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

[David J. Wilder]

> 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?

>   
> > 	Ingo
> >
> > ------------------------------------->
> > Subject: relay: fix timer madness
> > From: Ingo Molnar <mingo@xxxxxxx>
> >
> > 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@xxxxxxx>
> > ---
> >  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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