Re: [PATCH 2.6.12-rc5-mm2] [sched] add allowed CPUs check into find_idlest_group()

[Nick Piggin <nickpiggin@yahoo.com.au>]

M.Baris Demiray wrote:
> Hello,
> following patch adds check for allowed CPUs into
> sched.c:find_idlest_group() -as told in comment line that had
> removed-. But, I have several questions about that comment.
>
> Firstly, I've understood it as "Check whether process p is allowed to
> run on each CPU of to-be-found idlest group"; is that right?

Close.

Probably it would be better to take the intersection of
(group->cpumask, p->cpus_allowed), and skip the group if
the intersection is empty.

In addition to that, do a patch for find_idlest_cpu that
skips cpus that aren't allowed. You needn't do the cpumask
check each time round the loop, again just take the
intersection of the group->cpumask and p->cpus_allowed, and
loop over that.

Wanna do a patch for that?


> If so, isn't it more appropriate to do check in find_idlest_cpu()?
> Because, we're only interested in CPUs that are in idlest group
> but doing a check in find_idlest_group() also checks for CPUs
> that are not in idlest group (since we're traversing all the groups
> in given domain). Checking this after finding the idlest group
> (in find_idlest_cpu() with ordinary call order as in
> sched_balance_self()) will save us from extra overhead.
>
> Although I've questions in my mind, I'm sending a patch following
> that comment. Any explanation and comment on patch will be
> appreciated.

I don't think it does anything ;)

> Regards.
>
> Signed-off-by: M.Baris Demiray <baris@labristeknoloji.com>
>
> --- linux-2.6.12-rc5-mm2/kernel/sched.c.orig    2005-06-05 
> 12:31:04.000000000 +0000
> +++ linux-2.6.12-rc5-mm2/kernel/sched.c    2005-06-05 16:49:49.000000000 
> +0000
> @@ -1040,7 +1040,12 @@
>          int i;
>
>          local_group = cpu_isset(this_cpu, group->cpumask);
> -        /* XXX: put a cpus allowed check */
> +
> +        /* Check whether all CPUs in the group is allowed to run on */
> +        for_each_cpu_mask(i, group->cpumask) {
> +            if (!cpu_isset(i, p->cpus_allowed))
> +                continue;
> +        }
>
>          /* Tally up the load of all CPUs in the group */
>          avg_load = 0;


--
SUSE Labs, Novell Inc.

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