Re: [PATCH 1/5] Add a global synchronization point for pvclock

Avi Kivity
2010-04-17T18:48:18Z

On 04/15/2010 09:37 PM, Glauber Costa wrote:
> In recent stress tests, it was found that pvclock-based systems
> could seriously warp in smp systems. Using ingo's time-warp-test.c,
> I could trigger a scenario as bad as 1.5mi warps a minute in some systems.
> (to be fair, it wasn't that bad in most of them). Investigating further, I
> found out that such warps were caused by the very offset-based calculation
> pvclock is based on.
>
> This happens even on some machines that report constant_tsc in its tsc flags,
> specially on multi-socket ones.
>
> Two reads of the same kernel timestamp at approx the same time, will likely
> have tsc timestamped in different occasions too. This means the delta we
> calculate is unpredictable at best, and can probably be smaller in a cpu
> that is legitimately reading clock in a forward ocasion.
>
> Some adjustments on the host could make this window less likely to happen,
> but still, it pretty much poses as an intrinsic problem of the mechanism.
>
> A while ago, I though about using a shared variable anyway, to hold clock
> last state, but gave up due to the high contention locking was likely
> to introduce, possibly rendering the thing useless on big machines. I argue,
> however, that locking is not necessary.
>
> We do a read-and-return sequence in pvclock, and between read and return,
> the global value can have changed. However, it can only have changed
> by means of an addition of a positive value. So if we detected that our
> clock timestamp is less than the current global, we know that we need to
> return a higher one, even though it is not exactly the one we compared to.
>
> OTOH, if we detect we're greater than the current time source, we atomically
> replace the value with our new readings. This do causes contention on big
> boxes (but big here means *BIG*), but it seems like a good trade off, since
> it provide us with a time source guaranteed to be stable wrt time warps.
>
> After this patch is applied, I don't see a single warp in time during 5 days
> of execution, in any of the machines I saw them before.
>
>

Please define a cpuid bit that makes this optional. When we eventually
enable it in the future, it will allow a wider range of guests to enjoy it.

>
> +static u64 last_value = 0;
>

Needs to be atomic64_t.

> +
> cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
> {
> struct pvclock_shadow_time shadow;
> unsigned version;
> cycle_t ret, offset;
> + u64 last;
>
>
> + do {
> + last = last_value;
>

Otherwise, this assignment can see a partial update.

> + if (ret< last)
> + return last;
> + } while (unlikely(cmpxchg64(&last_value, last, ret) != ret));
> +
> return ret;
> }
>
>

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.