Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755590AbeAJNYn (ORCPT + 1 other); Wed, 10 Jan 2018 08:24:43 -0500 Received: from mx2.suse.de ([195.135.220.15]:38569 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752996AbeAJNYm (ORCPT ); Wed, 10 Jan 2018 08:24:42 -0500 From: Petr Mladek To: Steven Rostedt , Sergey Senozhatsky Cc: akpm@linux-foundation.org, linux-mm@kvack.org, Cong Wang , Dave Hansen , Johannes Weiner , Mel Gorman , Michal Hocko , Vlastimil Babka , Peter Zijlstra , Linus Torvalds , Jan Kara , Mathieu Desnoyers , Tetsuo Handa , rostedt@home.goodmis.org, Byungchul Park , Sergey Senozhatsky , Tejun Heo , Pavel Machek , linux-kernel@vger.kernel.org Subject: [PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes Date: Wed, 10 Jan 2018 14:24:17 +0100 Message-Id: <20180110132418.7080-2-pmladek@suse.com> X-Mailer: git-send-email 2.13.6 In-Reply-To: <20180110132418.7080-1-pmladek@suse.com> References: <20180110132418.7080-1-pmladek@suse.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: From: Steven Rostedt From: Steven Rostedt (VMware) This patch implements what I discussed in Kernel Summit. I added lockdep annotation (hopefully correctly), and it hasn't had any splats (since I fixed some bugs in the first iterations). It did catch problems when I had the owner covering too much. But now that the owner is only set when actively calling the consoles, lockdep has stayed quiet. Here's the design again: I added a "console_owner" which is set to a task that is actively writing to the consoles. It is *not* the same as the owner of the console_lock. It is only set when doing the calls to the console functions. It is protected by a console_owner_lock which is a raw spin lock. There is a console_waiter. This is set when there is an active console owner that is not current, and waiter is not set. This too is protected by console_owner_lock. In printk() when it tries to write to the consoles, we have: if (console_trylock()) console_unlock(); Now I added an else, which will check if there is an active owner, and no current waiter. If that is the case, then console_waiter is set, and the task goes into a spin until it is no longer set. When the active console owner finishes writing the current message to the consoles, it grabs the console_owner_lock and sees if there is a waiter, and clears console_owner. If there is a waiter, then it breaks out of the loop, clears the waiter flag (because that will release the waiter from its spin), and exits. Note, it does *not* release the console semaphore. Because it is a semaphore, there is no owner. Another task may release it. This means that the waiter is guaranteed to be the new console owner! Which it becomes. Then the waiter calls console_unlock() and continues to write to the consoles. If another task comes along and does a printk() it too can become the new waiter, and we wash rinse and repeat! By Petr Mladek about possible new deadlocks: The thing is that we move console_sem only to printk() call that normally calls console_unlock() as well. It means that the transferred owner should not bring new type of dependencies. As Steven said somewhere: "If there is a deadlock, it was there even before." We could look at it from this side. The possible deadlock would look like: CPU0 CPU1 console_unlock() console_owner = current; spin_lockA() printk() spin = true; while (...) call_console_drivers() spin_lockA() This would be a deadlock. CPU0 would wait for the lock A. While CPU1 would own the lockA and would wait for CPU0 to finish calling the console drivers and pass the console_sem owner. But if the above is true than the following scenario was already possible before: CPU0 spin_lockA() printk() console_unlock() call_console_drivers() spin_lockA() By other words, this deadlock was there even before. Such deadlocks are prevented by using printk_deferred() in the sections guarded by the lock A. Signed-off-by: Steven Rostedt (VMware) [pmladek@suse.com: Commit message about possible deadlocks] --- kernel/printk/printk.c | 108 ++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 107 insertions(+), 1 deletion(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index b9006617710f..7e6459abba43 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -86,8 +86,15 @@ EXPORT_SYMBOL_GPL(console_drivers); static struct lockdep_map console_lock_dep_map = { .name = "console_lock" }; +static struct lockdep_map console_owner_dep_map = { + .name = "console_owner" +}; #endif +static DEFINE_RAW_SPINLOCK(console_owner_lock); +static struct task_struct *console_owner; +static bool console_waiter; + enum devkmsg_log_bits { __DEVKMSG_LOG_BIT_ON = 0, __DEVKMSG_LOG_BIT_OFF, @@ -1753,8 +1760,56 @@ asmlinkage int vprintk_emit(int facility, int level, * semaphore. The release will print out buffers and wake up * /dev/kmsg and syslog() users. */ - if (console_trylock()) + if (console_trylock()) { console_unlock(); + } else { + struct task_struct *owner = NULL; + bool waiter; + bool spin = false; + + printk_safe_enter_irqsave(flags); + + raw_spin_lock(&console_owner_lock); + owner = READ_ONCE(console_owner); + waiter = READ_ONCE(console_waiter); + if (!waiter && owner && owner != current) { + WRITE_ONCE(console_waiter, true); + spin = true; + } + raw_spin_unlock(&console_owner_lock); + + /* + * If there is an active printk() writing to the + * consoles, instead of having it write our data too, + * see if we can offload that load from the active + * printer, and do some printing ourselves. + * Go into a spin only if there isn't already a waiter + * spinning, and there is an active printer, and + * that active printer isn't us (recursive printk?). + */ + if (spin) { + /* We spin waiting for the owner to release us */ + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_); + /* Owner will clear console_waiter on hand off */ + while (READ_ONCE(console_waiter)) + cpu_relax(); + + spin_release(&console_owner_dep_map, 1, _THIS_IP_); + printk_safe_exit_irqrestore(flags); + + /* + * The owner passed the console lock to us. + * Since we did not spin on console lock, annotate + * this as a trylock. Otherwise lockdep will + * complain. + */ + mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_); + console_unlock(); + printk_safe_enter_irqsave(flags); + } + printk_safe_exit_irqrestore(flags); + + } } return printed_len; @@ -2141,6 +2196,7 @@ void console_unlock(void) static u64 seen_seq; unsigned long flags; bool wake_klogd = false; + bool waiter = false; bool do_cond_resched, retry; if (console_suspended) { @@ -2229,14 +2285,64 @@ void console_unlock(void) console_seq++; raw_spin_unlock(&logbuf_lock); + /* + * While actively printing out messages, if another printk() + * were to occur on another CPU, it may wait for this one to + * finish. This task can not be preempted if there is a + * waiter waiting to take over. + */ + raw_spin_lock(&console_owner_lock); + console_owner = current; + raw_spin_unlock(&console_owner_lock); + + /* The waiter may spin on us after setting console_owner */ + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_); + stop_critical_timings(); /* don't trace print latency */ call_console_drivers(ext_text, ext_len, text, len); start_critical_timings(); + + raw_spin_lock(&console_owner_lock); + waiter = READ_ONCE(console_waiter); + console_owner = NULL; + raw_spin_unlock(&console_owner_lock); + + /* + * If there is a waiter waiting for us, then pass the + * rest of the work load over to that waiter. + */ + if (waiter) + break; + + /* There was no waiter, and nothing will spin on us here */ + spin_release(&console_owner_dep_map, 1, _THIS_IP_); + printk_safe_exit_irqrestore(flags); if (do_cond_resched) cond_resched(); } + + /* + * If there is an active waiter waiting on the console_lock. + * Pass off the printing to the waiter, and the waiter + * will continue printing on its CPU, and when all writing + * has finished, the last printer will wake up klogd. + */ + if (waiter) { + WRITE_ONCE(console_waiter, false); + /* The waiter is now free to continue */ + spin_release(&console_owner_dep_map, 1, _THIS_IP_); + /* + * Hand off console_lock to waiter. The waiter will perform + * the up(). After this, the waiter is the console_lock owner. + */ + mutex_release(&console_lock_dep_map, 1, _THIS_IP_); + printk_safe_exit_irqrestore(flags); + /* Note, if waiter is set, logbuf_lock is not held */ + return; + } + console_locked = 0; /* Release the exclusive_console once it is used */ -- 2.13.6