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

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Message-ID: <20070726132055.GC19398@in.ibm.com>
Date:	Thu, 26 Jul 2007 18:50:55 +0530
From:	ankita@...ibm.com (Ankita Garg)
To:	Ingo Molnar <mingo@...e.hu>
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)

Hi Ingo,

On Thu, Jul 26, 2007 at 01:05:04PM +0200, Ingo Molnar wrote:
> 
> * 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?

This patch did not fix my problem. I still get similar traces...

> 
> 	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;

-- 
Regards,
Ankita Garg (ankita@...ibm.com)
Linux Technology Center
IBM India Systems & Technology Labs, 
Bangalore, India   
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