From: Steven Rostedt <rostedt@goodmis.org>
To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org,
Cong Wang <xiyou.wangcong@gmail.com>,
Dave Hansen <dave.hansen@intel.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
Petr Mladek <pmladek@suse.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Vlastimil Babka <vbabka@suse.cz>,
"yuwang.yuwang" <yuwang.yuwang@alibaba-inc.com>,
Peter Zijlstra <peterz@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Jan Kara <jack@suse.cz>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: [PATCH v2] printk: Add console owner and waiter logic to load balance console writes
Date: Thu, 2 Nov 2017 13:06:05 -0400 [thread overview]
Message-ID: <20171102130605.05e987e8@gandalf.local.home> (raw)
In-Reply-To: <20171102115625.13892e18@gandalf.local.home>
From: Steven Rostedt (VMware) <rostedt@goodmis.org>
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 an 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!
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
Changes from v1:
Use "spin_acquire" instead of "mutex_acquire" for the
console_lock_dep annotation. It may make a difference in the future,
and it does act like a spinlock and not a mutex. (thanks to Peter
Ziljstra).
Remove all READ/WRITE_ONCE() from inside the spin lock protected
areas, as they are unnecessary.
Add a WRITE_ONCE() for clearing console_owner, because that's done
outside the spin lock protection.
Index: linux-trace.git/kernel/printk/printk.c
===================================================================
--- linux-trace.git.orig/kernel/printk/printk.c
+++ linux-trace.git/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
* 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 = console_owner;
+ waiter = console_waiter;
+ if (!waiter && owner && owner != current) {
+ 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) {
@@ -2215,6 +2271,20 @@ skip:
goto skip;
}
+ /*
+ * 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.
+ */
+
+ /* The waiter may spin on us after this */
+ spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+
+ raw_spin_lock(&console_owner_lock);
+ console_owner = current;
+ raw_spin_unlock(&console_owner_lock);
+
len += msg_print_text(msg, false, text + len, sizeof(text) - len);
if (nr_ext_console_drivers) {
ext_len = msg_print_ext_header(ext_text,
@@ -2232,11 +2302,48 @@ skip:
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 = 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 */
--
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next prev parent reply other threads:[~2017-11-02 17:06 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-26 11:28 [PATCH] mm: don't warn about allocations which stall for too long Tetsuo Handa
2017-10-26 11:41 ` Michal Hocko
2017-11-08 10:30 ` peter enderborg
2017-11-09 8:52 ` Michal Hocko
2017-11-09 9:34 ` peter enderborg
2017-11-09 10:09 ` Michal Hocko
2017-11-09 10:19 ` Tetsuo Handa
2017-10-26 14:37 ` Johannes Weiner
2017-10-31 19:32 ` Steven Rostedt
2017-11-01 8:30 ` Vlastimil Babka
2017-11-01 13:38 ` Petr Mladek
2017-11-01 15:36 ` Steven Rostedt
2017-11-02 11:46 ` Petr Mladek
2017-11-02 14:49 ` Steven Rostedt
2017-11-01 15:33 ` Steven Rostedt
2017-11-01 17:42 ` Vlastimil Babka
2017-11-01 17:54 ` Steven Rostedt
2017-11-02 8:53 ` Sergey Senozhatsky
2017-11-02 9:14 ` Sergey Senozhatsky
2017-11-02 14:55 ` Steven Rostedt
2017-11-02 12:55 ` Michal Hocko
2017-11-02 15:56 ` Steven Rostedt
2017-11-02 17:06 ` Steven Rostedt [this message]
2017-11-02 17:10 ` [PATCH v2] printk: Add console owner and waiter logic to load balance console writes Steven Rostedt
2017-11-02 17:38 ` Steven Rostedt
2017-11-03 10:19 ` Jan Kara
2017-11-03 11:18 ` Steven Rostedt
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