linux-kernel - Re: [PATCH 2/2] More precise timestamps for nested writes

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Date:	Wed, 15 Apr 2015 11:16:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Suresh E. Warrier" <warrier@...ux.vnet.ibm.com>
Cc:	lklml <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 2/2] More precise timestamps for nested writes

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.
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