* Ingo Molnar ([email protected]) wrote:
>
> * Ankita Garg <[email protected]> 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?
>
Hi Ingo,
I did not have this problem with LTTng, since I only touch atomic
variables in tracing code. However, I iterate on a rcu list of active
traces in a timer interrupt to see if subbuffers must be read and, if
yes, I send a wakeup to my user-space daemon. I had to change the
protection around this rcu list read from preempt disable to mutex lock
in this timer because try_to_wakeup is called in it, which takes a
spinlock.
Note: I don't use relay code at my tracing site.
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
>
> ------------------------------------->
> Subject: relay: fix timer madness
> From: Ingo Molnar <[email protected]>
>
> 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 <[email protected]>
> ---
> 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
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