Messages in this thread | | | Subject | Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention | From | Dave Hansen <> | Date | Thu, 11 Aug 2016 12:32:46 -0700 |
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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?
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