Hi Thomas, On Tue, Jan 14, 2020 at 02:45:00PM +0100, Thomas Gleixner wrote: > Ming, > > Ming Lei <ming.lei@xxxxxxxxxx> writes: > > On Fri, Jan 10, 2020 at 08:43:14PM +0100, Thomas Gleixner wrote: > >> Ming Lei <ming.lei@xxxxxxxxxx> writes: > >> > That is why I try to exclude isolated CPUs from interrupt effective affinity, > >> > turns out the approach is simple and doable. > >> > >> Yes, it's doable. But it still is inconsistent behaviour. Assume the > >> following configuration: > >> > >> 8 CPUs CPU0,1 assigned for housekeeping > >> > >> With 8 queues the proposed change does nothing because each queue is > >> mapped to exactly one CPU. > > > > That is expected behavior for this RT case, given userspace won't submit > > IO from isolated CPUs. > > What is _this_ RT case? We really don't implement policy for a specific > use case. If the kernel implements a policy then it has to be generally > useful and practical. Maybe the word of 'RT case' isn't accurate, I thought isolated CPUs is only used for realtime cases, at least that is Peter's usage, maybe I was wrong. But it can be generic for all isolated CPUs cases, in which users don't want managed interrupts to disturb the isolated CPU cores. > > >> With 4 queues you get the following: > >> > >> CPU0,1 queue 0 > >> CPU2,3 queue 1 > >> CPU4,5 queue 2 > >> CPU6,7 queue 3 > >> > >> No effect on the isolated CPUs either. > >> > >> With 2 queues you get the following: > >> > >> CPU0,1,2,3 queue 0 > >> CPU4,5,6,7 queue 1 > >> > >> So here the isolated CPUs 2 and 3 get the isolation, but 4-7 > >> not. That's perhaps intended, but definitely not documented. > > > > That is intentional change, given no IO will be submitted from 4-7 > > most of times in RT case, so it is fine to select effective CPU from > > isolated CPUs in this case. As peter mentioned, IO may just be submitted > > from isolated CPUs during booting. Once the system is setup, no IO > > comes from isolated CPUs, then no interrupt is delivered to isolated > > CPUs, then meet RT's requirement. > > Again. This is a specific usecase. Is this generally applicable? As mentioned above, it can be applied for all isolated CPUs, when users don't want managed interrupts to disturb these CPU cores. > > > We can document this change somewhere. > > Yes, this needs to be documented very clearly with that command line > parameter. OK, will do that in formal post. Thanks, Ming
