linux-kernel - Re: [Question] Hooks for scheduler tracing (CFS)

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Message-ID: <20070726110504.GB7673@elte.hu>
Date:	Thu, 26 Jul 2007 13:05:04 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Ankita Garg <ankita@...ibm.com>
Cc:	Arjan van de Ven <arjan@...radead.org>, linux@...mer.net,
	LKML <linux-kernel@...r.kernel.org>,
	RT-Users <linux-rt-users@...r.kernel.org>,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: [Question] Hooks for scheduler tracing (CFS)


* Ankita Garg <ankita@...ibm.com> wrote:

>         local_irq_save(flags);
>         buf = _stp_chan->buf[smp_processor_id()];
>         if (unlikely(buf->offset + length > _stp_chan->subbuf_size))
>                 length = relay_switch_subbuf(buf, length);
>         memcpy(buf->data + buf->offset, data, length);
>         buf->offset += length;
>         local_irq_restore(flags);

oh, what a fine piece of s^H^H :-/ Who in their right mind calls this 
from _tracing_ code:

                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);

and the comment is utter rubbish: __mod_timer() can lock up just as 
much. Just use an adaptive-polling method to drive the draining of the 
relay buffer, instead of mucking with timers from within the tracing 
code. Whoever implemented this has absolutely zero clue i have to say 
...

the smp_mb() is rubbish too.

could you try the patch below, does it fix the problem?

	Ingo

------------------------------------->
Subject: relay: fix timer madness
From: Ingo Molnar <mingo@...e.hu>

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 <mingo@...e.hu>
---
 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;
-
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