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authorSteven Rostedt (VMware) <rostedt@goodmis.org>2018-01-10 14:24:17 +0100
committerPetr Mladek <pmladek@suse.com>2018-01-16 17:21:09 +0100
commitdbdda842fe96f8932bae554f0adf463c27c42bc7 (patch)
treea021144a822752ced36f244466103c50a9f9c6d9 /kernel/printk
parent11ca75d2d6d18d5a7ee4d7ec1da6f864f5c8c8be (diff)
printk: Add console owner and waiter logic to load balance console writes
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. By Steven Rostedt: To demonstrate the issue, this module has been shown to lock up a system with 4 CPUs and a slow console (like a serial console). It is also able to lock up a 8 CPU system with only a fast (VGA) console, by passing in "loops=100". The changes in this commit prevent this module from locking up the system. #include <linux/module.h> #include <linux/delay.h> #include <linux/sched.h> #include <linux/mutex.h> #include <linux/workqueue.h> #include <linux/hrtimer.h> static bool stop_testing; static unsigned int loops = 1; static void preempt_printk_workfn(struct work_struct *work) { int i; while (!READ_ONCE(stop_testing)) { for (i = 0; i < loops && !READ_ONCE(stop_testing); i++) { preempt_disable(); pr_emerg("%5d%-75s\n", smp_processor_id(), " XXX NOPREEMPT"); preempt_enable(); } msleep(1); } } static struct work_struct __percpu *works; static void finish(void) { int cpu; WRITE_ONCE(stop_testing, true); for_each_online_cpu(cpu) flush_work(per_cpu_ptr(works, cpu)); free_percpu(works); } static int __init test_init(void) { int cpu; works = alloc_percpu(struct work_struct); if (!works) return -ENOMEM; /* * This is just a test module. This will break if you * do any CPU hot plugging between loading and * unloading the module. */ for_each_online_cpu(cpu) { struct work_struct *work = per_cpu_ptr(works, cpu); INIT_WORK(work, &preempt_printk_workfn); schedule_work_on(cpu, work); } return 0; } static void __exit test_exit(void) { finish(); } module_param(loops, uint, 0); module_init(test_init); module_exit(test_exit); MODULE_LICENSE("GPL"); Link: http://lkml.kernel.org/r/20180110132418.7080-2-pmladek@suse.com Cc: akpm@linux-foundation.org Cc: linux-mm@kvack.org Cc: Cong Wang <xiyou.wangcong@gmail.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Mel Gorman <mgorman@suse.de> Cc: Michal Hocko <mhocko@kernel.org> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Jan Kara <jack@suse.cz> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Cc: Byungchul Park <byungchul.park@lge.com> Cc: Tejun Heo <tj@kernel.org> Cc: Pavel Machek <pavel@ucw.cz> Cc: linux-kernel@vger.kernel.org Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> [pmladek@suse.com: Commit message about possible deadlocks] Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Petr Mladek <pmladek@suse.com>
Diffstat (limited to 'kernel/printk')
-rw-r--r--kernel/printk/printk.c108
1 files changed, 107 insertions, 1 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5d81206a572d..040fb948924e 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 @@ skip:
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 */