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Date: Wed, 15 Apr 2015 11:16:26 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Suresh E. Warrier" <warrier@linux.vnet.ibm.com>
Cc: lklml <linux-kernel@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@redhat.com>, Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH 2/2] More precise timestamps for nested writes
Message-ID: <20150415091626.GF17717@twins.programming.kicks-ass.net>
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On Mon, Apr 13, 2015 at 09:38:01PM -0500, Suresh E. Warrier wrote:

> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index c9b3005..0a2d862 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -482,6 +482,7 @@ struct ring_buffer_per_cpu {
>  	unsigned long			read_bytes;
>  	u64				write_stamp;
>  	u64				read_stamp;
> +	u64				last_stamp;
>  	/* ring buffer pages to update, > 0 to add, < 0 to remove */
>  	int				nr_pages_to_update;
>  	struct list_head		new_pages; /* new pages to add */

So what is wrong with something like:

u64 rb_get_delta(struct ring_buffer_per_cpu *cpu_buffer)
{
	u64 last, now;

	for (;;) {
		last = cpu_buffer->last_stamp;
		now = cpu_buffer->buffer->clock();

		if (cmpxchg_local(&cpu_buffer->last_stamp, last, now) == last)
			break;

		cpu_relax();
	}

	return now - last;
}

Of course, LL/SC on power stinks, but on most archs this is actually
fairly fast.