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SubjectRe: [patch] sched: fix SMT scheduler regression in find_busiest_queue()
* Suresh Siddha <suresh.b.siddha@intel.com> [2010-02-12 17:14:22]:

> PeterZ/Ingo,
>
> Ling Ma and Yanmin reported this SMT scheduler regression which lead to
> the condition where both the SMT threads on a core are busy while the
> other cores in the same socket are completely idle, causing major
> performance regression. I have appended a fix for this. This is
> relatively low risk fix and if you agree with both the fix and
> risk-assessment, can we please push this to Linus so that we can address
> this in 2.6.33.

Hi Suresh,

I have been looking at this issue in order to make
sched_smt_powersavings work. In my simple tests I find that the
default behavior is to have one task per core first since the total
cpu power of the core will be 1178 (589*2) that is not sufficient to
keep two tasks balanced in the group.

In the scenario that you have described, even though the group has
been identified as busiest, the find_busiest_queue() will return null
since wl will be 1780 {load(1024)*SCHED_LOAD_SCALE/power(589)} leading
to wl being greater than imbalance.
The fix that you have posted will solve the problem described.
However we need to make sched_smt_powersavings also work by increasing
the group capacity and allowing two tasks to run in a core.

As Peter mentioned, SD_PREFER_SIBLING flag is meant to spread the work
across group at any sched domain so that the solution will work for
pre-Nehalem quad cores also. But it still needs some work to get it
right. Please refer to my earlier bug report at:

http://lkml.org/lkml/2010/2/8/80

The solution you have posted will not work for non-HT quad cores where
we want the tasks to be spread across cache domains for best
performance though not a severe performance regression as in the case
of Nehalem.

I will test your solution in different scenarios and post updates.

Thanks,
Vaidy


> thanks,
> suresh
> ---
>
> 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]
> ---
>
> 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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