Messages in this thread | | | Subject | Re: [PATCH 2/9] IB: add a proper completion queue abstraction | From | Bart Van Assche <> | Date | Wed, 18 Nov 2015 10:20:14 -0800 |
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On 11/17/2015 11:55 PM, Sagi Grimberg wrote: >>> +static void ib_cq_poll_work(struct work_struct *work) >>> +{ >>> + struct ib_cq *cq = container_of(work, struct ib_cq, work); >>> + int completed; >>> + >>> + completed = __ib_process_cq(cq, IB_POLL_BUDGET_WORKQUEUE); >>> + if (completed >= IB_POLL_BUDGET_WORKQUEUE || >>> + ib_req_notify_cq(cq, IB_POLL_FLAGS) > 0) >>> + queue_work(ib_comp_wq, &cq->work); >>> +} >>> + >>> +static void ib_cq_completion_workqueue(struct ib_cq *cq, void *private) >>> +{ >>> + queue_work(ib_comp_wq, &cq->work); >>> +} >> >> The above code will cause all polling to occur on the context of the CPU >> that received the completion interrupt. This approach is not powerful >> enough. For certain workloads throughput is higher if work completions >> are processed by another CPU core on the same CPU socket. Has it been >> considered to make the CPU core on which work completions are processed >> configurable ? > > The workqueue is unbound. This means that the functionality you are > you are asking for exists.
Hello Sagi,
Are you perhaps referring to the sysfs CPU mask that allows to control workqueue affinity ? I expect that setting the CPU mask for an entire pool through sysfs will lead to suboptimal results. What I have learned by tuning target systems is that there is a significant performance difference (> 30% IOPS) between a configuration where each completion thread is pinned to exactly one CPU compared to allowing the scheduler to choose a CPU.
Controlling the CPU affinity of worker threads with the taskset command is not possible since the function create_worker() in kernel/workqueue.c calls kthread_bind_mask(). That function sets PF_NO_SETAFFINITY. From sched.h:
#define PF_NO_SETAFFINITY 0x04000000 /* Userland is not allowed to meddle with cpus_allowed */
Bart.
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