linux-kernel - Re: [PATCH 2/2] x86/e820: Use pr_debug to avoid spamming dmesg log with debug messages

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Date:   Wed, 5 May 2021 12:58:31 -0400
From:   Jason Baron <jbaron@...mai.com>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] x86/e820: Use pr_debug to avoid spamming dmesg log
 with debug messages



On 5/3/21 3:40 PM, Heiner Kallweit wrote:
> e820 emits quite some debug messages to the dmesg log. Let's restrict
> this to cases where the debug output is actually requested. Switch to
> pr_debug() for this purpose and make sure by checking the return code
> that pr_cont() is only called if applicable.
> 
> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
> ---
>  arch/x86/kernel/e820.c | 27 ++++++++++++++++-----------
>  1 file changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> index bc0657f0d..67ad4d8f0 100644
> --- a/arch/x86/kernel/e820.c
> +++ b/arch/x86/kernel/e820.c
> @@ -465,6 +465,7 @@ __e820__range_update(struct e820_table *table, u64 start, u64 size, enum e820_ty
>  	u64 end;
>  	unsigned int i;
>  	u64 real_updated_size = 0;
> +	int printed;
>  
>  	BUG_ON(old_type == new_type);
>  
> @@ -472,11 +473,13 @@ __e820__range_update(struct e820_table *table, u64 start, u64 size, enum e820_ty
>  		size = ULLONG_MAX - start;
>  
>  	end = start + size;
> -	printk(KERN_DEBUG "e820: update [mem %#010Lx-%#010Lx] ", start, end - 1);
> -	e820_print_type(old_type);
> -	pr_cont(" ==> ");
> -	e820_print_type(new_type);
> -	pr_cont("\n");
> +	printed = pr_debug("e820: update [mem %#010Lx-%#010Lx] ", start, end - 1);
> +	if (printed > 0) {
> +		e820_print_type(old_type);
> +		pr_cont(" ==> ");
> +		e820_print_type(new_type);
> +		pr_cont("\n");
> +	}


Hi Heiner,

We've been doing these like:

DEFINE_DYNAMIC_DEBUG_METADATA(e820_dbg, "e820 verbose mode");

.
.
.

if (DYNAMIC_DEBUG_BRANCH(e820_debg)) {
    printk(KERN_DEBUG "e820: update [mem %#010Lx-%#010Lx] ", start, end - 1);
    e820_print_type(old_type);
    pr_cont(" ==> ");
    e820_print_type(new_type);
    pr_cont("\n");
}


You could then have one DEFINE_DYNAMIC_DEBUG_METADATA statement - such that it enables
it all in one go, or do separate ones that enable it how you see fit.

Would that work here?

Thanks,

-Jason

>  
>  	for (i = 0; i < table->nr_entries; i++) {
>  		struct e820_entry *entry = &table->entries[i];
> @@ -540,7 +543,7 @@ static u64 __init e820__range_update_kexec(u64 start, u64 size, enum e820_type o
>  /* Remove a range of memory from the E820 table: */
>  u64 __init e820__range_remove(u64 start, u64 size, enum e820_type old_type, bool check_type)
>  {
> -	int i;
> +	int printed, i;
>  	u64 end;
>  	u64 real_removed_size = 0;
>  
> @@ -548,10 +551,12 @@ u64 __init e820__range_remove(u64 start, u64 size, enum e820_type old_type, bool
>  		size = ULLONG_MAX - start;
>  
>  	end = start + size;
> -	printk(KERN_DEBUG "e820: remove [mem %#010Lx-%#010Lx] ", start, end - 1);
> -	if (check_type)
> -		e820_print_type(old_type);
> -	pr_cont("\n");
> +	printed = pr_debug("e820: remove [mem %#010Lx-%#010Lx] ", start, end - 1);
> +	if (printed > 0) {
> +		if (check_type)
> +			e820_print_type(old_type);
> +		pr_cont("\n");
> +	}
>  
>  	for (i = 0; i < e820_table->nr_entries; i++) {
>  		struct e820_entry *entry = &e820_table->entries[i];
> @@ -1230,7 +1235,7 @@ void __init e820__reserve_resources_late(void)
>  		if (start >= end)
>  			continue;
>  
> -		printk(KERN_DEBUG "e820: reserve RAM buffer [mem %#010llx-%#010llx]\n", start, end);
> +		pr_debug("e820: reserve RAM buffer [mem %#010llx-%#010llx]\n", start, end);
>  		reserve_region_with_split(&iomem_resource, start, end, "RAM buffer");
>  	}
>  }
>