Re: A couple of TSC questions

[Waiman Long]

On 4/2/23 22:00, Paul E. McKenney wrote:
> On Sun, Apr 02, 2023 at 09:04:04PM -0400, Waiman Long wrote:
> > On 3/31/23 13:16, Paul E. McKenney wrote:
> > > On Tue, Mar 28, 2023 at 02:58:54PM -0700, Paul E. McKenney wrote:
> > > > On Mon, Mar 27, 2023 at 10:19:54AM +0800, Feng Tang wrote:
> > > > > On Fri, Mar 24, 2023 at 05:47:33PM -0700, Paul E. McKenney wrote:
> > > > > > On Wed, Mar 22, 2023 at 01:14:48PM +0800, Feng Tang wrote:
> > > [ . . . ]
> > >
> > > > > > > > Second, we are very occasionally running into console messages like this:
> > > > > > > >
> > > > > > > > Measured 2 cycles TSC warp between CPUs, turning off TSC clock.
> > > > > > > >
> > > > > > > > This comes from check_tsc_sync_source() and indicates that one CPU's
> > > > > > > > TSC read produced a later time than a later read from some other CPU.
> > > > > > > > I am beginning to suspect that these can be caused by unscheduled delays
> > > > > > > > in the TSC synchronization code, but figured I should ask you if you have
> > > > > > > > ever seen these.  And of course, if so, what the usual causes might be.
> > > > > > > I haven't seen this error myself or got similar reports. Usually it
> > > > > > > should be easy to detect once happened, as falling back to HPET
> > > > > > > will trigger obvious performance degradation.
> > > > > > And that is exactly what happened.  ;-)
> > > > > >
> > > > > > > Could you give more detail about when and how it happens, and the
> > > > > > > HW info like how many sockets the platform has.
> > > > > > We are in early days, so I am checking for other experiences.
> > > > > >
> > > > > > > CC Thomas, Waiman, as they discussed simliar case here:
> > > > > > > https://lore.kernel.org/lkml/87h76ew3sb.ffs@tglx/T/#md4d0a88fb708391654e78312ffa75b481690699f
> > > > > > Fun!  ;-)
> > > > Waiman, do you recall what fraction of the benefit was provided by the
> > > > first patch, that is, the one that grouped the sync_lock, last_tsc,
> > > > max_warp, nr_warps, and random_warps global variables into a single
> > > > struct?
> > The purpose of the first patch is just to avoid false cacheline sharing
> > between the watchdog cpu and another cpu that happens to access a nearby
> > data in the same cacheline.
> >
> > Now I realize that I should have followed up with this patch series. The
> > problem reported in that patch series happen on one system only, I believe.
> Thus far I am seeing eight systems, but out of a large number.  So this
> is very much preliminary.
>
> > > And what we are seeing is unlikely to be due to cache-latency-induced
> > > delays.  We see a very precise warp, for example, one system always
> > > has 182 cycles of TSC warp, another 273 cycles, and a third 469 cycles.
> > > Another is at the insanely large value of about 2^64/10, and shows some
> > > variation, but that variation is only about 0.1%.
> > >
> > > But any given system only sees warp on about half of its reboots.
> > > Perhaps due to the automation sometimes power cycling?
> > >
> > > There are few enough affected systems that investigation will take
> > > some time.
> > Maybe the difference in wrap is due to NUMA distance of the running cpu from
> > the node where the data reside. It will be interesting to see if my patch
> > helps.
> Almost all of them are single-socket systems.
>
> If the problem sticks with a few systems, I should be able to test
> patches no problem.  If it is randomly distributed across the fleet, a
> bit more prework analysis will be called for.  But what is life without
> a challenge?  ;-)

If it is happening on a single socket system, maybe it is caused by 
false cacheline sharing. It is hard to tell unless we find a way to 
reproduce it.

Cheers,
Longman