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Subject: Re: [PATCH] lib/Kconfig.debug: add a comment to PROVE_LOCKING impact
To:     Lukasz Luba <l.luba@partner.samsung.com>,
        linux-kernel@vger.kernel.org
Cc:     b.zolnierkie@samsung.com, mingo@redhat.com
References: <CGME20181009153959eucas1p1ab3c10ba4868a4e97a1bff24d8f34c41@eucas1p1.samsung.com>
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <c328b3c5-ef09-5cf7-80f5-4b6a8cc9a553@redhat.com>
Date:   Tue, 9 Oct 2018 13:29:35 -0400
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On 10/09/2018 11:39 AM, Lukasz Luba wrote:
> This patch add some comment related to performance impact,
> which can be really big (x3 times slower context switch).
>
> Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
> ---
>  lib/Kconfig.debug | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 4966c4f..9e67a2a3 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1088,6 +1088,9 @@ config PROVE_LOCKING
>  	 the proof of observed correctness is also maintained for an
>  	 arbitrary combination of these separate locking variants.
>  
> +	 This feature enables LOCKDEP which can harm system performance
> +	 even x3 times.
> +
>  	 For more details, see Documentation/locking/lockdep-design.txt.
>  
>  config LOCK_STAT
> @@ -1112,6 +1115,10 @@ config LOCK_STAT
>  	 CONFIG_LOCK_STAT defines "contended" and "acquired" lock events.
>  	 (CONFIG_LOCKDEP defines "acquire" and "release" events.)
>  
> +	 This feature enables LOCKDEP which can harm system performance
> +	 even x3 times.
> +	 For more details, see Documentation/locking/lockdep-design.txt.
> +
>  config DEBUG_RT_MUTEXES
>  	bool "RT Mutex debugging, deadlock detection"
>  	depends on DEBUG_KERNEL && RT_MUTEXES
> @@ -1175,6 +1182,10 @@ config DEBUG_LOCK_ALLOC
>  	 spin_lock_init()/mutex_init()/etc., or whether there is any lock
>  	 held during task exit.
>  
> +	 This feature enables LOCKDEP which can harm system performance
> +	 even x3 times.
> +	 For more details, see Documentation/locking/lockdep-design.txt.
> +
>  config LOCKDEP
>  	bool
>  	depends on DEBUG_KERNEL && LOCK_DEBUGGING_SUPPORT

I don't mind seeing warning message about the performance impact of
enabling lockdep. However, the actual slowdown depends on many factors
including the type of systems, workloads and so on. I am a bit hesitant
to quote an actual figure like x3 in the help text as it may not be
correct in many cases.

BTW, my recent lockdep patch (https://lkml.org/lkml/2018/10/2/1410)
should be able to reduce the performance impact of enabling lockdep.
Could you try to see if that help in your case?

Cheers,
Longman