linux-kernel - Re: [RFC/PATCH] init/main.c: Simplify initcall

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Message-ID: <20150119191924.GA19153@redhat.com>
Date:	Mon, 19 Jan 2015 20:19:25 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Fabian Frederick <fabf@...net.be>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC/PATCH] init/main.c: Simplify initcall_blacklisted()

On 01/17, Rasmus Villemoes wrote:
>
> Using kasprintf to get the function name makes us look up the name
> twice, along with all the vsnprintf overhead of parsing the format
> string etc. It also means there is an allocation failure case to deal
> with. Since symbol_string in vsprintf.c would anyway allocate an array
> of size KSYM_SYMBOL_LEN on the stack, that might as well be done up
> here.
> 
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
> 
> Notes:
>     I don't know how expensive it is to do the symbol lookup for each
>     initcall. It might be worthwhile adding an
>     
>       if (list_empty(&blacklisted_initcalls))
>         return false;
>     
>     at the very beginning of initcall_blacklisted(), since this is a debug
>     feature and the blacklist is indeed usually empty.

If we want to optimize this... I am wondering if we can change
initcall_blacklist()

	-	entry->buf = alloc_bootmem(strlen(str_entry) + 1);
	+	ebtry->fn = kallsyms_lookup_name(str_entry);

and then change initcall_blacklisted() to just compare the pointers.

Oleg.

>  init/main.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/init/main.c b/init/main.c
> index 61b993767db5..286602b677de 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -733,22 +733,18 @@ static bool __init_or_module initcall_blacklisted(initcall_t fn)
>  {
>  	struct list_head *tmp;
>  	struct blacklist_entry *entry;
> -	char *fn_name;
> +	char fn_name[KSYM_SYMBOL_LEN];
>  
> -	fn_name = kasprintf(GFP_KERNEL, "%pf", fn);
> -	if (!fn_name)
> -		return false;
> +	sprint_symbol_no_offset(fn_name, (unsigned long)fn);
>  
>  	list_for_each(tmp, &blacklisted_initcalls) {
>  		entry = list_entry(tmp, struct blacklist_entry, next);
>  		if (!strcmp(fn_name, entry->buf)) {
>  			pr_debug("initcall %s blacklisted\n", fn_name);
> -			kfree(fn_name);
>  			return true;
>  		}
>  	}
>  
> -	kfree(fn_name);
>  	return false;
>  }
>  #else
> -- 
> 2.1.3
> 

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