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

["George Spelvin" <linux@horizon.com>]

> 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!

> 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.

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