linux-kernel - Re: [PATCH] Documentation: scheduler: fix outdated information on arch SD flags and sched

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Date:   Thu, 12 Nov 2020 12:50:05 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Barry Song <song.bao.hua@...ilicon.com>
Cc:     corbet@....net, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>, Adrian Freund <adrian@...und.io>
Subject: Re: [PATCH] Documentation: scheduler: fix outdated information on arch SD flags and sched_domain


Hi,

On 12/11/20 11:34, Barry Song wrote:
> This document seems to be out of date for many, many years. Even it has
> misspelled from the first day.
> ARCH_HASH_SCHED_TUNE should be ARCH_HAS_SCHED_TUNE
> ARCH_HASH_SCHED_DOMAIN should be ARCH_HAS_SCHED_DOMAIN
>
> But since v2.6.14, kernel completely deleted the relevant code and even
> arch_init_sched_domains() was deleted.
>
> Right now, kernel is asking architectures to call set_sched_topology() to
> override the default sched domains.
>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Adrian Freund <adrian@...und.io>
> Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
> ---
>  Documentation/scheduler/sched-domains.rst | 12 +++---------
>  1 file changed, 3 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/scheduler/sched-domains.rst b/Documentation/scheduler/sched-domains.rst
> index 5c4b7f4f0062..434d4e7e86c5 100644
> --- a/Documentation/scheduler/sched-domains.rst
> +++ b/Documentation/scheduler/sched-domains.rst
> @@ -69,15 +69,9 @@ The implementor should read comments in include/linux/sched.h:
>  struct sched_domain fields, SD_FLAG_*, SD_*_INIT to get an idea of
>  the specifics and what to tune.
>
> -Architectures may retain the regular override the default SD_*_INIT flags
> -while using the generic domain builder in kernel/sched/core.c if they wish to
> -retain the traditional SMT->SMP->NUMA topology (or some subset of that). This
> -can be done by #define'ing ARCH_HASH_SCHED_TUNE.
> -
> -Alternatively, the architecture may completely override the generic domain
> -builder by #define'ing ARCH_HASH_SCHED_DOMAIN, and exporting your
> -arch_init_sched_domains function. This function will attach domains to all
> -CPUs using cpu_attach_domain.
> +Architectures may override the generic domain builder and the default
> +SD_*_INIT flags by define'ing an array of sched_domain_topology_level and

That's way before my time, but IIRC those macros are also gone since

  143e1e28cb40 ("sched: Rework sched_domain topology definition")

The only way to override flags for a given topology level is to create a
new topology_level array and pass it through set_sched_topology() - reason
being you can't redefine cpu_{smt, core, *}_flags().

> +calling set_sched_topology() with this array as the parameter.
>

>  The sched-domains debugging infrastructure can be enabled by enabling
>  CONFIG_SCHED_DEBUG. This enables an error checking parse of the sched domains

This part about debug bits is also somewhat dusty. Mind adding the
following to your patch?

@Peter: would you object to getting rid of that sched_debug thing and
always do the sched_domain_debug() faff whenever we have
CONFIG_SCHED_DEBUG? Maybe keep sched_debug for the NUMA sort, but even then
that already requires having CONFIG_SCHED_DEBUG...

---
diff --git a/Documentation/scheduler/sched-domains.rst b/Documentation/scheduler/sched-domains.rst
index 434d4e7e86c5..0ebf8afb24ae 100644
--- a/Documentation/scheduler/sched-domains.rst
+++ b/Documentation/scheduler/sched-domains.rst
@@ -74,6 +74,8 @@ SD_*_INIT flags by define'ing an array of sched_domain_topology_level and
 calling set_sched_topology() with this array as the parameter.

 The sched-domains debugging infrastructure can be enabled by enabling
-CONFIG_SCHED_DEBUG. This enables an error checking parse of the sched domains
-which should catch most possible errors (described above). It also prints out
-the domain structure in a visual format.
+CONFIG_SCHED_DEBUG and adding 'sched_debug' to your cmdline. If you forgot to
+tweak your cmdline, you can also flip the /sys/kernel/debug/sched_debug
+knob. This enables an error checking parse of the sched domains which should
+catch most possible errors (described above). It also prints out the domain
+structure in a visual format.