Re: [RFC,PATCH 11/15] knfsd: RDMA transport core

[Tom Tucker <tom@opengridcomputing.com>]

On Mon, 2007-05-21 at 17:11 +1000, Neil Brown wrote:
> On Friday May 18, tom@opengridcomputing.com wrote:
> > 
> > This file implements the core transport data management and I/O
> > path. The I/O path for RDMA involves receiving callbacks on interrupt 
> > context. Since all the svc transport locks are _bh locks we enqueue the 
> > transport on a list, schedule a tasklet to dequeue data indications from 
> > the RDMA completion queue. The tasklet in turn takes _bh locks to 
> > enqueue receive data indications on a list for the transport. The 
> > svc_rdma_recvfrom transport function dequeues data from this list in an 
> > NFSD thread context.
> 
> Cannot we simply change the usage of ->sp_lock to always disable
> interrupts?
> That would make this much simpler.  How much would it cost?

I don't think they are particularly expensive to acquire, but they add
incremental interrupt latency to the system. My impression is that the
general guidance from Linus et al is to avoid them if not absolutely
necessary. Trond's comments I think reflected this viewpoint as well,
but perhaps I'm just solving a problem that doesn't exist.

> 
> Alternatively, why can the network layer deliver these notification in
> "bh" context, but the ib layer wants to deliver them in irq context?
> Does doing it in irq context have lower latency or something?
> 

Yes, latency was certainly the design criteria in OFA. In practice, I
think we're talking nanoseconds (i.e. interrupt handler vs. tasklet),
however, when you're working with minimum latencies in the 2.8us range,
nanoseconds become statistically significant.

For our purposes, I don't think the latency is an issue. 

> NeilBrown


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