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Date: Mon, 7 Aug 2017 19:18:28 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Prarit Bhargava <prarit@redhat.com>
Cc: linux-kernel@vger.kernel.org, Mark Salyzyn <salyzyn@android.com>,
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Subject: Re: [PATCH v4] printk: Add monotonic, boottime, and realtime
 timestamps
Message-ID: <20170807171828.w7cefxruiljzlicd@hirez.programming.kicks-ass.net>
References: <1502121162-27981-1-git-send-email-prarit@redhat.com>
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On Mon, Aug 07, 2017 at 11:52:42AM -0400, Prarit Bhargava wrote:
> +static u64 printk_get_ts(void)
> +{
> +	u64 mono, offset_real;
> +
> +	if (printk_time <= PRINTK_TIME_LOCAL)
> +		return local_clock();
> +
> +	if (printk_time == PRINTK_TIME_BOOT)
> +		return ktime_get_boot_log_ts();
> +
> +	mono = ktime_get_real_log_ts(&offset_real);
> +
> +	if (printk_time == PRINTK_TIME_MONO)
> +		return mono;
> +
> +	return mono + offset_real;
> +}
>  
>  static size_t print_time(u64 ts, char *buf)
>  {
> @@ -1643,7 +1750,7 @@ static bool cont_add(int facility, int level, enum log_flags flags, const char *
>  		cont.facility = facility;
>  		cont.level = level;
>  		cont.owner = current;
> -		cont.ts_nsec = local_clock();
> +		cont.ts_nsec = printk_get_ts();
>  		cont.flags = flags;
>  	}
>  

So you really want to do all those branches every single time you
printk() ? I know printk() is somewhat of a slow path, but shees that's
ugly.

Why not have that setup function of yours set a function pointer?