linux-kernel - Re: [RFC PATCH] Introduce timekeeper latch synchronization

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Date:	Fri, 13 Sep 2013 18:13:22 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	John Stultz <john.stultz@...aro.org>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	lttng-dev@...ts.lttng.org
Subject: Re: [RFC PATCH] Introduce timekeeper latch synchronization

On Thu, 12 Sep 2013, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> [...]
> > Yep, that's good. I suppose if there's multiple use sites we can jump
> > through another few hoops to get rid of the specific struct foo
> > assumptions by storing sizeof() whatever we do use and playing pointer
> > math games.
> > 
> > But for now with the time stuff as only user this looks ok.
> 
> OK! Here is the full implementation of the idea against Linux
> timekeeper, ntp, and PPS. It appears that ntp and PPS were relying on
> the timekeeper seqlock too. And guess what, after booting my laptop with
> this kernel there still no smoke coming out of it after a good 5 minutes
> of testing. ;-)
> 
> Comments are welcome.

First of all this needs to be split into several patches.

>  static void update_wall_time(void)
 ...
> -	write_seqcount_begin(&timekeeper_seq);
>  	/* Update clock->cycle_last with the new value */
>  	clock->cycle_last = tk->cycle_last;
> -	/*
> -	 * Update the real timekeeper.
> -	 *
> -	 * We could avoid this memcpy by switching pointers, but that
> -	 * requires changes to all other timekeeper usage sites as
> -	 * well, i.e. move the timekeeper pointer getter into the
> -	 * spinlocked/seqcount protected sections. And we trade this
> -	 * memcpy under the timekeeper_seq against one before we start
> -	 * updating.
> -	 */
> -	memcpy(real_tk, tk, sizeof(*tk));
> -	timekeeping_update(real_tk, action);

So you're dropping the timekeeping_update() call here. I wonder how
this works.

> +	timekeeper_write_begin(&latch_timekeeper, &prev, &tk);
> +	*tk = *prev;

Do we really need to do the copy on all call sites, instead of doing
it in timekeeper_write_begin() ?

Thanks,

	tglx
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