Date:   Mon, 13 Dec 2021 09:58:44 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH -V2] numa balancing: move some document to make it
 consistent with the code

Valentin Schneider <valentin.schneider@....com> writes:

> On 09/12/21 08:44, Huang Ying wrote:
>> After commit 8a99b6833c88 ("sched: Move SCHED_DEBUG sysctl to
>> debugfs"), some NUMA balancing sysctls enclosed with SCHED_DEBUG has
>> been moved to debugfs.  This patch move the document for these
>> sysctls from
>>
>>   Documentation/admin-guide/sysctl/kernel.rst
>>
>> to
>>
>>   Documentation/scheduler/debug.txt
>>
>
> AFAIA new documentation files should be written in reST, and the "source"
> file is .rst so the new one should be too (as much as Peter hates it).
>
> Also, most files in there are named sched-*.rst, does that want to be
> sched-debug.rst ?

OK.  Will do that.

>> to make the document consistent with the code.
>>
>> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
>> Fixes: 8a99b6833c88 ("sched: Move SCHED_DEBUG sysctl to debugfs")
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Cc: Valentin Schneider <valentin.schneider@....com>
>> Cc: stable@...r.kernel.org # since v5.13
>
>> diff --git a/Documentation/scheduler/debug.txt b/Documentation/scheduler/debug.txt
>> new file mode 100644
>> index 000000000000..848d83c3123c
>> --- /dev/null
>> +++ b/Documentation/scheduler/debug.txt
>> @@ -0,0 +1,48 @@
>> +Scheduler debugfs
>> +
>
> How about a small intro?
>
> ---
> diff --git a/Documentation/scheduler/debug.txt b/Documentation/scheduler/debug.txt
> index 848d83c3123c..08600de5b90e 100644
> --- a/Documentation/scheduler/debug.txt
> +++ b/Documentation/scheduler/debug.txt
> @@ -1,4 +1,10 @@
> +=================
>  Scheduler debugfs
> +=================
> +
> +Booting a kernel with CONFIG_SCHED_DEBUG=y will give access to scheduler
> +-specific debug files under /sys/kernel/debug/sched. Some of those files are
> +described below.
>  
>  numa_balancing
>  --------------
> ---
>
>> +numa_balancing
>> +--------------
>
> I think you got the heading ordering wrong, see
>   Documentation/doc-guide/sphinx.rst#Specific guidelines for the kernel documentation
>
> IIRC Sphinx/reST only requires heading ordering to be consistent within a
> given file, but having consistency throughout the project simplifies
> reviewing/contributing. In this case, headings with "=" must appear before
> headings with "-".

Thanks for reminding.  Will change it in the next version.

Best Regards,
Huang, Ying

>> +
>> +`numa_balancing` directory is used to hold files to control NUMA
>> +balancing feature.  If the system overhead from the feature is too
>> +high then the rate the kernel samples for NUMA hinting faults may be
>> +controlled by the `scan_period_min_ms, scan_delay_ms,
>> +scan_period_max_ms, scan_size_mb` files.
>> +
>> +
>> +scan_period_min_ms, scan_delay_ms, scan_period_max_ms, scan_size_mb
>> +===================================================================
>> +
>> +Automatic NUMA balancing scans tasks address space and unmaps pages to
>> +detect if pages are properly placed or if the data should be migrated to a
>> +memory node local to where the task is running.  Every "scan delay" the task
>> +scans the next "scan size" number of pages in its address space. When the
>> +end of the address space is reached the scanner restarts from the beginning.
>> +
>> +In combination, the "scan delay" and "scan size" determine the scan rate.
>> +When "scan delay" decreases, the scan rate increases.  The scan delay and
>> +hence the scan rate of every task is adaptive and depends on historical
>> +behaviour. If pages are properly placed then the scan delay increases,
>> +otherwise the scan delay decreases.  The "scan size" is not adaptive but
>> +the higher the "scan size", the higher the scan rate.
>> +
>> +Higher scan rates incur higher system overhead as page faults must be
>> +trapped and potentially data must be migrated. However, the higher the scan
>> +rate, the more quickly a tasks memory is migrated to a local node if the
>> +workload pattern changes and minimises performance impact due to remote
>> +memory accesses. These files control the thresholds for scan delays and
>> +the number of pages scanned.
>> +
>> +``scan_period_min_ms`` is the minimum time in milliseconds to scan a
>> +tasks virtual memory. It effectively controls the maximum scanning
>> +rate for each task.
>> +
>> +``scan_delay_ms`` is the starting "scan delay" used for a task when it
>> +initially forks.
>> +
>> +``scan_period_max_ms`` is the maximum time in milliseconds to scan a
>> +tasks virtual memory. It effectively controls the minimum scanning
>> +rate for each task.
>> +
>> +``scan_size_mb`` is how many megabytes worth of pages are scanned for
>> +a given scan.
>> --
>> 2.30.2

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