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

[Dave Hansen]

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.

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.

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?