linux-kernel - Re: [PATCH] MIPS: Loongson: Make default kernel log buffer size as 128KB for Loongson3

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Date:   Wed, 16 Oct 2019 10:23:10 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Paul Burton <paulburton89@...il.com>,
        Huacai Chen <chenhc@...ote.com>,
        Paul Burton <paul.burton@...s.com>
Cc:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: Loongson: Make default kernel log buffer size as
 128KB for Loongson3

On 10/16/2019 02:53 AM, Paul Burton wrote:
> Hi Tiezhu & Huacai,
>
> On Tue, Oct 15, 2019 at 12:00:25PM +0800, Tiezhu Yang wrote:
>> On 10/15/2019 11:36 AM, Huacai Chen wrote:
>>> On Tue, Oct 15, 2019 at 10:12 AM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>>>> When I update kernel with loongson3_defconfig based on the Loongson 3A3000
>>>> platform, then using dmesg command to show kernel ring buffer, the initial
>>>> kernel messages have disappeared due to the log buffer is too small, it is
>>>> better to change the default kernel log buffer size from 16KB to 128KB.
>>>>
>>>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>>>> ---
>>>>    arch/mips/configs/loongson3_defconfig | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/mips/configs/loongson3_defconfig b/arch/mips/configs/loongson3_defconfig
>>>> index 90ee008..3aa2201 100644
>>>> --- a/arch/mips/configs/loongson3_defconfig
>>>> +++ b/arch/mips/configs/loongson3_defconfig
>>>> @@ -12,7 +12,7 @@ CONFIG_TASKSTATS=y
>>>>    CONFIG_TASK_DELAY_ACCT=y
>>>>    CONFIG_TASK_XACCT=y
>>>>    CONFIG_TASK_IO_ACCOUNTING=y
>>>> -CONFIG_LOG_BUF_SHIFT=14
>>>> +CONFIG_LOG_BUF_SHIFT=17
>>> Hi, Tiezhu,
>>>
>>> Why you choose 128KB but not 64KB or 256KB? I found 64KB is enough for
>>> our cases. And if you really need more, I think 256KB could be better
>>> because there are many platforms choose 256KB.
>> Hi Huacai,
>>
>> Thanks for your reply and suggestion, I will send a v2 patch.
> Thanks for the patches.
>
> I actually have a slight preference for 128KB if you've no specific
> need, since 128KB is the default. Some quick grepping says that of 405
> defconfigs in tree (as of v5.4-rc3), we have:
>
>    LOG_BUF_SHIFT  Count
>               12  1
> 	     13  3
> 	     14  235
> 	     15  18
> 	     16  39
> 	     17  90
> 	     18  13
> 	     19  2
> 	     20  4
>
> ie. 16KiB is by far the most common, then second most common is the
> default 128KiB. 256KiB is comparatively rare.

Hi Paul,

Thank you very much for your detailed explanation. I think 128KB is 
enough to save
the boot messages because the dmesg output size is about 40KB after 
reboot in my
system.

>
> However, I don't think your v1 patch is quite right Tiezhu - since 17 is
> the default it shouldn't be specified in the defconfig at all. Did you
> manually make the change in the loongson3_defconfig file? If so please
> take a look at the savedefconfig make target & try something like this:
>
>    make ARCH=mips loongson3_defconfig
>    make ARCH=mips menuconfig
>    # Change LOG_BUF_SHIFT
>    make ARCH=mips savedefconfig
>    mv defconfig arch/mips/configs/loongson3_defconfig
>    git add -i arch/mips/configs/loongson3_defconfig
>    # Stage the relevant changes, drop the others
>
> You should end up with the CONFIG_LOG_BUF_SHIFT line just getting
> deleted.

Yes, you are right, I will make a new patch followed with your steps and 
send a v3
patch: just delete the CONFIG_LOG_BUF_SHIFT line to use the default 
value 128KB and
update the commit message.

>
> If on the other hand you really do prefer 256KiB for these systems
> please describe why in the commit message. It could be something as
> simple as "we have lots of memory so using 256KiB isn't a big deal, and
> gives us a better chance of preserving boot messages until they're
> examined". But if your log is getting this big before you look at it (or
> before something like systemd copies it into its journal), there's
> probably something fishy going on.

Hi Huacai,

What do you think? If you have any more suggestion, please let me know.

Thanks,

Tiezhu Yang

>
> Thanks,
>      Paul