Messages in this thread | | | Date | Sun, 15 Jul 2001 13:31:12 -0700 (PDT) | From | Davide Libenzi <> | Subject | Re: CPU affinity & IPI latency |
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On 15-Jul-2001 Andi Kleen wrote: > On Fri, Jul 13, 2001 at 03:43:05PM -0700, Mike Kravetz wrote: >> - Define a new field in the task structure 'saved_cpus_allowed'. >> With a little collapsing of existing fields, there is room to put >> this on the same cache line as 'cpus_allowed'. >> - In reschedule_idle() if we determine that the best CPU for a task >> is the CPU it is associated with (p->processor), then temporarily >> bind the task to that CPU. The task is temporarily bound to the >> CPU by overwriting the 'cpus_allowed' field such that the task can >> only be scheduled on the target CPU. Of course, the original >> value of 'cpus_allowed' is saved in 'saved_cpus_allowed'. >> - In schedule(), the loop which examines all tasks on the runqueue >> will restore the value of 'cpus_allowed'. > > This sounds racy, at least with your simple description. Even other CPUs > could walk their run queue while the IPI is being processed and reset the > cpus_allowed flag too early. I guess it would need a check in the > schedule loop that restores cpus_allowed that the current CPU is the only > on set in that task's cpus_allowed. This unfortunately would hang the > process if the CPU for some reason cannot process schedules timely, so > it may be needed to add a timestamp also to the task_struct that allows > the restoration of cpus_allowed even from non target CPUs after some > time.
I previously expressed ( maybe in this thread, I don't remember ) my idea of not touching ( hacking ) the current scheduler to let it fit to SMP, that IMHO, has different problems and needs different answers. Anyway, as I said a couple of messages ago, the solution of this problem could be to move the moved task in a private ( per CPU ) wakeup list that, when the idle comes up, it'll be ran.
- Davide
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