linux-kernel - Re: [PATCH] kernel/sched/topology: Clarify root domain(s) debug string

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Date:   Sun, 27 May 2018 18:04:25 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     peterz@...radead.org, mingo@...hat.com,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH] kernel/sched/topology: Clarify root domain(s) debug
 string

On Thu, May 24, 2018 at 05:29:36PM +0200, Juri Lelli wrote:
> When scheduler debug is enabled, building scheduling domains outputs
> information about how the domains are laid out and to which root domain
> each CPU (or sets of CPUs) belongs, e.g.:
> 
>  CPU0 attaching sched-domain(s):
>   domain-0: span=0-5 level=MC
>    groups: 0:{ span=0 }, 1:{ span=1 }, 2:{ span=2 }, 3:{ span=3 }, 4:{ span=4 }, 5:{ span=5 }
>  CPU1 attaching sched-domain(s):
>   domain-0: span=0-5 level=MC
>    groups: 1:{ span=1 }, 2:{ span=2 }, 3:{ span=3 }, 4:{ span=4 }, 5:{ span=5 }, 0:{ span=0 }
> 
>  [...]
> 
>  span: 0-5 (max cpu_capacity = 1024)
> 
> The fact that latest line refers to CPUs 0-5 root domain doesn't however look

                last line?

> immediately obvious to me: one might wonder why span 0-5 is reported "again".
> 
> Make it more clear by adding "root domain" to it, as to end with the
> following.
> 
>  CPU0 attaching sched-domain(s):
>   domain-0: span=0-5 level=MC
>    groups: 0:{ span=0 }, 1:{ span=1 }, 2:{ span=2 }, 3:{ span=3 }, 4:{ span=4 }, 5:{ span=5 }
>  CPU1 attaching sched-domain(s):
>   domain-0: span=0-5 level=MC
>    groups: 1:{ span=1 }, 2:{ span=2 }, 3:{ span=3 }, 4:{ span=4 }, 5:{ span=5 }, 0:{ span=0 }
> 
>  [...]
> 
>  root domain span: 0-5 (max cpu_capacity = 1024)
> 
> Signed-off-by: Juri Lelli <juri.lelli@...hat.com>

I played the sched_load_balance flag to trigger this and it makes sense to
improve the print with 'root domain'.

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

One thing I believe is a bit weird is sched_load_balance also can affect the
wake-up path, because a NULL sd is attached to the rq if sched_load_balance
is set to 0.

This could turn off the "for_each_domain(cpu, tmp)" loop in
select_task_rq_fair and hence we would always end up in the
select_idle_sibling path for those CPUs.

It also means that "XXX always" can/should be removed because sd can very
well be NULL for other sd_flag types as well, not just sd_flag ==
SD_BALANCE_WAKE. I'll send a patch to remove that comment as I just tested
this is true.

thanks,

 - Joel