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    SubjectRe: [patch] sched: fix SMT scheduler regression in find_busiest_queue()
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    On Fri, 2010-02-12 at 17:14 -0800, Suresh Siddha wrote:

    > From: Suresh Siddha <suresh.b.siddha@intel.com>
    > Subject: sched: fix SMT scheduler regression in find_busiest_queue()
    >
    > Fix a SMT scheduler performance regression that is leading to a scenario
    > where SMT threads in one core are completely idle while both the SMT threads
    > in another core (on the same socket) are busy.
    >
    > This is caused by this commit (with the problematic code highlighted)
    >
    > commit bdb94aa5dbd8b55e75f5a50b61312fe589e2c2d1
    > Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Date: Tue Sep 1 10:34:38 2009 +0200
    >
    > sched: Try to deal with low capacity
    >
    > @@ -4203,15 +4223,18 @@ find_busiest_queue()
    > ...
    > for_each_cpu(i, sched_group_cpus(group)) {
    > + unsigned long power = power_of(i);
    >
    > ...
    >
    > - wl = weighted_cpuload(i);
    > + wl = weighted_cpuload(i) * SCHED_LOAD_SCALE;
    > + wl /= power;
    >
    > - if (rq->nr_running == 1 && wl > imbalance)
    > + if (capacity && rq->nr_running == 1 && wl > imbalance)
    > continue;
    >
    > On a SMT system, power of the HT logical cpu will be 589 and
    > the scheduler load imbalance (for scenarios like the one mentioned above)
    > can be approximately 1024 (SCHED_LOAD_SCALE). The above change of scaling
    > the weighted load with the power will result in "wl > imbalance" and
    > ultimately resulting in find_busiest_queue() return NULL, causing
    > load_balance() to think that the load is well balanced. But infact
    > one of the tasks can be moved to the idle core for optimal performance.
    >
    > We don't need to use the weighted load (wl) scaled by the cpu power to
    > compare with imabalance. In that condition, we already know there is only a
    > single task "rq->nr_running == 1" and the comparison between imbalance,
    > wl is to make sure that we select the correct priority thread which matches
    > imbalance. So we really need to compare the imabalnce with the original
    > weighted load of the cpu and not the scaled load.
    >
    > But in other conditions where we want the most hammered(busiest) cpu, we can
    > use scaled load to ensure that we consider the cpu power in addition to the
    > actual load on that cpu, so that we can move the load away from the
    > guy that is getting most hammered with respect to the actual capacity,
    > as compared with the rest of the cpu's in that busiest group.
    >
    > Fix it.
    >
    > Reported-by: Ma Ling <ling.ma@intel.com>
    > Initial-Analysis-by: Zhang, Yanmin <yanmin_zhang@linux.intel.com>
    > Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
    > Cc: stable@kernel.org [2.6.32.x]

    A reproduction case would have been nice, I've been playing with busy
    loops and plotting the cpus on paper, but I didn't manage to reproduce.

    Still, I went through the logic and it seems to make sense, so:

    Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

    Ingo, sed -e 's/sched\.c/sched_fair.c/g', makes it apply to tip/master
    and should provide means of solving the rebase/merge conflict.

    > ---
    >
    > diff --git a/kernel/sched.c b/kernel/sched.c
    > index 3a8fb30..bef5369 100644
    > --- a/kernel/sched.c
    > +++ b/kernel/sched.c
    > @@ -4119,12 +4119,23 @@ find_busiest_queue(struct sched_group *group, enum cpu_idle_type idle,
    > continue;
    >
    > rq = cpu_rq(i);
    > - wl = weighted_cpuload(i) * SCHED_LOAD_SCALE;
    > - wl /= power;
    > + wl = weighted_cpuload(i);
    >
    > + /*
    > + * When comparing with imbalance, use weighted_cpuload()
    > + * which is not scaled with the cpu power.
    > + */
    > if (capacity && rq->nr_running == 1 && wl > imbalance)
    > continue;
    >
    > + /*
    > + * For the load comparisons with the other cpu's, consider
    > + * the weighted_cpuload() scaled with the cpu power, so that
    > + * the load can be moved away from the cpu that is potentially
    > + * running at a lower capacity.
    > + */
    > + wl = (wl * SCHED_LOAD_SCALE) / power;
    > +
    > if (wl > max_load) {
    > max_load = wl;
    > busiest = rq;
    >
    >




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