Messages in this thread | | | Date | Wed, 3 Jan 2007 16:41:53 +0200 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH v4 01/13] Linux RDMA Core Changes |
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> > > @@ -1373,7 +1374,7 @@ int ib_peek_cq(struct ib_cq *cq, int wc_ > > > static inline int ib_req_notify_cq(struct ib_cq *cq, > > > enum ib_cq_notify cq_notify) > > > { > > > - return cq->device->req_notify_cq(cq, cq_notify); > > > + return cq->device->req_notify_cq(cq, cq_notify, NULL); > > > } > > > > > > /** > > > > Can't say I like this adding overhead in data path operations (and note this > > can't be optimized out). And kernel consumers work without passing it in, so it > > hurts kernel code even for Chelsio. Granted, the cost is small here, but these > > things do tend to add up. > > > > It seems all Chelsio needs is to pass in a consumer index - so, how about a new > > entry point? Something like void set_cq_udata(struct ib_cq *cq, struct ib_udata *udata)? > > > > Adding a new entry point would hurt chelsio's user mode performance if > if then requires 2 kernel transitions to rearm the cq.
No, it won't need 2 transitions - just an extra function call, so it won't hurt performance - it would improve performance.
ib_uverbs_req_notify_cq would call
ib_uverbs_req_notify_cq() { ib_set_cq_udata(cq, udata) ib_req_notify_cq(cq, cmd.solicited_only ? IB_CQ_SOLICITED : IB_CQ_NEXT_COMP); }
This way kernel consumers don't incur any overhead, and in userspace users extra function call is dwarfed by system call overhead.
> Passing in user data is sort of SOP for these sorts of verbs.
I don't see other examples. Where we did pass extra user data is in non-data pass verbs such as create QP.
This is most inner tight loop in many ULPs, so we should be very careful about adding code there - these things do add up. See recent IRQ API update in kernel.
> How much does passing one more param cost for kernel users?
Donnu. I just reviewed the code. It really should be up to patch submitter to check the performance effect of his patch, if there might be any.
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