linux-kernel - Re: Runtime overhead of PREEMPT

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Message-ID: <15e4768144a74b093ad5a43f6e5c263fd98775fb.camel@gmx.de>
Date:   Fri, 20 Oct 2023 16:34:20 +0200
From:   Mike Galbraith <efault@....de>
To:     Emanuele Rocca <ema@...ian.org>
Cc:     linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>
Subject: Re: Runtime overhead of PREEMPT_DYNAMIC

On Fri, 2023-10-20 at 16:11 +0200, Mike Galbraith wrote:
> On Fri, 2023-10-20 at 15:40 +0200, Emanuele Rocca wrote:
> > Hi Mike,
> >
> > Here is the full diff between the kernel configurations I used. The only
> > change I made was setting CONFIG_PREEMPT_DYNAMIC=n, everything else was
> > a consequence of that AFAICT, but please do let me know if you see
> > anything that shouldn't be there.
> >
> > --- config-6.5.0-0.preempt-dynamic-amd64        2023-10-11 15:30:02.000000000 +0200
> > +++ config-6.5.0-0.a.test-amd64 2023-10-11 14:30:02.000000000 +0200
> >
> > @@ -10597,7 +10596,6 @@
> >  # end of Scheduler Debugging
> >  
> >  # CONFIG_DEBUG_TIMEKEEPING is not set
> > -CONFIG_DEBUG_PREEMPT=y
> >  
> >  #
> >  # Lock Debugging (spinlocks, mutexes, etc...)
>
> Seems you had also turned on DEBUG_PREEMPT in the dynamic setup, which
> adds some overhead.. but not a metric ton.

Hm, I don't recall the overhead as being that bad, but thar she blows.

i7-4790                                       avg         cmdline
6.5.8-voluntary 3685.08 3679.93 3704.98   3689.99  1.000
6.5.8-dynamic   3571.62 3568.61 3550.55   3563.59   .965
                3052.06 3032.74 3019.93   3034.91   .822  +DEBUG_PREEMPT
                3651.37 3599.87 3615.18   3622.14   .981  preempt=none
                3459.58 3514.09 3539.88   3504.51   .949  preempt=full