> > > > ib_set_cq_udata() would transition into the kernel to pass in the
> > > > consumer's index. In addition, ib_req_notify_cq would also transition
> > > > into the kernel since its not a bypass function for chelsio.
> > >
> > > We misunderstand each other.
> > >
> > > ib_uverbs_req_notify_cq is in drivers/infiniband/core/uverbs_cmd.c -
> > > all this code runs inside the IB_USER_VERBS_CMD_REQ_NOTIFY_CQ command,
> > > so there is a single user to kernel transition.
> > >
> >
> > Oh I see.
> >
> > This seems like a lot of extra code to avoid passing one extra arg to
> > the driver's req_notify_cq verb. I'd appreciate other folk's input on
> > how important they think this is.
> >
> > If you insist, then I'll run some tests specifically in kernel mode and
> > see how this affects mthca's req_notify performance.
>
> This might be an interesting datapoint.
>
Here's what I measured:
Without extra param (1000 iterations in cycles):
ave 101.283 min 91 max 247
With extra param (1000 iterations in cycles):
ave 103.311 min 91 max 221
Convert cycles to ns (3466.727 MHz CPU):
Without: 101.283 / 3466.727 = .02922us == 29.22ns
With: 103.311 / 3466.727 = .02980us == 29.80ns
So I measure a .58ns average increase for passing in the additional
parameter.
Here is a snipit of the test:
spin_lock_irq(&lock);
do_gettimeofday(&start_tv);
for (i=0; i<1000; i++) {
cycles_start[i] = get_cycles();
ib_req_notify_cq(cb->cq, IB_CQ_NEXT_COMP);
cycles_stop[i] = get_cycles();
}
do_gettimeofday(&stop_tv);
spin_unlock_irq(&lock);
if (stop_tv.tv_usec < start_tv.tv_usec) {
stop_tv.tv_usec += 1000000;
stop_tv.tv_sec -= 1;
}
for (i=0; i < 1000; i++) {
cycles_t v = cycles_stop[i] - cycles_start[i];
sum += v;
if (v > max)
max = v;
if (min == 0 || v < min)
min = v;
}
printk(KERN_ERR PFX "FOO delta sec %lu usec %lu sum %llu min %llu max
%llu\n",
stop_tv.tv_sec - start_tv.tv_sec,
stop_tv.tv_usec - start_tv.tv_usec,
(unsigned long long)sum, (unsigned long long)min,
(unsigned long long)max);
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