linux-kernel - Re: [PATCH 4/4] timekeeping: Use printk_deferred when holding timekeeping seqlock

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Message-ID: <822806193.15621.1399988364196.JavaMail.zimbra@efficios.com>
Date:	Tue, 13 May 2014 13:39:24 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	George Spelvin <linux@...izon.com>
Cc:	john stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, tglx@...utronix.de
Subject: Re: [PATCH 4/4] timekeeping: Use printk_deferred when holding
 timekeeping seqlock

----- Original Message -----
> From: "George Spelvin" <linux@...izon.com>
> To: linux@...izon.com, "mathieu desnoyers" <mathieu.desnoyers@...icios.com>
> Cc: "john stultz" <john.stultz@...aro.org>, linux-kernel@...r.kernel.org, peterz@...radead.org, tglx@...utronix.de
> Sent: Tuesday, May 13, 2014 9:29:18 AM
> Subject: Re: [PATCH 4/4] timekeeping: Use printk_deferred when holding timekeeping seqlock
> 
> > We could expose a new clock type (besides monotonic and realtime) that is
> > documented as non-strictly monotonic. It may return a time very slightly in
> > the past if readers race with clock source frequency change. The caller
> > could
> > handle this situation (e.g. in user-space) by keeping its own per-cpu or
> > per-thread "last clock value" data structure (something we cannot do in a
> > vDSO) if it really cares about per-cpu/thread clock monotonicity.
> 
> That the first of two options I proposed.  The problem, with respect to
> the immediate problem of debugging during a write deadlocking, is
> that it makes a more complex API which callers must understand the
> subtleties of.
> 
> Perhaps necessary, but definitely a minus.
> 
> > This could be implemented with the scheme I proposed as a prototype here:
> > 
> >    https://lkml.org/lkml/2013/9/14/136
> 
> I'm working my way though it.  I definitely like the first patch!

Thanks! :)

> 
> > Thoughts ?
> 
> I was trying to tackle the "hard problem" of making *all* time reads
> non-blocking, with monotonicity guarantees.  There has to be *some* bound
> on blocking times (in particular, time between reading hardware tiemrs
> and translating them to real time), but they can be reasonably long.

What I gathered from my past discussion with John on this topic is that
virtualization blows away pretty much any assumption we can make on
"update should be reasonably short". A virtualized CPU can be preempted
for a rather long time (AFAIU not possible to bound).

> I think I have an idea that could work, but given the hairiness of
> the timeeeping code, implementing it would be a major project.

Indeed, timekeeping is not for the faint of heart. ;-)

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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