linux-kernel - Re: [PATCH RFC] remove jump_label optimization for perf sched events

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Date:	Thu, 24 Nov 2011 14:23:00 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	Jason Baron <jbaron@...hat.com>, rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC] remove jump_label optimization for perf sched events

On Mon, 2011-11-21 at 15:17 +0200, Gleb Natapov wrote:
> So how about throttling it like the patch below does until stop_machine()
> no longer needed for patching (and it is possible that new way of patching
> will still have significant overhead).
> 
> Signed-off-by: Gleb Natapov <gleb@...hat.com>
> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
> index 66f23dc..a4687f6 100644
> --- a/include/linux/jump_label.h
> +++ b/include/linux/jump_label.h
> @@ -3,12 +3,15 @@
>  
>  #include <linux/types.h>
>  #include <linux/compiler.h>
> +#include <linux/workqueue.h>
>  
>  #if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)
>  
>  struct jump_label_key {
>         atomic_t enabled;
>         struct jump_entry *entries;
> +       unsigned long rl_timeout;
> +       struct delayed_work rl_work;
>  #ifdef CONFIG_MODULES
>         struct jump_label_mod *next;


I'm not sure its worth it doing in generic code like this, it bloats the
jump_label_key structure quite significantly (and there's tons of those
around) for only 1 real user.

If we want to do this in generic code, I think its best to introduce
something like:

struct jump_label_key_deferred {
	struct jump_label_key key;
	unsigned long timeout;
	struct delayed_work work;
}

But is there really any other user for this? All the trace bits are root
only iirc and kvm itself only sets them on the guest kernel I think for
paravirt, so that's not a problem.


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