linux-kernel - Re: [RFC PATCH] sched_clock: also call register_current_timer

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Date:	Fri, 2 May 2014 17:50:12 +0100
From:	Will Deacon <will.deacon@....com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Russell King <linux@....linux.org.uk>,
	John Stultz <john.stultz@...aro.org>,
	Theodore Ts o <tytso@....edu>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] sched_clock: also call
 register_current_timer_delay() if possible

On Wed, Apr 30, 2014 at 05:56:53PM +0100, Sebastian Andrzej Siewior wrote:
> * Will Deacon | 2014-04-30 14:26:28 [+0100]:
> >I don't think that's the problem I was referring to. What I mean is that a
> >clocksource might overflow at any number of bits, so the delay calculation
> >needs to take this into account when it does:
> >
> >	while ((get_cycles() - start) < cycles)
> >
> >because a premature overflow from get_cycles() will cause us to return
> >early. The solution is to mask the result of the subtraction before the
> >comparison to match the width of the clock.
> 
> So I got this:

[...]

> Is this what you had in mind? If so, there is one user of
> register_current_timer_delay() which I didn't convert. That is
> arch_timer_delay_timer_register(). It returns arch_counter_get_cntvct()
> which seems to return an u64 (which is truncated to 32bit). However
> arch_counter_register() registers the clocksource with 56bits. So this
> does not look too good, right?

That should be fine, I think there's only an issue if you can overflow
twice during a single delay operation, so the thing would need to be
ticking at quite a frequency for that to happen!

> The other thing I noticed is
> |arch/arm/include/asm/timex.h:typedef unsigned long cycles_t;
> 
> This works for clocksource because timekeeping is using
> |include/linux/clocksource.h:typedef u64 cycle_t;
> 
> instead.
> Do I assume correct, that the arch_timer is really providing a number
> wider than 32bit? Shouldn't I then promote cycles_t to 64bit if that
> timer is active? Unless you have better suggestions of course :)

The architected timer is guaranteed to be at least 56 bits wide, but I
think we can safely truncate delay sources to 32-bit.

So actually, we only have a problem if people want to register delay clocks
smaller than 32-bit. Maybe it's simpler to enforce at least 32-bit precision
and don't bother with the registration if the clock is smaller than that?
You could use sizeof(cycles_t) to parameterise that.

Will
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