linux-kernel - Re: [PATCH 13/15] x86/kconfig/64: Enable popular scheduler, cgroups and namespaces options in the defconfig

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Message-ID: <aBuI36FCDbj20x28@gmail.com>
Date: Wed, 7 May 2025 18:22:55 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Yafang Shao <laoar.shao@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, ardb@...nel.org, arnd@...db.de,
	bp@...en8.de, dwmw@...zon.co.uk, hpa@...or.com,
	linux-kernel@...r.kernel.org, michal.lkml@...kovi.net,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	vkuznets@...hat.com, yamada.masahiro@...ionext.com
Subject: Re: [PATCH 13/15] x86/kconfig/64: Enable popular scheduler, cgroups
 and namespaces options in the defconfig


* Yafang Shao <laoar.shao@...il.com> wrote:

> On Wed, May 7, 2025 at 3:06 PM Ingo Molnar <mingo@...nel.org> wrote:
> >
> >
> > * Yafang Shao <laoar.shao@...il.com> wrote:
> >
> > > Hello Mingo,
> > >
> > > > +CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
> > > > +CONFIG_IRQ_TIME_ACCOUNTING=y
> > >
> > > Enabling CONFIG_IRQ_TIME_ACCOUNTING=y can lead to user-visible behavioral
> > > changes. For more context, please refer to the related discussion here:
> > > https://lore.kernel.org/all/20241222024734.63894-1-laoar.shao@gmail.com/ .
> >
> > Yeah. I actually agree with your series. It (re-)includes IRQ/softirq
> > time in task CPU usage statistics even under IRQ_TIME_ACCOUNTING=y,
> > while still keeping the finegrained IRQ/softirq statistics as well,
> > correct?
> 
> Correct.
> 
> >
> > The Kconfig option is also arguably rather misleading:
> >
> > config IRQ_TIME_ACCOUNTING
> >         bool "Fine granularity task level IRQ time accounting"
> >         depends on HAVE_IRQ_TIME_ACCOUNTING && !VIRT_CPU_ACCOUNTING_NATIVE
> >         help
> >           Select this option to enable fine granularity task irq time
> >           accounting. This is done by reading a timestamp on each
> >           transitions between softirq and hardirq state, so there can be a
> >           small performance impact.
> >
> > It only warns about a small performance impact, but doesn't warn that
> > CPU accounting is changed in an incompatible fashion that surprises
> > tooling...
> 
> Yes, this breaks our userspace tools.

Okay, so 2 out of your 3 fixes are upstream already:

  763a744e24a8 ("sched: Don't account irq time if sched_clock_irqtime is disabled")
  a6fd16148fdd ("sched, psi: Don't account irq time if sched_clock_irqtime is disabled")

But we don't have this one yet:

  [PATCH v8 4/4] sched: Fix cgroup irq time for CONFIG_IRQ_TIME_ACCOUNTING

  https://lore.kernel.org/r/20250103022409.2544-5-laoar.shao@gmail.com

which is also essential to fully fix the tooling regression, right?

I think this last patch fell between the cracks, I didn't see any 
fundamental objections against the fix.

Since the patch does not apply cleanly anymore, mind sending a fresh 
-v9 version against v6.15-rc5 or so?

Thanks,

	Ingo