Re: [topic change] jiffies as a time value

From: George Anzinger
Date: Tue Mar 15 2005 - 18:15:29 EST

john stultz wrote:
On Mon, 2005-03-14 at 15:40 -0800, George Anzinger wrote:

john stultz wrote:

On Sat, 2005-03-12 at 16:49 -0800, Matt Mackall wrote:

+ /* finally, update legacy time values */
+ write_seqlock_irqsave(&xtime_lock, x_flags);
+ xtime = ns2timespec(system_time + wall_time_offset);
+ wall_to_monotonic = ns2timespec(wall_time_offset);
+ wall_to_monotonic.tv_sec = -wall_to_monotonic.tv_sec;
+ wall_to_monotonic.tv_nsec = -wall_to_monotonic.tv_nsec;
+ /* XXX - should jiffies be updated here? */

Excellent question.

Indeed. Currently jiffies is used as both a interrupt counter and a
time unit, and I'm trying make it just the former. If I emulate it then
it stops functioning as a interrupt counter, and if I don't then I'll
probably break assumptions about jiffies being a time unit. So I'm not
sure which is the easiest path to go until all the users of jiffies are
audited for intent.

Really? Who counts interrupts??? The timer code treats jiffies as a unit of time. You will need to rewrite that to make it otherwise.

Ug. I'm thin on time this week, so I was hoping to save this discussion
for later, but I guess we can get into it now.

Well, assuming timer interrupts actually occur HZ times a second, yes
one could (and current practice, one does) implicitly interpret jiffies
as being a valid notion of time. However with SMIs, bad drivers that
disable interrupts for too long, and virtualization the reality is that
that assumption doesn't hold.

We do have the lost-ticks compensation code that tries to help this, but
that conflicts with some virtualization implementations. Suspend/resume
tries to compensate jiffies for ticks missed over time suspended, but
I'm not sure how accurate it really is (additionally, looking at it now,
it assumes jiffies is only 32bits).

Adding to that, the whole jiffies doesn't really increment at HZ, but
ACTHZ confusion, or bad drivers that assume HZ=100, we get a fair amount
of trouble stemming from folks using jiffies as a time value. Because
in reality, it is just a interrupt counter.

Well, currently, in x86 systems it causes wall clock to advance a very well defined amount. That it is not exactly 1/HZ is something we need to live with...

So now, if new timeofday code emulates jiffies, we have to decide if it
emulates jiffies at HZ or ACTHZ? Also there could be issues with jiffies
possibly jittering from it being incremented every tick and then set to
the proper time when the timekeeping code runs.

I think your overlooking timers. We have a given resolution for timers and some code, at least, expects timers to run with that resolution. This REQUIRES interrupts at resolution frequency. We can argue about what that interrupt event is called (currently a jiffies interrupt) and disparage the fact that hardware can not give us "nice" numbers for the resolution, but we do need the interrupts. That there are bad places in the code where interrupts are delayed is not really important in this discussion. For what it worth, the RT patch Ingo is working on is getting latencies down in the 10s of microseconds region.

We also need, IMNSHO to recognize that, at lest with some hardware, that interrupt IS in fact the clock and is the only reasonable way we have of reading it. This is true, for example, on the x86. The TSC we use as a fill in for between interrupts is not stable in the long term and should only be used to interpolate over 1 to 10 ticks or so.

I'm not sure which is the best way to go, but it sounds that emulating
it is probably the easiest. I just deferred the question with a comment
until now because its not completely obvious. Any suggestions on the
above questions (I'm guessing the answers are: use ACTHZ, and the jitter
won't hurt that bad).

But then you have another problem. To correctly function, times need to expire on time (hay how bout that) not some time later. To do this we need an interrupt source. To this point in time, the jiffies interrupt has been the indication that one or more timer may have expired. While we don't need to "count" the interrupts, we DO need them to expire the timers AND they need to be on time.

Well, something Nish Aravamudan has been working on is converting the
common users of jiffies (drivers) to start using human time units. These
very well understood units (which avoid HZ/ACTHZ/HZ=100 assumptions) can
then be accurately changed to jiffies (or possibly some other time unit)
internally. It would even be possible for soft-timers to expire based
upon the actual high-res time value, rather then the low-res tick-
counter(which is something else Nish has been playing with). When that
occurs we can easily start doing other interesting things that I believe
you've already been working on in your HRT code, such as changing the
timer interrupt frequency dynamically, or working with multiple timer
interrupt sources.

This is also what is done in things like posix timers. The fact remains that, at least in the posix timers case, the resolution is exported to the user and implies certain things. I am not sure we are explicitly exporting the resolution in the kernel, but, down under the code, there is a resolution AND it impacts what one should expect of timers.

So basically, lots of interesting questions and possibilities and I very
much look forward to your input and suggestions.

It may help to understand that MOST internal timers (i.e. timers the kernel sets) never expire. They are canceled by the caller because they were really "dead man" timers, i.e. "it better happen be this time or we are hurting".

Users, on the other hand, for the most part set up timers to allow bits of code to run either periodically or at some specified time. These are the folks who care about latency and being on time.

User also, of course, also use "dead man" timers such as, for example, the time out on select. By their nature the "dead man" timer, usually, does not have strict on time requirements.
George Anzinger george@xxxxxxxxxx

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