[PATCH] kernel: disable CONFIG_RCU_EXPERT and friends

[Hauke Mehrtens]

On 8/22/21 10:01 AM, Rui Salvaterra wrote:
> Based on the existing documentation [1][2], I dare anyone to demonstrate that
> we need to fine-tune these RCU parameters. The (performance) breakage potential
> for doing so is immense, so let's just please put down this loaded footgun.
> Disable CONFIG_RCU_EXPERT and dependent symbols.
> 
> [1] https://www.kernel.org/doc/Documentation/RCU/Design/Data-Structures/Data-Structures.html
> [2] https://lwn.net/Articles/777214/
> 
> Signed-off-by: Rui Salvaterra <rsalvaterra at gmail.com>

There are also some targets which explicitly unset CONFIG_RCU_EXPERT:
$ git grep CONFIG_RCU_EXPERT
target/linux/gemini/config-5.10:# CONFIG_RCU_EXPERT is not set
target/linux/gemini/config-5.4:# CONFIG_RCU_EXPERT is not set
target/linux/generic/config-5.10:CONFIG_RCU_EXPERT=y
target/linux/generic/config-5.4:CONFIG_RCU_EXPERT=y
target/linux/ipq807x/config-default:# CONFIG_RCU_EXPERT is not set
target/linux/layerscape/armv7/config-5.4:# CONFIG_RCU_EXPERT is not set
target/linux/layerscape/armv8_64b/config-5.4:# CONFIG_RCU_EXPERT is not set
target/linux/mediatek/mt7623/config-5.10:# CONFIG_RCU_EXPERT is not set
target/linux/oxnas/ox810se/config-default:# CONFIG_RCU_EXPERT is not set
target/linux/oxnas/ox820/config-default:# CONFIG_RCU_EXPERT is not set
target/linux/rockchip/armv8/config-5.10:# CONFIG_RCU_EXPERT is not set
target/linux/rockchip/armv8/config-5.4:# CONFIG_RCU_EXPERT is not set
target/linux/tegra/config-5.10:# CONFIG_RCU_EXPERT is not set
target/linux/tegra/config-5.4:# CONFIG_RCU_EXPERT is not set

This should be removed from the target config file.

CONFIG_RCU_STALL_COMMON=y and CONFIG_RCU_NEED_SEGCBLIST=y are also set 
in most target configurations. Should we move them to the generic 
configuration?

Hauke


> ---
>   target/linux/generic/config-5.10 | 7 +------
>   target/linux/generic/config-5.4  | 6 +-----
>   2 files changed, 2 insertions(+), 11 deletions(-)
> 
> diff --git a/target/linux/generic/config-5.10 b/target/linux/generic/config-5.10
> index f944f1718f..91f137fbc3 100644
> --- a/target/linux/generic/config-5.10
> +++ b/target/linux/generic/config-5.10
> @@ -4631,12 +4631,8 @@ CONFIG_PWRSEQ_SIMPLE=y
>   CONFIG_RCU_CPU_STALL_TIMEOUT=60
>   # CONFIG_RCU_EQS_DEBUG is not set
>   # CONFIG_RCU_EXPEDITE_BOOT is not set
> -CONFIG_RCU_EXPERT=y
> -CONFIG_RCU_FANOUT=32
> -CONFIG_RCU_FANOUT_LEAF=16
> -# CONFIG_RCU_FAST_NO_HZ is not set
> +# CONFIG_RCU_EXPERT is not set
>   CONFIG_RCU_KTHREAD_PRIO=0
> -# CONFIG_RCU_NOCB_CPU is not set
>   # CONFIG_RCU_PERF_TEST is not set
>   # CONFIG_RCU_REF_SCALE_TEST is not set
>   # CONFIG_RCU_SCALE_TEST is not set
> @@ -5995,7 +5991,6 @@ CONFIG_SYSVIPC_SYSCTL=y
>   # CONFIG_TARGET_CORE is not set
>   # CONFIG_TASKSTATS is not set
>   # CONFIG_TASKS_RCU is not set
> -CONFIG_TASKS_TRACE_RCU_READ_MB=y
>   # CONFIG_TASK_XACCT is not set
>   # CONFIG_TC35815 is not set
>   # CONFIG_TCG_ATMEL is not set
> diff --git a/target/linux/generic/config-5.4 b/target/linux/generic/config-5.4
> index 45d8f0976e..06f5e43a6d 100644
> --- a/target/linux/generic/config-5.4
> +++ b/target/linux/generic/config-5.4
> @@ -4262,12 +4262,8 @@ CONFIG_PWRSEQ_SIMPLE=y
>   CONFIG_RCU_CPU_STALL_TIMEOUT=60
>   # CONFIG_RCU_EQS_DEBUG is not set
>   # CONFIG_RCU_EXPEDITE_BOOT is not set
> -CONFIG_RCU_EXPERT=y
> -CONFIG_RCU_FANOUT=32
> -CONFIG_RCU_FANOUT_LEAF=16
> -# CONFIG_RCU_FAST_NO_HZ is not set
> +# CONFIG_RCU_EXPERT is not set
>   CONFIG_RCU_KTHREAD_PRIO=0
> -# CONFIG_RCU_NOCB_CPU is not set
>   # CONFIG_RCU_PERF_TEST is not set
>   # CONFIG_RCU_TORTURE_TEST is not set
>   CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY=3
> 

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