Re: [PATCH 2/9] IB: add a proper completion queue abstraction

[Bart Van Assche]

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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/