linux-kernel - Re: [tip:locking/core] locking/atomics: Check generated headers are up-to-date

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Date:   Wed, 21 Nov 2018 09:02:58 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     mark.rutland@....com, tglx@...utronix.de, hpa@...or.com,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        boqun.feng@...il.com, will.deacon@....com
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:locking/core] locking/atomics: Check generated headers are
 up-to-date


* tip-bot for Mark Rutland <tipbot@...or.com> wrote:

> Commit-ID:  8d32588077bdc390420cfa6946f407033a20d7a8
> Gitweb:     https://git.kernel.org/tip/8d32588077bdc390420cfa6946f407033a20d7a8
> Author:     Mark Rutland <mark.rutland@....com>
> AuthorDate: Tue, 4 Sep 2018 11:48:29 +0100
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Thu, 1 Nov 2018 11:01:10 +0100
> 
> locking/atomics: Check generated headers are up-to-date
> 
> Now that all the generated atomic headers are in place, it would be good
> to ensure that:
> 
> a) the headers are up-to-date when scripting changes.
> 
> b) developers don't directly modify the generated headers.
> 
> To ensure both of these properties, let's add a Kbuild step to check
> that the generated headers are up-to-date.
> 
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: catalin.marinas@....com
> Cc: Will Deacon <will.deacon@....com>
> Cc: linuxdrivers@...otech.com
> Cc: dvyukov@...gle.com
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: arnd@...db.de
> Cc: aryabinin@...tuozzo.com
> Cc: glider@...gle.com
> Link: http://lkml.kernel.org/r/20180904104830.2975-6-mark.rutland@arm.com
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  Kbuild                          | 18 ++++++++++++++++--
>  scripts/atomic/check-atomics.sh | 19 +++++++++++++++++++
>  2 files changed, 35 insertions(+), 2 deletions(-)

These scripts are *awfully* slow to be run at every kernel build - even a 
reasonably fast machine:

  model name	: Intel(R) Xeon(R) CPU E5-2680 v2 @ 2.80GHz

... they are adding 3-4 seconds to the build time:

[Before]:

  galatea:~/linux/linux> perf stat --null --repeat 3 make kernel/sched/core.o
  CALL    scripts/checksyscalls.sh
  DESCEND  objtool
  CALL    scripts/checksyscalls.sh
  DESCEND  objtool
  CALL    scripts/checksyscalls.sh
  DESCEND  objtool

 Performance counter stats for 'make kernel/sched/core.o' (3 runs):

          1.201874 +- 0.000371 seconds time elapsed  ( +-  0.03% )


[After]:

  galatea:~/linux/linux> perf stat --null --repeat 3 make kernel/sched/core.o

  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh
  DESCEND  objtool
  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh
  DESCEND  objtool
  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh
  DESCEND  objtool

 Performance counter stats for 'make kernel/sched/core.o' (3 runs):

            4.5987 +- 0.0109 seconds time elapsed  ( +-  0.24% )

Could we please get this fixed so that proper dependencies are checked 
and it's only regenerated when needed? This slowdown makes additive-build 
kernel development quite painful, as ~5 seconds is in the 'too long' 
category already, while 1.2 seconds is basically instantaneous.

I cannot even imagine the slowdown on a truly slow box where kernel 
development *has* to be additive.

Thanks,

	Ingo