On Thu, Jan 28, 2021 at 05:02:41PM +0100, Thomas Gleixner wrote:
> On Wed, Jan 27 2021 at 09:19, Marcelo Tosatti wrote:
> > On Wed, Jan 27, 2021 at 11:57:16AM +0000, Robin Murphy wrote:
> >> > + hk_flags = HK_FLAG_DOMAIN | HK_FLAG_MANAGED_IRQ;
> >> > + mask = housekeeping_cpumask(hk_flags);
> >>
> >> AFAICS, this generally resolves to something based on cpu_possible_mask
> >> rather than cpu_online_mask as before, so could now potentially return an
> >> offline CPU. Was that an intentional change?
> >
> > Robin,
> >
> > AFAICS online CPUs should be filtered.
>
> The whole pile wants to be reverted. It's simply broken in several ways.
I was asking for your comments on interaction with CPU hotplug :-)
Anyway...
So housekeeping_cpumask has multiple meanings. In this case:
HK_FLAG_DOMAIN | HK_FLAG_MANAGED_IRQ
domain
Isolate from the general SMP balancing and scheduling
algorithms. Note that performing domain isolation this way
is irreversible: it's not possible to bring back a CPU to
the domains once isolated through isolcpus. It's strongly
advised to use cpusets instead to disable scheduler load
balancing through the "cpuset.sched_load_balance" file.
It offers a much more flexible interface where CPUs can
move in and out of an isolated set anytime.
You can move a process onto or off an "isolated" CPU via
the CPU affinity syscalls or cpuset.
<cpu number> begins at 0 and the maximum value is
"number of CPUs in system - 1".
managed_irq
Isolate from being targeted by managed interrupts
which have an interrupt mask containing isolated
CPUs. The affinity of managed interrupts is
handled by the kernel and cannot be changed via
the /proc/irq/* interfaces.
This isolation is best effort and only effective
if the automatically assigned interrupt mask of a
device queue contains isolated and housekeeping
CPUs. If housekeeping CPUs are online then such
interrupts are directed to the housekeeping CPU
so that IO submitted on the housekeeping CPU
cannot disturb the isolated CPU.
If a queue's affinity mask contains only isolated
CPUs then this parameter has no effect on the
interrupt routing decision, though interrupts are
only delivered when tasks running on those
isolated CPUs submit IO. IO submitted on
housekeeping CPUs has no influence on those
queues.
So as long as the meaning of the flags are respected, seems
alright.
Nitesh, is there anything preventing this from being fixed
in userspace ? (as Thomas suggested previously).
