Re: Using sched_clock() for polling time limit

[Ben Hutchings]

On Sat, 2013-06-29 at 21:50 +0300, Eliezer Tamir wrote:
> On 28/06/2013 19:51, Ben Hutchings wrote:
> > On Fri, 2013-06-28 at 15:59 +0300, Eliezer Tamir wrote:
> >> Our use of sched_clock is OK because we don't mind the side effects
> >> of calling it and occasionally waking up on a different CPU.
> >
> > Sure about that?  Jitter matters too.
> >
> Pretty sure, this is a limit on how long we poll, it's for fairness to
> the rest of the system not for performance of this code.
> 
> What matters is that on average you are bounded by something close to
> what the user specified. If once in a while you run less because of
> clock jitter, or even twice the specified time, it's no big deal.

So what is the maximum time difference in sched_clock() values between
CPUs in the same system?

> So I don't see how jitter would matter.
> 
> And if your workload is jitter sensitive, you should probably be
> pinning tasks to CPUs anyway.

Yes, they should be pinned.  But I think a single task that isn't pinned
could poll for significantly longer than intended and the effect
wouldn't be limited to that one task.

> >> When CONFIG_DEBUG_PREEMPT is on, disable preempt before calling
> >> sched_clock() so we don't trigger a debug_smp_processor_id() warning.
> > [...]
> >
> > I think this is papering over a problem.  The warning is there for a
> > good reason.
> 
> I think we understand the warning, and that we are OK with the effects.
> 
> looking at how other users in the kernel solved this issue
> It seems like this is what they do.
> for example trace/ring_buffer.c:ring_buffer_time_Stamp()

Well, this seems to be debug information, not actually used for timing.

> Also kvm_clock_read() and xen_clokcsource_read() seem to disable preempt
> just to silence this warning.

I can't see what you're referring to.

> If they really cared about reading the value on one CPU, then being
> scheduled on another they should have disabled interrupts.
> or am I missing something?
> 
> > Would functions like these make it possible to use sched_clock() safely
> > for polling?  (I didn't spend much time thinking about the names.)
[...]
> I don't understand, preempt_disable() only makes prevents preempt
> from taking the CPU away from you, you could still lose it for
> other reasons.
> You would really need to disable interrupts in this region to be
> sure that it all executed on the same CPU.

Hmm, yes you're right.  Anyway, what I was trying to suggest was that
you would record the current CPU along with the initial timestamp, and
then abort low-latency polling if either the task is moved onto a
different CPU or the time limit was reached.  Checking the CPU first
means that the sched_clock() comparison is valid.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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