kthread;
++ struct rcuwait rcuwait;
+ };
+
+ #ifdef CONFIG_LOCKDEP
+--- a/kernel/printk/internal.h
++++ b/kernel/printk/internal.h
+@@ -90,6 +90,7 @@ enum nbcon_prio nbcon_get_default_prio(v
+ void nbcon_atomic_flush_pending(void);
+ bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
+ int cookie);
++void nbcon_kthread_create(struct console *con);
+
+ /*
+ * Check if the given console is currently capable and allowed to print
+@@ -108,6 +109,8 @@ static inline bool console_is_usable(str
+ if (flags & CON_NBCON) {
+ if (!con->write_atomic)
+ return false;
++ if (!con->write_thread)
++ return false;
+ } else {
+ if (!con->write)
+ return false;
+@@ -124,12 +127,35 @@ static inline bool console_is_usable(str
+ return true;
+ }
+
++/**
++ * nbcon_kthread_wake - Wake up a printk thread
++ * @con: Console to operate on
++ */
++static inline void nbcon_kthread_wake(struct console *con)
++{
++ /*
++ * Guarantee any new records can be seen by tasks preparing to wait
++ * before this context checks if the rcuwait is empty.
++ *
++ * The full memory barrier in rcuwait_wake_up() pairs with the full
++ * memory barrier within set_current_state() of
++ * ___rcuwait_wait_event(), which is called after prepare_to_rcuwait()
++ * adds the waiter but before it has checked the wait condition.
++ *
++ * This pairs with nbcon_kthread_func:A.
++ */
++ rcuwait_wake_up(&con->rcuwait); /* LMM(nbcon_kthread_wake:A) */
++}
++
+ #else
+
+ #define PRINTK_PREFIX_MAX 0
+ #define PRINTK_MESSAGE_MAX 0
+ #define PRINTKRB_RECORD_MAX 0
+
++static inline void nbcon_kthread_wake(struct console *con) { }
++static inline void nbcon_kthread_create(struct console *con) { }
++
+ /*
+ * In !PRINTK builds we still export console_sem
+ * semaphore and some of console functions (console_unlock()/etc.), so
+--- a/kernel/printk/nbcon.c
++++ b/kernel/printk/nbcon.c
+@@ -10,6 +10,7 @@
+ #include <linux/export.h>
+ #include <linux/init.h>
+ #include <linux/irqflags.h>
++#include <linux/kthread.h>
+ #include <linux/minmax.h>
+ #include <linux/percpu.h>
+ #include <linux/preempt.h>
+@@ -837,6 +838,7 @@ EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
+ /**
+ * nbcon_emit_next_record - Emit a record in the acquired context
+ * @wctxt: The write context that will be handed to the write function
++ * @use_atomic: True if the write_atomic callback is to be used
+ *
+ * Return: True if this context still owns the console. False if
+ * ownership was handed over or taken.
+@@ -850,7 +852,7 @@ EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
+ * When true is returned, @wctxt->ctxt.backlog indicates whether there are
+ * still records pending in the ringbuffer,
+ */
+-static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
++static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_atomic)
+ {
+ struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+ struct console *con = ctxt->console;
+@@ -899,8 +901,14 @@ static bool nbcon_emit_next_record(struc
+ nbcon_state_read(con, &cur);
+ wctxt->unsafe_takeover = cur.unsafe_takeover;
+
+- if (con->write_atomic) {
++ if (use_atomic &&
++ con->write_atomic) {
+ con->write_atomic(con, wctxt);
++
++ } else if (!use_atomic &&
++ con->write_thread) {
++ con->write_thread(con, wctxt);
++
+ } else {
+ /*
+ * This function should never be called for legacy consoles.
+@@ -936,6 +944,118 @@ static bool nbcon_emit_next_record(struc
+ return nbcon_context_exit_unsafe(ctxt);
+ }
+
++/**
++ * nbcon_kthread_should_wakeup - Check whether a printer thread should wakeup
++ * @con: Console to operate on
++ * @ctxt: The acquire context that contains the state
++ * at console_acquire()
++ *
++ * Return: True if the thread should shutdown or if the console is
++ * allowed to print and a record is available. False otherwise.
++ *
++ * After the thread wakes up, it must first check if it should shutdown before
++ * attempting any printing.
++ */
++static bool nbcon_kthread_should_wakeup(struct console *con, struct nbcon_context *ctxt)
++{
++ bool ret = false;
++ short flags;
++ int cookie;
++
++ if (kthread_should_stop())
++ return true;
++
++ cookie = console_srcu_read_lock();
++
++ flags = console_srcu_read_flags(con);
++ if (console_is_usable(con, flags)) {
++ /* Bring the sequence in @ctxt up to date */
++ ctxt->seq = nbcon_seq_read(con);
++
++ ret = prb_read_valid(prb, ctxt->seq, NULL);
++ }
++
++ console_srcu_read_unlock(cookie);
++ return ret;
++}
++
++/**
++ * nbcon_kthread_func - The printer thread function
++ * @__console: Console to operate on
++ */
++static int nbcon_kthread_func(void *__console)
++{
++ struct console *con = __console;
++ struct nbcon_write_context wctxt = {
++ .ctxt.console = con,
++ .ctxt.prio = NBCON_PRIO_NORMAL,
++ };
++ struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
++ unsigned long flags;
++ short con_flags;
++ bool backlog;
++ int cookie;
++ int ret;
++
++wait_for_event:
++ /*
++ * Guarantee this task is visible on the rcuwait before
++ * checking the wake condition.
++ *
++ * The full memory barrier within set_current_state() of
++ * ___rcuwait_wait_event() pairs with the full memory
++ * barrier within rcuwait_has_sleeper().
++ *
++ * This pairs with rcuwait_has_sleeper:A and nbcon_kthread_wake:A.
++ */
++ ret = rcuwait_wait_event(&con->rcuwait,
++ nbcon_kthread_should_wakeup(con, ctxt),
++ TASK_INTERRUPTIBLE); /* LMM(nbcon_kthread_func:A) */
++
++ if (kthread_should_stop())
++ return 0;
++
++ /* Wait was interrupted by a spurious signal, go back to sleep. */
++ if (ret)
++ goto wait_for_event;
++
++ do {
++ backlog = false;
++
++ cookie = console_srcu_read_lock();
++
++ con_flags = console_srcu_read_flags(con);
++
++ if (console_is_usable(con, con_flags)) {
++ 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)) {
++ /*
++ * 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, flags);
++ }
++
++ console_srcu_read_unlock(cookie);
++
++ } while (backlog);
++
++ goto wait_for_event;
++}
++
+ /* Track the nbcon emergency nesting per CPU. */
+ static DEFINE_PER_CPU(unsigned int, nbcon_pcpu_emergency_nesting);
+ static unsigned int early_nbcon_pcpu_emergency_nesting __initdata;
+@@ -1012,7 +1132,7 @@ static bool nbcon_atomic_emit_one(struct
+ * The higher priority printing context takes over responsibility
+ * to print the pending records.
+ */
+- if (!nbcon_emit_next_record(wctxt))
++ if (!nbcon_emit_next_record(wctxt, true))
+ return false;
+
+ nbcon_context_release(ctxt);
+@@ -1113,7 +1233,7 @@ static int __nbcon_atomic_flush_pending_
+ * handed over or taken over. In both cases the context is no
+ * longer valid.
+ */
+- if (!nbcon_emit_next_record(&wctxt))
++ if (!nbcon_emit_next_record(&wctxt, true))
+ return -EAGAIN;
+
+ if (!ctxt->backlog) {
+@@ -1159,11 +1279,11 @@ static void nbcon_atomic_flush_pending_c
+ local_irq_restore(flags);
+
+ /*
+- * If flushing was successful but more records are available, this
+- * context must flush those remaining records because there is no
+- * other context that will do it.
++ * If flushing was successful but more records are available this
++ * context must flush those remaining records if the printer thread
++ * is not available to do it.
+ */
+- if (!err && prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
++ if (!err && !con->kthread && prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
+ stop_seq = prb_next_reserve_seq(prb);
+ goto again;
+ }
+@@ -1315,6 +1435,63 @@ void nbcon_cpu_emergency_flush(void)
+ }
+ }
+
++/*
++ * nbcon_kthread_stop - Stop a printer thread
++ * @con: Console to operate on
++ */
++static void nbcon_kthread_stop(struct console *con)
++{
++ lockdep_assert_console_list_lock_held();
++
++ if (!con->kthread)
++ return;
++
++ kthread_stop(con->kthread);
++ con->kthread = NULL;
++}
++
++/**
++ * nbcon_kthread_create - Create a printer thread
++ * @con: Console to operate on
++ *
++ * If it fails, let the console proceed. The atomic part might
++ * be usable and useful.
++ */
++void nbcon_kthread_create(struct console *con)
++{
++ struct task_struct *kt;
++
++ lockdep_assert_console_list_lock_held();
++
++ if (!(con->flags & CON_NBCON) || !con->write_thread)
++ return;
++
++ if (con->kthread)
++ return;
++
++ /*
++ * Printer threads cannot be started as long as any boot console is
++ * registered because there is no way to synchronize the hardware
++ * registers between boot console code and regular console code.
++ */
++ if (have_boot_console)
++ return;
++
++ kt = kthread_run(nbcon_kthread_func, con, "pr/%s%d", con->name, con->index);
++ if (IS_ERR(kt)) {
++ con_printk(KERN_ERR, con, "failed to start printing thread\n");
++ return;
++ }
++
++ con->kthread = kt;
++
++ /*
++ * It is important that console printing threads are scheduled
++ * shortly after a printk call and with generous runtime budgets.
++ */
++ sched_set_normal(con->kthread, -20);
++}
++
+ /**
+ * nbcon_alloc - Allocate buffers needed by the nbcon console
+ * @con: Console to allocate buffers for
+@@ -1360,6 +1537,7 @@ void nbcon_init(struct console *con, u64
+ /* nbcon_alloc() must have been called and successful! */
+ BUG_ON(!con->pbufs);
+
++ rcuwait_init(&con->rcuwait);
+ nbcon_seq_force(con, init_seq);
+ nbcon_state_set(con, &state);
+ }
+@@ -1372,6 +1550,7 @@ void nbcon_free(struct console *con)
+ {
+ struct nbcon_state state = { };
+
++ nbcon_kthread_stop(con);
+ nbcon_state_set(con, &state);
+
+ /* Boot consoles share global printk buffers. */
+@@ -1440,6 +1619,7 @@ void nbcon_driver_release(struct console
+ */
+ cookie = console_srcu_read_lock();
+ if (console_is_usable(con, console_srcu_read_flags(con)) &&
++ !con->kthread &&
+ prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
+ __nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb), false);
+ }
+--- a/kernel/printk/printk.c
++++ b/kernel/printk/printk.c
+@@ -2685,6 +2685,8 @@ void suspend_console(void)
+ void resume_console(void)
+ {
+ struct console *con;
++ short flags;
++ int cookie;
+
+ if (!console_suspend_enabled)
+ return;
+@@ -2701,6 +2703,14 @@ void resume_console(void)
+ */
+ synchronize_srcu(&console_srcu);
+
++ cookie = console_srcu_read_lock();
++ for_each_console_srcu(con) {
++ flags = console_srcu_read_flags(con);
++ if (flags & CON_NBCON)
++ nbcon_kthread_wake(con);
++ }
++ console_srcu_read_unlock(cookie);
++
+ pr_flush(1000, true);
+ }
+
+@@ -3021,6 +3031,13 @@ static bool console_flush_all(bool do_co
+ u64 printk_seq;
+ bool progress;
+
++ /*
++ * console_flush_all() is only for legacy consoles,
++ * unless the nbcon console has no kthread printer.
++ */
++ if ((flags & CON_NBCON) && con->kthread)
++ continue;
++
+ if (!console_is_usable(con, flags))
+ continue;
+ any_usable = true;
+@@ -3314,9 +3331,23 @@ EXPORT_SYMBOL(console_stop);
+
+ void console_start(struct console *console)
+ {
++ short flags;
++
+ console_list_lock();
+ console_srcu_write_flags(console, console->flags | CON_ENABLED);
++ flags = console->flags;
+ console_list_unlock();
++
++ /*
++ * Ensure that all SRCU list walks have completed. The related
++ * printing context must be able to see it is enabled so that
++ * it is guaranteed to wake up and resume printing.
++ */
++ synchronize_srcu(&console_srcu);
++
++ if (flags & CON_NBCON)
++ nbcon_kthread_wake(console);
++
+ __pr_flush(console, 1000, true);
+ }
+ EXPORT_SYMBOL(console_start);