On 09/04/2013 01:59 AM, Paul E. McKenney wrote:
> On Tue, Sep 03, 2013 at 01:41:03PM +0800, Chen Gang wrote:
>> Hello Maintainers:
>>
>> Is this issue finished ?
>>
>> If need additional help from me (e.g. some test things, or others, if
>> you have no time, can let me try), please let me know, I should try.
>
> Ah, sorry, here is the patch.
>
Thanks.
:-)
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> rcu: Micro-optimize rcu_cpu_has_callbacks()
>
> 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.c...@asianux.com>
> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index b1b959d..38596be 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -2756,10 +2756,13 @@ 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;
> @@ -3326,8 +3329,8 @@ 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
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