[ltt-dev] [PATCH 10/10] call_rcu: avoid useless futex wakeups

[compudj@krystal.dyndns.org (Mathieu Desnoyers)]

* Paolo Bonzini (pbonzini at redhat.com) wrote:
> If the call_rcu thread is waiting for entries to accumulate, it is
> not necessary to wake it up.  Use the BUSY flag to detect this.

The crdp->futex value being -1, I think this case is already being taken
care of.

Thanks,

Mathieu

> 
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> ---
>  urcu-call-rcu-impl.h |    8 +++++---
>  1 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
> index d8570e3..165956e 100644
> --- a/urcu-call-rcu-impl.h
> +++ b/urcu-call-rcu-impl.h
> @@ -514,10 +514,12 @@ void call_rcu(struct rcu_head *head,
>  	head->func = func;
>  	crdp = get_call_rcu_data();
>  	cds_wfq_enqueue(&crdp->cbs, &head->next);
> -	/* Write list before writing the flags.  */
> +	/* Write list before checking/writing the flags.  */
>  	cmm_smp_mb();
> -	uatomic_or(&crdp->flags, URCU_CALL_RCU_BUSY);
> -	wake_call_rcu_thread(crdp);
> +	if ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_BUSY) == 0) {
> +		uatomic_or(&crdp->flags, URCU_CALL_RCU_BUSY);
> +		wake_call_rcu_thread(crdp);
> +	}
>  }
>  
>  /*
> -- 
> 1.7.4.4
> 
> 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com