Re: Performance regression: thread wakeup time (latency) increased up to 3x

[Norbert <nbrtt01@gmail.com>]

On 10/18/21 18:56, Norbert wrote:
> On 10/18/21 04:25, Peter Zijlstra wrote:
>> On Fri, Oct 15, 2021 at 09:08:58PM -0700, Norbert wrote:
>>
>>>>>> On Fri, Oct 15, 2021 at 12:43:45AM -0700, Norbert wrote:
>>>>>>> Performance regression: thread wakeup time (latency) increased up 
>>>>>>> to 3x.
>>>>>>>
>>>>>>> Happened between 5.13.8 and 5.14.0. Still happening at least on 
>>>>>>> 5.14.11.
>>
>>> So git-bisect finally identified the following commit.
>>> The performance difference came in a single step. Times were 
>>> consistent with
>>> my first post either the slow time or the fast time,
>>> as far as I could tell during the bisection.
>>>
>>> It is a bit unfortunate that this comes from an attempt to reduce OS 
>>> noise.
>>>
>>> -----------------------------------------------------
>>> commit a5183862e76fdc25f36b39c2489b816a5c66e2e5
>>> Author: Yunfeng Ye <yeyunfeng@huawei.com>
>>> Date:   Thu May 13 01:29:16 2021 +0200
>>>
>>>      tick/nohz: Conditionally restart tick on idle exit
>>>
>>>      In nohz_full mode, switching from idle to a task will 
>>> unconditionally
>>>      issue a tick restart. If the task is alone in the runqueue or is 
>>> the
>>>      highest priority, the tick will fire once then eventually stop. 
>>> But that
>>>      alone is still undesired noise.
>>>
>>>      Therefore, only restart the tick on idle exit when it's strictly
>>>      necessary.
>>>
>>>      Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
>>>      Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
>>>      Signed-off-by: Ingo Molnar <mingo@kernel.org>
>>>      Acked-by: Peter Zijlstra <peterz@infradead.org>
>>>      Link:
>>> https://lore.kernel.org/r/20210512232924.150322-3-frederic@kernel.org
>>> -----------------------------------------------------
>>>
>>> Is there anything else to do to complete this report?
>>
>> So it _could_ be you're seeing increased use of deeper idle states due
>> to less noise. I'm forever forgetting what the most friendly tool is for
>> checking that (powertop can I think), Rafael?
>>
>> One thing to try is boot with idle=halt and see if that makes a
>> different.
>>
>> Also, let me Cc all the people involved.. the thread starts:
>>
>>    
>> https://lkml.kernel.org/r/035c23b4-118e-6a35-36d9-1b11e3d679f8@gmail.com
>>
> 
> 
> Booting with idle=halt results in a thread wakeup time of around 2000 
> ns, so in the middle between the kernel 5.13 value of 1080 ns and the 
> kernel 5.14/5.15 value of around 3550 ns. The wake call time remains at 
> 740 ns (meaning as bad as without this setting). I'm not sure how much 
> that says or doesn't say. By the way, using cpufreq.off=1 seems to have 
> no effect at all.
> 
> In the meantime I verified the finding from the git bisection by 
> manually reverting the changes from this commit in the source code of 
> the 5.15-rc5 code base. By doing so the timings for the 
> isolated/nohz_full CPUs come back almost to the (good) 5.13 values (both 
> wakeup and wake-call).
> 
> However the timings for the non-isolated CPUs are unaffected and remain
> with the worse performance of 1.3x for the wakeup and 1.4x for the wake 
> call. So this apparently requires a separate independent git-bisect and 
> is probably a second separate issue (if it is also due to a single change).
> 
> I've tried a bit to narrow down the cause of the 3.3x slowdown but am 
> still trying to find my way through the maze of little functions... :-).


On the thought that it might enter deeper idle/wait/sleep states:

The benchmark executes this test in a quite tight loop, except that so 
far it waited 1000 ns (with a mix of pause and rdtsc) before calling 
futex-wake, to make sure the other thread fully enters the futex-wait 
without taking any shortcuts.

Except when this "prepare time" is reduced to less than even 350 ns or 
so, the timings remain the same (they go up before they start going 
down). Surely in this situation the thread is at least not supposed to 
enter deeper states for such short waiting times.

Best, Norbert