Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention

[Waiman Long]

On 08/11/2016 03:32 PM, Dave Hansen wrote:
> On 08/10/2016 11:29 AM, Waiman Long wrote:
> > +static cycle_t read_hpet(struct clocksource *cs)
> > +{
> > +	int seq;
> > +
> > +	seq = READ_ONCE(hpet_save.seq);
> > +	if (!HPET_SEQ_LOCKED(seq)) {
> ...
> > +	}
> > +
> > +	/*
> > +	 * Wait until the locked sequence number changes which indicates
> > +	 * that the saved HPET value is up-to-date.
> > +	 */
> > +	while (READ_ONCE(hpet_save.seq) == seq) {
> > +		/*
> > +		 * Since reading the HPET is much slower than a single
> > +		 * cpu_relax() instruction, we use two here in an attempt
> > +		 * to reduce the amount of cacheline contention in the
> > +		 * hpet_save.seq cacheline.
> > +		 */
> > +		cpu_relax();
> > +		cpu_relax();
> > +	}
> > +
> > +	return (cycle_t)READ_ONCE(hpet_save.hpet);
> > +}
> It's a real bummer that this all has to be open-coded.  I have to wonder
> if there were any alternatives that you tried that were simpler.

What do you mean by "open-coded"? Do you mean the function can be inlined?


> Is READ_ONCE()/smp_store_release() really strong enough here?  It
> guarantees ordering, but you need ordering *and* a guarantee that your
> write is visible to the reader.  Don't you need actual barriers for
> that?  Otherwise, you might be seeing a stale HPET value, and the spin
> loop that you did waiting for it to be up-to-date was worthless.  The
> seqlock code, uses barriers, btw.

The cmpxchg() and smp_store_release() act as the lock/unlock sequence 
with the proper barriers. Another important point is that the hpet value 
is visible to the other readers  before the sequence number. This is 
what the smp_store_release() is providing. cmpxchg is an actual barrier, 
even though smp_store_release() is not. However, the x86 architecture 
will guarantee the writes are in order, I think.

> Also, since you're fundamentally reading a second-hand HPET value, does
> that have any impact on the precision of the HPET as a timesource?  Or,
> is it so coarse already that this isn't an issue?

There can always be unexpected latency in the returned time value, such 
as an interrupt or NMI. I think as long as the time won't go backward, 
it should be fine.

Cheers,
Longman