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

> 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@elte.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@elte.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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