Messages in this thread | | | Date | Mon, 09 Jun 2008 13:59:25 -0700 | From | Max Krasnyanskiy <> | Subject | Re: [patch] sched: prevent bound kthreads from changing cpus_allowed |
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David Rientjes wrote: >> 2) Sometimes calls to kthread_bind are binding to any online cpu, such as in: >> >> drivers/infiniband/hw/ehca/ehca_irq.c: kthread_bind(cct->task, any_online_cpu(cpu_online_map)); >> >> In such cases, the PF_THREAD_BOUND seems inappropriate. The caller of >> kthread_bind() really doesn't seem to care where that thread is bound; >> they just want it on a CPU that is still online. >> > > This particular case is simply moving the thread to any online cpu so that > it survives long enough for the subsequent kthread_stop() in > destroy_comp_task(). So I don't see a problem with this instance. > > A caller to kthread_bind() can always remove PF_THREAD_BOUND itself upon > return, but I haven't found any cases in the tree where that is currently > necessary. And doing that would defeat the semantics of kthread_bind() > where these threads are supposed to be bound to a specific cpu and not > allowed to run on others.
Actually I have another use case here. Above example in particular may be ok but it does demonstrate the issue nicely. Which is that in some cases kthreads are bound to a CPU but do not have a strict "must run here" requirement and could be moved if needed. For example I need an ability to move workqueue threads. Workqueue threads do kthread_bind(). So how about we add something like kthread_bind_strict() which would set PF_THREAD_BOUND ? We could also simply add flags argument to the kthread_bind() which would be better imo but requires more changes. ie It'd look like kthread_bind(..., cpu, KTHREAD_BIND_STRICT);
Things like migration threads, stop machine, etc would use the strict version and everything else would use non-strict bind.
--- On the related note (this seems like the right crowd :). What do people think about kthreads and cpusets in general. We currently have a bit of a disconnect in the logic. 1. kthreads can be put into a cpuset at which point their cpus_allowed mask is updated properly 2. kthread's cpus_allowed mask is updated properly when cpuset setup changes (cpus added, removed, etc). 3. kthreads inherit cpuset from a parent (kthreadd for example) _but_ they either do kthread_bind() or set_cpus_allowed() and both of those simply ignore inherited cpusets.
Notice how scenario #3 does not fit into the overall picture. The behaviour is inconsistent. How about this: - Split sched_setaffinity into
sched_setaffinity() { task *p = get_task_by_pid(); return task_setaffinity(p); }
task_setaffinity(task, cpumask, flags) { if (flags & FORCE) { // Used for kthreads that require strict binding. // Detach the task from the current cpuset // and put it into the root cpuset. // Set PF_THREAD_BOUND. }
// Rest of the original sched_setaffinity logic }
- Have kthreads call task_setaffinity() instead of set_cpus_allowed() directly. That way the behaviour will be consistent across the board.
Comments ?
Max
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