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    SubjectRe: [patch 1/2] sched, timers: move calc_load() to scheduler
    On Thu, 14 May 2009, Peter Zijlstra wrote:
    > On Thu, 2009-05-14 at 11:21 +0000, Thomas Gleixner wrote:
    > > plain text document attachment (move-calc-load-to-scheduler-v1.patch)
    >
    > > +/*
    > > + * calc_load - update the avenrun load estimates 10 ticks after the
    > > + * CPUs have updated calc_load_tasks.
    > > + */
    > > +void calc_global_load(void)
    > > +{
    > > + unsigned long upd = calc_load_update + 10;
    > > + long active;
    > > +
    > > + if (time_before(jiffies, upd))
    > > + return;
    > >
    > > + active = atomic_long_read(&calc_load_tasks);
    > > + active = active > 0 ? active * FIXED_1 : 0;
    > >
    > > + avenrun[0] = calc_load(avenrun[0], EXP_1, active);
    > > + avenrun[1] = calc_load(avenrun[1], EXP_5, active);
    > > + avenrun[2] = calc_load(avenrun[2], EXP_15, active);
    > > +
    > > + calc_load_update += LOAD_FREQ;
    > > +}
    >
    > > @@ -1211,7 +1160,8 @@ static inline void update_times(unsigned
    > > void do_timer(unsigned long ticks)
    > > {
    > > jiffies_64 += ticks;
    > > - update_times(ticks);
    > > + update_wall_time();
    > > + calc_global_load();
    > > }
    >
    > I can see multiple cpus fall into calc_global_load() concurrently, which
    > would 'age' the load faster than expected.
    >
    > Should we plug that hole?

    They can't. do_timer() is called by exactly one CPU under xtime
    lock. What we removed is the loop over all online CPUs to retrieve the
    number of active tasks.

    Thanks,

    tglx


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