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
