* David J. Wilder (dwilder@us.ibm.com) wrote:

quoted

I guess they might have to switch to such an asynchronous delivery
system if they want to do this properly. Simply put, your polling
solution is exactly what I do, but I check a flag set by the writer
instead of waking up the readers unconditionally.

Mathieu
 

Ingo's solution could call waitqueue_active() inside wakeup_readers() to 
determine if there are waiters. Right?

Yes and no.. What would be part of it, but also checking if there are
buffers ready to be read would reduce the unnecessary polling even more.
Just checking for waiting readers would be rather inefficient in a
scenario of infrequent events where the readers are always waiting for
data; they would be awakened at every timer tick.

Mathieu

quoted

quoted

Ingo

------------------------------------->
Subject: relay: fix timer madness
From: Ingo Molnar <redacted>

remove timer calls (!!!) from deep within the tracing infrastructure.
This was totally bogus code that can cause lockups and worse.
Poll the buffer every 2 jiffies for now.

Signed-off-by: Ingo Molnar <redacted>
---
kernel/relay.c |   14 +++++---------
1 file changed, 5 insertions(+), 9 deletions(-)

Index: linux-rt-rebase.q/kernel/relay.c
===================================================================

--- linux-rt-rebase.q.orig/kernel/relay.c
+++ linux-rt-rebase.q/kernel/relay.c

@@ -319,6 +319,10 @@ static void wakeup_readers(unsigned long

{
	struct rchan_buf *buf = (struct rchan_buf *)data;
	wake_up_interruptible(&buf->read_wait);
+	/*
+	 * Stupid polling for now:
+	 */
+	mod_timer(&buf->timer, jiffies + 1);
}

/**

@@ -336,6 +340,7 @@ static void __relay_reset(struct rchan_b

		init_waitqueue_head(&buf->read_wait);
		kref_init(&buf->kref);
		setup_timer(&buf->timer, wakeup_readers, (unsigned long)buf);
+		mod_timer(&buf->timer, jiffies + 1);
	} else
		del_timer_sync(&buf->timer);

@@ -604,15 +609,6 @@ size_t relay_switch_subbuf(struct rchan_

		buf->subbufs_produced++;
		buf->dentry->d_inode->i_size += buf->chan->subbuf_size -
			buf->padding[old_subbuf];
-		smp_mb();
-		if (waitqueue_active(&buf->read_wait))
-			/*
-			 * Calling wake_up_interruptible() from here
-			 * will deadlock if we happen to be logging
-			 * from the scheduler (trying to re-grab
-			 * rq->lock), so defer it.
-			 */
-			__mod_timer(&buf->timer, jiffies + 1);
	}

	old = buf->data;
   

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68