Re: [PATCH tip/core/rcu 08/11] rcu: Micro-optimize rcu_cpu_has_callbacks()

[Chen Gang]

Thank you for your whole work, firstly  :-).


And your suggestion about testing (in our discussion) is also valuable
to me.

I need start LTP in q4. After referenced your suggestion, my first step
for using/learning LTP is not mainly for finding kernel issues, but for
testing kernel (to improve my kernel testing efficiency).

When I want to find issues by reading code, I will consider about LTP
too (I will try to find issues which can be tested by LTP).


On 09/25/2013 09:29 AM, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> The for_each_rcu_flavor() loop unconditionally scans all flavors, even
> when the first flavor might have some non-lazy callbacks.  Once the
> loop has seen a non-lazy callback, further passes through the loop
> cannot change the state.  This is not a huge problem, given that there
> can be at most three RCU flavors (RCU-bh, RCU-preempt, and RCU-sched),
> but this code is on the path to idle, so speeding it up even a small
> amount would have some benefit.
> 
> This commit therefore does two things:
> 
> 1.	Rearranges the order of the list of RCU flavors in order to
> 	place the most active flavor first in the list.  The most active
> 	RCU flavor is RCU-preempt, or, if there is no RCU-preempt,
> 	RCU-sched.
> 
> 2.	Reworks the for_each_rcu_flavor() to exit early when the first
> 	non-lazy callback is seen, or, in the case where the caller
> 	does not care about non-lazy callbacks (RCU_FAST_NO_HZ=n),
> 	when the first callback is seen.
> 
> Reported-by: Chen Gang <gang.chen@xxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcutree.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index e6f2e8f..49464ad 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -2727,10 +2727,13 @@ static int rcu_cpu_has_callbacks(int cpu, bool *all_lazy)
>  
>  	for_each_rcu_flavor(rsp) {
>  		rdp = per_cpu_ptr(rsp->rda, cpu);
> -		if (rdp->qlen != rdp->qlen_lazy)
> +		if (!rdp->nxtlist)
> +			continue;
> +		hc = true;
> +		if (rdp->qlen != rdp->qlen_lazy || !all_lazy) {
>  			al = false;
> -		if (rdp->nxtlist)
> -			hc = true;
> +			break;
> +		}
>  	}
>  	if (all_lazy)
>  		*all_lazy = al;
> @@ -3297,8 +3300,8 @@ void __init rcu_init(void)
>  
>  	rcu_bootup_announce();
>  	rcu_init_geometry();
> -	rcu_init_one(&rcu_sched_state, &rcu_sched_data);
>  	rcu_init_one(&rcu_bh_state, &rcu_bh_data);
> +	rcu_init_one(&rcu_sched_state, &rcu_sched_data);
>  	__rcu_init_preempt();
>  	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
>  
> 


-- 
Chen Gang

-- 
Chen Gang
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