Re: [PATCH] RDMA/core: reduce IB_POLL_BATCH constant

[Chuck Lever]

> On Feb 21, 2018, at 8:44 AM, Sagi Grimberg <sagi@xxxxxxxxxxx> wrote:
> 
> 
>>> On Tue, 2018-02-20 at 21:59 +0100, Arnd Bergmann wrote:
>>>> /* # of WCs to poll for with a single call to ib_poll_cq */
>>>> -#define IB_POLL_BATCH			16
>>>> +#define IB_POLL_BATCH			8
>>> 
>>> The purpose of batch polling is to minimize contention on the cq spinlock.
>>> Reducing the IB_POLL_BATCH constant may affect performance negatively. Has
>>> the performance impact of this change been verified for all affected drivers
>>> (ib_srp, ib_srpt, ib_iser, ib_isert, NVMeOF, NVMeOF target, SMB Direct, NFS
>>> over RDMA, ...)?
>> Only the users of the DIRECT polling method use an on-stack
>> array of ib_wc's. This is only the SRP drivers.
>> The other two modes have use of a dynamically allocated array
>> of ib_wc's that hangs off the ib_cq. These shouldn't need any
>> reduction in the size of this array, and they are the common
>> case.
>> IMO a better solution would be to change ib_process_cq_direct
>> to use a smaller on-stack array, and leave IB_POLL_BATCH alone.
> 
> The only reason why I added this array on-stack was to allow consumers
> that did not use ib_alloc_cq api to call it, but that seems like a
> wrong decision when thinking it over again (as probably these users
> did not set the wr_cqe correctly).
> 
> How about we make ib_process_cq_direct use the cq wc array and add
> a WARN_ON statement (and fail it gracefully) if the caller used this
> API without calling ib_alloc_cq?

Agreed, I prefer that all three modes use dynamically allocated
memory for that array.


> --
> diff --git a/drivers/infiniband/core/cq.c b/drivers/infiniband/core/cq.c
> index bc79ca8215d7..cd3e9e124834 100644
> --- a/drivers/infiniband/core/cq.c
> +++ b/drivers/infiniband/core/cq.c
> @@ -25,10 +25,10 @@
> #define IB_POLL_FLAGS \
>        (IB_CQ_NEXT_COMP | IB_CQ_REPORT_MISSED_EVENTS)
> 
> -static int __ib_process_cq(struct ib_cq *cq, int budget, struct ib_wc *poll_wc)
> +static int __ib_process_cq(struct ib_cq *cq, int budget)
> {
>        int i, n, completed = 0;
> -       struct ib_wc *wcs = poll_wc ? : cq->wc;
> +       struct ib_wc *wcs = cq->wc;
> 
>        /*
>         * budget might be (-1) if the caller does not
> @@ -72,9 +72,9 @@ static int __ib_process_cq(struct ib_cq *cq, int budget, struct ib_wc *poll_wc)
>  */
> int ib_process_cq_direct(struct ib_cq *cq, int budget)
> {
> -       struct ib_wc wcs[IB_POLL_BATCH];
> -
> -       return __ib_process_cq(cq, budget, wcs);
> +       if (unlikely(WARN_ON_ONCE(!cq->wc)))
> +               return 0;
> +       return __ib_process_cq(cq, budget);
> }
> EXPORT_SYMBOL(ib_process_cq_direct);
> 
> @@ -88,7 +88,7 @@ static int ib_poll_handler(struct irq_poll *iop, int budget)
>        struct ib_cq *cq = container_of(iop, struct ib_cq, iop);
>        int completed;
> 
> -       completed = __ib_process_cq(cq, budget, NULL);
> +       completed = __ib_process_cq(cq, budget);
>        if (completed < budget) {
>                irq_poll_complete(&cq->iop);
>                if (ib_req_notify_cq(cq, IB_POLL_FLAGS) > 0)
> @@ -108,7 +108,7 @@ 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, NULL);
> +       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);
> --

--
Chuck Lever