Attachment 910105 Details for Bug 1109967 – Why flag PF_NO_SETAFFINITY is cleared on select_fallback_rq when the fixed CPU is not on NUMA node 0

Why flag PF_NO_SETAFFINITY is cleared on select_fallback_rq when the fixed CPU is not on NUMA node 0

why_the_flag_is_cleared_on_numa_node_non_0.txt (text/plain), 1.68 KB, created by Daniel Bristot de Oliveira on 2014-06-18 17:14:22 UTC

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Creator: Daniel Bristot de Oliveira

Created: 2014-06-18 17:14:22 UTC

Size: 1.68 KB

patch

obsolete

>select_task_rq()
>the cpu is always initialized as 0.
>and then it enters in the if (because the tasks are not allowed to run on cpu 0, unless it is for the CPU 0 :-))
>
>    call the select_fallback_rq(task_cpu(p), p);
>            the first argument (cpu) is always 0 because set_task_cpu(fixed CPU) was not called yet.
>
>            int nid = cpu_to_node(cpu); <----- here we have a logical error, because it will always return node 0, because cpu is always = 0;
>           
>            /*
>             * If the node that the cpu is on has been offlined, cpu_to_node()
>             * will return -1. There is no cpu on the node, and we should
>             * select the cpu on the other node.
>             */
>            if (nid != -1) {                           <----- it will always enter here because nid = 0.
>                    nodemask = cpumask_of_node(nid);   <----- here we have another error, if the desired CPU is not on the same NUMA node that CPU0.
>
>                    /* Look for allowed, online CPU in same node. */
>                    for_each_cpu(dest_cpu, nodemask) {
>                            if (!cpu_online(dest_cpu))
>                                    continue;
>                            if (!cpu_active(dest_cpu))
>                                    continue;
>                            if (cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p))) <---- if tsk_cpus_allowed(p) returns a CPU on the numa node 0, it will match.
>                                    return dest_cpu;                             <---- when it returns here, the flag is not cleared.
>                    }
>            }
>
>            if the allowed cpu is not on the numa node 0, it will continue and the flag will be cleared.
>

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Attachments on bug 1109967: 909211 | 910105 | 911799