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    SubjectRe: [PATCH tip/core/rcu 31/86] rcu: further lower priority in rcu_yield()
    On Sun, May 01, 2011 at 07:51:04PM +0200, Mike Galbraith wrote:
    > On Sun, 2011-05-01 at 06:21 -0700, Paul E. McKenney wrote:
    > > From: Paul E. McKenney <paul.mckenney@linaro.org>
    > >
    > > Although rcu_yield() dropped from real-time to normal priority, there
    > > is always the possibility that the competing tasks have been niced.
    > > So nice to 19 in rcu_yield() to help ensure that other tasks have a
    > > better chance of running.
    >
    > But.. that just prolongs the pain of overhead you _have_ to eat, no? In
    > a brief surge, fine, you can spread the cost out.. but how do you know
    > when it's ok to yield?

    I modeled this code on the existing code in ksoftirqd. But yes, this is
    a heuristic. I do believe that it is quite robust, but time will tell.

    > (When maintenance threads worrying about their CPU usage is worrisome.)

    Indeed. But I am not introducing this, just moving the existing checking
    from ksoftirqd.

    So I believe that I am OK here.

    Thanx, Paul

    > > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > > ---
    > > kernel/rcutree.c | 1 +
    > > 1 files changed, 1 insertions(+), 0 deletions(-)
    > >
    > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > > index 3295c7b..963b4b1 100644
    > > --- a/kernel/rcutree.c
    > > +++ b/kernel/rcutree.c
    > > @@ -1561,6 +1561,7 @@ static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
    > > mod_timer(&yield_timer, jiffies + 2);
    > > sp.sched_priority = 0;
    > > sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
    > > + set_user_nice(current, 19);
    > > schedule();
    > > sp.sched_priority = RCU_KTHREAD_PRIO;
    > > sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
    >
    >


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