linux-kernel - Re: [PATCH] sched/cpuacct: show only present CPUs to userspace

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Message-ID: <CAEivzxdKg20giNzsSpxhwCeWJM=JuD5=ti8MupibUoLEyOA4Zg@mail.gmail.com>
Date: Fri, 8 Nov 2024 14:00:56 +0100
From: Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: mingo@...hat.com, James Morse <james.morse@....com>, Gavin Shan <gshan@...hat.com>, 
	"Russell King (Oracle)" <rmk+kernel@...linux.org.uk>, Thomas Gleixner <tglx@...utronix.de>, 
	Simon Deziel <simon.deziel@...onical.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/cpuacct: show only present CPUs to userspace

On Mon, Oct 28, 2024 at 12:07 PM Jonathan Cameron
<Jonathan.Cameron@...wei.com> wrote:
>
> On Fri, 25 Oct 2024 17:35:56 +0200
> Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com> wrote:
>
> > Gentle ping.
> >
> > On Thu, Oct 17, 2024 at 12:22 PM Alexander Mikhalitsyn
> > <aleksandr.mikhalitsyn@...onical.com> wrote:
> > >
> > > After commit b0c69e1214bc ("drivers: base: Use present CPUs in GENERIC_CPU_DEVICES")
> > > changed which CPUs are shown in /sys/devices/system/cpu/ (only "present" ones)
> > > it also makes sense to change cpuacct cgroupv1 code not to report CPUs
> > > which are not present in the system as it confuses userspace.
> > > Let's make it consistent.
> > >
> > > A configuration when #(present CPUs) < #(possible CPUs) is easy to get with:
> > > qemu-system-x86_64
> > >         -smp 3,maxcpus=12 \
> > >         ...
> > >
>

Dear Jonathan,

> On a general basis, we definitely want these to line up, but I'm not familiar
> enough with this code to give more specific review.
>
> Other than that, I'm curious as to what userspace is tripping over this?

Originally it was reported me with regard to LXD
https://github.com/canonical/lxd/issues/13324

But I can imagine that any userspace can be confused there as what you
see in /proc/cpuinfo (and even /sys/devices/system/cpu/)
is not aligned with that you see in cpuacct.usage_all file). That's
extremely unexpected to see more CPUs in a cgroup-related file
than in /sys/devices/system/cpu/, IMHO.

Kind regards,
Alex

>
> Jonathan
>
> > > Cc: James Morse <james.morse@....com>
> > > Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > > Cc: Gavin Shan <gshan@...hat.com>
> > > Cc: "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>
> > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > Reported-by: Simon Deziel <simon.deziel@...onical.com>
> > > Closes: https://github.com/canonical/lxd/issues/13324
> > > Co-developed-by: Simon Deziel <simon.deziel@...onical.com>
> > > Signed-off-by: Simon Deziel <simon.deziel@...onical.com>
> > > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> > > ---
> > >  kernel/sched/cpuacct.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
> > > index 0de9dda09949..0f07fbfdb20e 100644
> > > --- a/kernel/sched/cpuacct.c
> > > +++ b/kernel/sched/cpuacct.c
> > > @@ -213,7 +213,7 @@ static int __cpuacct_percpu_seq_show(struct seq_file *m,
> > >         u64 percpu;
> > >         int i;
> > >
> > > -       for_each_possible_cpu(i) {
> > > +       for_each_present_cpu(i) {
> > >                 percpu = cpuacct_cpuusage_read(ca, i, index);
> > >                 seq_printf(m, "%llu ", (unsigned long long) percpu);
> > >         }
> > > @@ -247,7 +247,7 @@ static int cpuacct_all_seq_show(struct seq_file *m, void *V)
> > >                 seq_printf(m, " %s", cpuacct_stat_desc[index]);
> > >         seq_puts(m, "\n");
> > >
> > > -       for_each_possible_cpu(cpu) {
> > > +       for_each_present_cpu(cpu) {
> > >                 seq_printf(m, "%d", cpu);
> > >                 for (index = 0; index < CPUACCT_STAT_NSTATS; index++)
> > >                         seq_printf(m, " %llu",
> > > --
> > > 2.34.1
> > >
>