From nobody Thu Dec 18 05:58:35 2025 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 609A11D86D2 for ; Wed, 4 Sep 2024 12:05:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725451544; cv=none; b=g6NJrHj1TaKIjELgANrMJ1XNuAItuBHAN/DhqOJ61JWPTccwo7ySLvieQShJ6cg3WbZncx4c+rmjwrxTl8uzKQZolpe7Iaehs75iy+t0z46kWo3LsFKxF1gLJcvfc6AJkaev9O8rhXgOWng1eHj8eR4bT+Gmf/dtlpTMNNZXUpY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725451544; c=relaxed/simple; bh=4v5aeJofKfR0ZoCF2QIJZT4kzDlYxs0/6EpdRyCHtN8=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=NPKADY33TO03HbPDww+gdAoPJpxk3JMkA1NxQN7jzGFQ+K3QHSPEkryGYlRJiTXcVxm1tewo4CrHNvZ5f8gEKQaMoEFn5i/J44wPjez/2n6UkLS3Zhslupe3kAeWzUwstxbcKeLJ8fa11oakVC62Y+hKQ9Josp4YOzIIzGQnbuc= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=sA8QAbtH; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=fcvbKjzW; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="sA8QAbtH"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="fcvbKjzW" From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1725451540; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=rLkJir6xzSMNayX/D6HhMiMivMcjPhmzXeWP3W+BvCs=; b=sA8QAbtHmvF+fDYKJSVXD4XoAKa0joKAXNOfY813iPK+hKx81j6EIKygSn4FlWOaSUIG4h qM74BOLZSgb0PP/hNL2S/jVS4tq8j24Yai+ZzIBNvwpLqZUuRPpEP/TZp4c7CMgLN8AcR6 L+c6q4wgd60W4smcGPdDnnDvHpq2+QF3ubrWi+lH0bQHqi06QECUl3yhPIWRu9pOJlgnJe AU9TQsT4prWRw+6dwFaKgFIxFbj4N9G0pJEZ1wnwySNJa7aQyxFjTsnPjUilqwk3QjSKNx VKV61g7KA5YVqdE08G2dW+TpG2vMiQERKrhZVC874Czbk45vUZ9mxSgPCccpqg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1725451540; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=rLkJir6xzSMNayX/D6HhMiMivMcjPhmzXeWP3W+BvCs=; b=fcvbKjzWP6Vdb7bY2T4RE2oSlAIqSJObyW+7aWWj5KoUi5ln0lTOZykHxbfGYGZTZf4/cB mZB70dUesOKonnCA== To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: [PATCH printk v6 09/17] printk: nbcon: Rely on kthreads for normal operation Date: Wed, 4 Sep 2024 14:11:28 +0206 Message-Id: <20240904120536.115780-10-john.ogness@linutronix.de> In-Reply-To: <20240904120536.115780-1-john.ogness@linutronix.de> References: <20240904120536.115780-1-john.ogness@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Once the kthread is running and available (i.e. @printk_kthreads_running is set), the kthread becomes responsible for flushing any pending messages which are added in NBCON_PRIO_NORMAL context. Namely the legacy console_flush_all() and device_release() no longer flush the console. And nbcon_atomic_flush_pending() used by nbcon_cpu_emergency_exit() no longer flushes messages added after the emergency messages. The console context is safe when used by the kthread only when one of the following conditions are true: 1. Other caller acquires the console context with NBCON_PRIO_NORMAL with preemption disabled. It will release the context before rescheduling. 2. Other caller acquires the console context with NBCON_PRIO_NORMAL under the device_lock. 3. The kthread is the only context which acquires the console with NBCON_PRIO_NORMAL. This is satisfied for all atomic printing call sites: nbcon_legacy_emit_next_record() (#1) nbcon_atomic_flush_pending_con() (#1) nbcon_device_release() (#2) It is even double guaranteed when @printk_kthreads_running is set because then _only_ the kthread will print for NBCON_PRIO_NORMAL. (#3) Signed-off-by: John Ogness --- kernel/printk/internal.h | 26 ++++++++++++++++++++++ kernel/printk/nbcon.c | 17 +++++++++----- kernel/printk/printk.c | 48 +++++++++++++++++++++++++++++++++++++++- 3 files changed, 84 insertions(+), 7 deletions(-) diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h index a96d4114a1db..8166e24f8780 100644 --- a/kernel/printk/internal.h +++ b/kernel/printk/internal.h @@ -113,6 +113,13 @@ static inline bool console_is_usable(struct console *c= on, short flags, bool use_ /* The write_atomic() callback is optional. */ if (use_atomic && !con->write_atomic) return false; + + /* + * For the !use_atomic case, @printk_kthreads_running is not + * checked because the write_thread() callback is also used + * via the legacy loop when the printer threads are not + * available. + */ } else { if (!con->write) return false; @@ -176,6 +183,7 @@ static inline void nbcon_atomic_flush_pending(void) { } static inline bool nbcon_legacy_emit_next_record(struct console *con, bool= *handover, int cookie, bool use_atomic) { return false; } static inline void nbcon_kthread_wake(struct console *con) { } +static inline void nbcon_kthreads_wake(void) { } =20 static inline bool console_is_usable(struct console *con, short flags, bool use_atomic) { return false; } @@ -190,6 +198,7 @@ extern bool legacy_allow_panic_sync; /** * struct console_flush_type - Define available console flush methods * @nbcon_atomic: Flush directly using nbcon_atomic() callback + * @nbcon_offload: Offload flush to printer thread * @legacy_direct: Call the legacy loop in this context * @legacy_offload: Offload the legacy loop into IRQ * @@ -197,6 +206,7 @@ extern bool legacy_allow_panic_sync; */ struct console_flush_type { bool nbcon_atomic; + bool nbcon_offload; bool legacy_direct; bool legacy_offload; }; @@ -211,6 +221,22 @@ static inline void printk_get_console_flush_type(struc= t console_flush_type *ft) =20 switch (nbcon_get_default_prio()) { case NBCON_PRIO_NORMAL: + if (have_nbcon_console && !have_boot_console) { + if (printk_kthreads_running) + ft->nbcon_offload =3D true; + else + ft->nbcon_atomic =3D true; + } + + /* Legacy consoles are flushed directly when possible. */ + if (have_legacy_console || have_boot_console) { + if (!is_printk_legacy_deferred()) + ft->legacy_direct =3D true; + else + ft->legacy_offload =3D true; + } + break; + case NBCON_PRIO_EMERGENCY: if (have_nbcon_console && !have_boot_console) ft->nbcon_atomic =3D true; diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c index 784e5de88abf..5146ce9853a3 100644 --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -1494,6 +1494,7 @@ static int __nbcon_atomic_flush_pending_con(struct co= nsole *con, u64 stop_seq, static void nbcon_atomic_flush_pending_con(struct console *con, u64 stop_s= eq, bool allow_unsafe_takeover) { + struct console_flush_type ft; unsigned long flags; int err; =20 @@ -1523,10 +1524,12 @@ static void nbcon_atomic_flush_pending_con(struct c= onsole *con, u64 stop_seq, =20 /* * 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. + * context must flush those remaining records if the printer thread + * is not available do it. */ - if (prb_read_valid(prb, nbcon_seq_read(con), NULL)) { + printk_get_console_flush_type(&ft); + if (!ft.nbcon_offload && + prb_read_valid(prb, nbcon_seq_read(con), NULL)) { stop_seq =3D prb_next_reserve_seq(prb); goto again; } @@ -1754,17 +1757,19 @@ void nbcon_device_release(struct console *con) =20 /* * This context must flush any new records added while the console - * was locked. The console_srcu_read_lock must be taken to ensure - * the console is usable throughout flushing. + * was locked if the printer thread is not available to do it. The + * console_srcu_read_lock must be taken to ensure the console is + * usable throughout flushing. */ cookie =3D console_srcu_read_lock(); + printk_get_console_flush_type(&ft); if (console_is_usable(con, console_srcu_read_flags(con), true) && + !ft.nbcon_offload && prb_read_valid(prb, nbcon_seq_read(con), NULL)) { /* * If nbcon_atomic flushing is not available, fallback to * using the legacy loop. */ - printk_get_console_flush_type(&ft); if (ft.nbcon_atomic) { __nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb), false); } else if (ft.legacy_direct) { diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 55d75db00042..149c3e04c2b5 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2384,6 +2384,9 @@ asmlinkage int vprintk_emit(int facility, int level, if (ft.nbcon_atomic) nbcon_atomic_flush_pending(); =20 + if (ft.nbcon_offload) + nbcon_kthreads_wake(); + if (ft.legacy_direct) { /* * The caller may be holding system-critical or @@ -2732,6 +2735,7 @@ void suspend_console(void) =20 void resume_console(void) { + struct console_flush_type ft; struct console *con; =20 if (!console_suspend_enabled) @@ -2749,6 +2753,10 @@ void resume_console(void) */ synchronize_srcu(&console_srcu); =20 + printk_get_console_flush_type(&ft); + if (ft.nbcon_offload) + nbcon_kthreads_wake(); + pr_flush(1000, true); } =20 @@ -3060,6 +3068,7 @@ static inline void printk_kthreads_check_locked(void)= { } */ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *h= andover) { + struct console_flush_type ft; bool any_usable =3D false; struct console *con; bool any_progress; @@ -3071,12 +3080,22 @@ static bool console_flush_all(bool do_cond_resched,= u64 *next_seq, bool *handove do { any_progress =3D false; =20 + printk_get_console_flush_type(&ft); + cookie =3D console_srcu_read_lock(); for_each_console_srcu(con) { short flags =3D console_srcu_read_flags(con); u64 printk_seq; bool progress; =20 + /* + * console_flush_all() is only responsible for nbcon + * consoles when the nbcon consoles cannot print via + * their atomic or threaded flushing. + */ + if ((flags & CON_NBCON) && (ft.nbcon_atomic || ft.nbcon_offload)) + continue; + if (!console_is_usable(con, flags, !do_cond_resched)) continue; any_usable =3D true; @@ -3387,9 +3406,25 @@ EXPORT_SYMBOL(console_stop); =20 void console_start(struct console *console) { + struct console_flush_type ft; + bool is_nbcon; + console_list_lock(); console_srcu_write_flags(console, console->flags | CON_ENABLED); + is_nbcon =3D console->flags & CON_NBCON; 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); + + printk_get_console_flush_type(&ft); + if (is_nbcon && ft.nbcon_offload) + nbcon_kthread_wake(console); + __pr_flush(console, 1000, true); } EXPORT_SYMBOL(console_start); @@ -4115,6 +4150,8 @@ static bool __pr_flush(struct console *con, int timeo= ut_ms, bool reset_on_progre =20 /* Flush the consoles so that records up to @seq are printed. */ printk_get_console_flush_type(&ft); + if (ft.nbcon_atomic) + nbcon_atomic_flush_pending(); if (ft.legacy_direct) { console_lock(); console_unlock(); @@ -4152,8 +4189,10 @@ static bool __pr_flush(struct console *con, int time= out_ms, bool reset_on_progre * that they make forward progress, so only increment * @diff for usable consoles. */ - if (!console_is_usable(c, flags, true)) + if (!console_is_usable(c, flags, true) && + !console_is_usable(c, flags, false)) { continue; + } =20 if (flags & CON_NBCON) { printk_seq =3D nbcon_seq_read(c); @@ -4629,8 +4668,15 @@ EXPORT_SYMBOL_GPL(kmsg_dump_rewind); */ void console_try_replay_all(void) { + struct console_flush_type ft; + + printk_get_console_flush_type(&ft); if (console_trylock()) { __console_rewind_all(); + if (ft.nbcon_atomic) + nbcon_atomic_flush_pending(); + if (ft.nbcon_offload) + nbcon_kthreads_wake(); /* Consoles are flushed as part of console_unlock(). */ console_unlock(); } --=20 2.39.2