Re: [Buildroot] [PATCH] linux/linux.mk: forcibly (re)enable Make jobserver

On 7/30/26 20:14, Arnout Vandecappelle wrote:
>
>
> On 30/07/2026 20:06, Laszlo Ersek wrote:
>> Commit 0b9efc991f8a («linux: use BR2_MAKE», 2023-04-10) replaced
>> $(MAKE) with $(BR2_MAKE) in a number of recipes. As a consequence,
>> the child make is unable to discover the job server, and we get a
>> warning such as
>>
>>> warning: jobserver unavailable: using -j1. Add `+' to parent make rule.
>>
>> https://www.gnu.org/software/make/manual/html_node/Error-Messages.html
>> https://www.gnu.org/software/make/manual/html_node/MAKE-Variable.html
>>
>> Inform the parent Make, by prefixing the recipe with "+", that
>> $(BR2_MAKE) can deal with the job server.
>>
>> Cc: Arnout Vandecappelle <[email protected]>
>> Cc: Oleg Lyovin <[email protected]>
>> Cc: [email protected]
>> Signed-off-by: Laszlo Ersek <[email protected]>
>> ---
>>
>> Notes:
>>      Commit 0b9efc991f8a replaces multiple instances of $(MAKE) with
>>      $(BR2_MAKE); I'm only prefixing the one recipe with "+" that
>>      seems to cause the most noticeable slowdown for me (by missing
>>      out on a parallel kernel build).
>
>   Are you really sure this speeds things up?

Yes.

> The core of the work should be in the `make all` invocation on line
> 546. All the other invocations should be pretty much serial. In
> particular the rebuild-with-initramfs should do nothing more than
> re-linking and re-compressing the kernel because the exact same
> command was already done in the build step.

In my case,

- both "LINUX_BUILD_CMDS" -- the "all" target --,

- and "linux-rebuild-with-initramfs" -- the $(LINUX_TARGET_NAME), aka
   "Image" target --

seem to perform a similarly large [1] kernel build. The first one builds
object files concurrently already, the second one (without this patch)
doesn't.

[1] The "Image" build (from "linux-rebuild-with-initramfs") seems not to
execute the following types of recipes from the "all" target (from
"LINUX_BUILD_CMDS"):

- CC [M]
- LD [M]
- CHKSHA1 on various headers
- COPY on various headers
- DTC on various device tree files
- GZIP on Image.gz
- UPD and WRAP on various generated header files

I think these differences are expected. However, while
"linux-rebuild-with-initramfs" performs fewer build steps, according to
the above, it does build all the built-in object files anew, and that
happens way more slowly than in "LINUX_BUILD_CMDS", because
"linux-rebuild-with-initramfs" doesn't do it concurrently.

(FWIW, I'm based off of release 2025.05.)

Laszlo
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