linux-kernel - Re: A couple of TSC questions

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Date:   Sun, 2 Apr 2023 21:04:04 -0400
From:   Waiman Long <longman@...hat.com>
To:     paulmck@...nel.org, Feng Tang <feng.tang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: A couple of TSC questions


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.

> 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.

Cheers,
Longman