Messages in this thread | | | Date | Thu, 1 Apr 2010 12:27:40 -0700 | From | Andrew Morton <> | Subject | Re: [REGRESSION 2.6.30][PATCH 1/1] sched: defer idle accounting till after load update period |
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On Mon, 29 Mar 2010 09:41:12 -0400 Chase Douglas <chase.douglas@canonical.com> wrote:
> There's a period of 10 ticks where calc_load_tasks is updated by all the > cpus for the load avg. Usually all the cpus do this during the first > tick. If any cpus go idle, calc_load_tasks is decremented accordingly. > However, if they wake up calc_load_tasks is not incremented. Thus, if > cpus go idle during the 10 tick period, calc_load_tasks may be > decremented to a non-representative value. This issue can lead to > systems having a load avg of exactly 0, even though the real load avg > could theoretically be up to NR_CPUS. > > This change defers calc_load_tasks accounting after each cpu updates the > count until after the 10 tick period. > > BugLink: http://bugs.launchpad.net/bugs/513848 >
There was useful information in the [patch 0/1] email, such as the offending commit ID. If possible, it's best to avoid the [patch 0/n] thing altogether - that information either has to be moved into the [patch 1/n] changelog by someone (ie: me), or it just gets ommitted and lost.
> --- > kernel/sched.c | 16 ++++++++++++++-- > 1 files changed, 14 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched.c b/kernel/sched.c > index 9ab3cd7..c0aedac 100644 > --- a/kernel/sched.c > +++ b/kernel/sched.c > @@ -3064,7 +3064,8 @@ void calc_global_load(void) > */ > static void calc_load_account_active(struct rq *this_rq) > { > - long nr_active, delta; > + static atomic_long_t deferred; > + long nr_active, delta, deferred_delta; > > nr_active = this_rq->nr_running; > nr_active += (long) this_rq->nr_uninterruptible; > @@ -3072,6 +3073,17 @@ static void calc_load_account_active(struct rq *this_rq) > if (nr_active != this_rq->calc_load_active) { > delta = nr_active - this_rq->calc_load_active; > this_rq->calc_load_active = nr_active; > + > + /* Need to defer idle accounting during load update period: */ > + if (unlikely(time_before(jiffies, this_rq->calc_load_update) && > + time_after_eq(jiffies, calc_load_update))) { > + atomic_long_add(delta, &deferred); > + return; > + }
That seems a sensible way to avoid the gross-looking "10 ticks" thing.
What was the reason for "update the avenrun load estimates 10 ticks after the CPUs have updated calc_load_tasks"? Can we do something smarter there to fix this?
> + deferred_delta = atomic_long_xchg(&deferred, 0); > + delta += deferred_delta; > + > atomic_long_add(delta, &calc_load_tasks); > } > }
The global `deferred' is unfortunate from a design and possibly scalability POV. Can it be moved into the `struct rq'? That way it can become a plain old `unsigned long', too.
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