Re: [Patch v4 1/3] lib: Restrict cpumask_local_spread to houskeeping CPUs

[Nitesh Narayan Lal <nitesh@redhat.com>]

On 1/28/21 11:59 AM, Marcelo Tosatti wrote:
> 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).

I think it should be doable atleast for most of the devices.
However, I do wonder if there is a better way of fixing this generically
from the kernel?

Currently, as Thomas mentioned housekeeping_cpumask() is used at different
locations just to fix the issue corresponding to that component or driver.

-- 
Thanks
Nitesh