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

[Peter Zijlstra]

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@xxxxxxxxxx>
> 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@xxxxxxxxxx>
>     Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
>     Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
>     Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>     Link:
> https://lore.kernel.org/r/20210512232924.150322-3-frederic@xxxxxxxxxx
> -----------------------------------------------------
> 
> 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@xxxxxxxxx