----- Original Message ----- > From: "George Spelvin" <li...@horizon.com> > To: li...@horizon.com, "mathieu desnoyers" <mathieu.desnoy...@efficios.com> > Cc: "john stultz" <john.stu...@linaro.org>, linux-kernel@vger.kernel.org, > pet...@infradead.org, t...@linutronix.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
