Date:   Sat, 14 Nov 2020 00:50:18 +1300
From:   Barry Song <song.bao.hua@...ilicon.com>
To:     <corbet@....net>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC:     <linuxarm@...wei.com>, Barry Song <song.bao.hua@...ilicon.com>,
        "Ingo Molnar" <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>, Adrian Freund <adrian@...und.io>,
        Valentin Schneider <valentin.schneider@....com>
Subject: [PATCH v3] Documentation: scheduler: fix information on arch SD flags, sched_domain and sched_debug

This document seems to be out of date for many, many years.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

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.

On the other hand, to print the schedule debug information, users need to
set sched_debug cmdline or enable it by sysfs entry. So this patch also
adds the description for sched_debug.

Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Adrian Freund <adrian@...und.io>
Reviewed-by: Valentin Schneider <valentin.schneider@....com>
Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
---
 -v3:
 * add Reviewed-by of Valentin Schneider
 * add description of sched_debug

 Documentation/scheduler/sched-domains.rst | 26 ++++++++++-------------
 1 file changed, 11 insertions(+), 15 deletions(-)

diff --git a/Documentation/scheduler/sched-domains.rst b/Documentation/scheduler/sched-domains.rst
index 5c4b7f4f0062..8582fa5e9170 100644
--- a/Documentation/scheduler/sched-domains.rst
+++ b/Documentation/scheduler/sched-domains.rst
@@ -65,21 +65,17 @@ of the SMP domain will span the entire machine, with each group having the
 cpumask of a node. Or, you could do multi-level NUMA or Opteron, for example,
 might have just one domain covering its one NUMA level.
 
-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.
+The implementor should read comments in include/linux/sched/sd_flags.h:
+SD_* to get an idea of the specifics and what to tune for the SD flags
+of a sched_domain.
 
-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 flags
+for a given topology level by creating a sched_domain_topology_level array 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.
-- 
2.25.1

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