There's lots of work being scheduled on even isolated cpus. If you are not running a tickless kernel, you should see around 1000 local timer interrupts per second (by default). You will also see soft irqs (if you haven't affinitized them with some housekeeping cpu), non maskable interrupts/machine check errors, work queue tasks, etc. As for rcu, even with offloading you will see the isolated cores performing work required to schedule the callbacks on the offloaded cpus. You can solve that by switching to rcu callback polling, but my point here is, there's a number of different types of tasks that will run on isolated cpus.
On Mon, 22 May 2017, 11:41 Himanshu Sharma, <imhimansh...@gmail.com> wrote: > Thanks Wojciech for the quick reply. I read about rcu offloading and in my > testing, can confirm that kernel rcu threads are scheduled on core 0 even > if core 0 is isolated. Another thing I observed was that there are some > kworker threads which run on isolated cpus other than 0. Is this expected > behavior, because I used to think that isolated cpus are not touched by the > kernel. These kworker threads will definitely lead to context switches and > hamper performance a little bit. And I am afraid we can do nothing to get > rid of them. > > Himanshu Sharma > > > On Mon, May 22, 2017 at 2:08 PM, Wojciech Kudla <wojciech.ku...@gmail.com> > wrote: > >> There's a number of kernel tasks that are implicitly bound to cpu0. For >> an example of one have a look at rcu offloading and its restrictions. >> >> On Mon, 22 May 2017, 08:59 Himanshu Sharma, <imhimansh...@gmail.com> >> wrote: >> >>> Hi Michael >>> >>> Did you find a satisfactory reason for not isolating cpu 0, maybe some >>> low level OS code that is bound to run on core 0? I am also stuck at this >>> question right now and am thinking you might have an answer. >>> >>> Thanks >>> Himanshu >>> >>> On Wednesday, March 4, 2015 at 8:08:22 PM UTC+5:30, Michael Mattoss >>> wrote: >>>> >>>> Hi guys, >>>> >>>> I'm in the process of setting up a new dual-socket server for a low >>>> latency workload. >>>> The application will run exclusively on one CPU and everything else >>>> (i.e. OS, non-critical processes) will run on the other CPU to avoid cache >>>> pollution. >>>> I was wondering if it makes any difference as to which one of the >>>> 2 CPU's is chosen for the workload. >>>> Theoretically, there should be no difference but I was wondering if >>>> there is some low-level stuff (e.g. core OS code, system management >>>> interrupts handlers) that is statically allocated to CPU-0 as every system >>>> has at least 1 CPU. >>>> Of course, if that's the case then CPU-1 is the better choice. >>>> >>>> Any thoughts/suggestions? >>>> >>>> Thanks, >>>> Michael >>>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "mechanical-sympathy" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to mechanical-sympathy+unsubscr...@googlegroups.com. >>> For more options, visit https://groups.google.com/d/optout. >>> >> -- >> You received this message because you are subscribed to a topic in the >> Google Groups "mechanical-sympathy" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/mechanical-sympathy/SnJ6LTKCjEU/unsubscribe >> . >> To unsubscribe from this group and all its topics, send an email to >> mechanical-sympathy+unsubscr...@googlegroups.com. >> > >> For more options, visit https://groups.google.com/d/optout. >> > -- > You received this message because you are subscribed to the Google Groups > "mechanical-sympathy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to mechanical-sympathy+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "mechanical-sympathy" group. To unsubscribe from this group and stop receiving emails from it, send an email to mechanical-sympathy+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
