> On Aug 29, 2016, at 10:23 AM, Anna Schumaker <Anna.Schumaker@xxxxxxxxxx> wrote:
>
> Hi Chuck,
>
> On 08/23/2016 01:52 PM, Chuck Lever wrote:
>> For xprtrdma, the RPC Call and Reply buffers are involved in real
>> I/O operations.
>>
>> To start with, the DMA direction of the I/O for a Call is opposite
>> that of a Reply.
>>
>> In the current arrangement, the Reply buffer address is on a
>> four-byte alignment just past the call buffer. Would be friendlier
>> on some platforms if that was at a DMA cache alignment instead.
>>
>> Because the current arrangement allocates a single memory region
>> which contains both buffers, the RPC Reply buffer often contains a
>> page boundary in it when the Call buffer is large enough (which is
>> frequent).
>>
>> It would be a little nicer for setting up DMA operations (and
>> possible registration of the Reply buffer) if the two buffers were
>> separated, well-aligned, and contained as few page boundaries as
>> possible.
>>
>> Now, I could just pad out the single memory region used for the pair
>> of buffers. But frequently that would mean a lot of unused space to
>> ensure the Reply buffer did not have a page boundary.
>>
>> Add a separate pointer to rpc_rqst that points right to the RPC
>> Reply buffer. This makes no difference to xprtsock, but it will help
>> xprtrdma in subsequent patches.
>>
>> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>> ---
>> include/linux/sunrpc/xprt.h | 5 +++--
>> net/sunrpc/clnt.c | 2 +-
>> net/sunrpc/sched.c | 1 +
>> net/sunrpc/xprtrdma/transport.c | 1 +
>> 4 files changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
>> index 72c2aeb..46f069e 100644
>> --- a/include/linux/sunrpc/xprt.h
>> +++ b/include/linux/sunrpc/xprt.h
>> @@ -84,8 +84,9 @@ struct rpc_rqst {
>> struct list_head rq_list;
>>
>> void *rq_buffer; /* Call XDR encode buffer */
>> - size_t rq_callsize,
>> - rq_rcvsize;
>> + size_t rq_callsize;
>> + void *rq_rbuffer; /* Reply XDR decode buffer */
>> + size_t rq_rcvsize;
>
> Just a nit-picky question :) Is there any reason that you're adding the buffer between rq_callsize and rq_rcvsize? It seems like you could leave those alone and add the pointer either before or after them.
Hi Anna-
Keeping related fields together is usually more important than an extra
line or two in a commit. At the very least, the function of these fields
is more clear (to me, anyway) in this order.
Generally it's good practice to keep together structure fields that are
used at the same time. These four fields might appear in the same CPU
cacheline, though that can change as fields are introduced or removed
earlier in struct rpc_rqst.
An argument can be made that the code is just as easy to read this way:
void *rq_buffer, *rq_rbuffer;
size_t rq_callsize, rq_rcvsize;
If that's your preference as maintainer, I will change it in the next
version of this series.
> Thanks,
> Anna
>
>> size_t rq_xmit_bytes_sent; /* total bytes sent */
>> size_t rq_reply_bytes_recvd; /* total reply bytes */
>> /* received */
>> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
>> index ab467c0..fd389c0 100644
>> --- a/net/sunrpc/clnt.c
>> +++ b/net/sunrpc/clnt.c
>> @@ -1768,7 +1768,7 @@ rpc_xdr_encode(struct rpc_task *task)
>> req->rq_buffer,
>> req->rq_callsize);
>> xdr_buf_init(&req->rq_rcv_buf,
>> - (char *)req->rq_buffer + req->rq_callsize,
>> + req->rq_rbuffer,
>> req->rq_rcvsize);
>>
>> p = rpc_encode_header(task);
>> diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
>> index 6690ebc..5db68b3 100644
>> --- a/net/sunrpc/sched.c
>> +++ b/net/sunrpc/sched.c
>> @@ -891,6 +891,7 @@ int rpc_malloc(struct rpc_task *task)
>> dprintk("RPC: %5u allocated buffer of size %zu at %p\n",
>> task->tk_pid, size, buf);
>> rqst->rq_buffer = buf->data;
>> + rqst->rq_rbuffer = (char *)rqst->rq_buffer + rqst->rq_callsize;
>> return 0;
>> }
>> EXPORT_SYMBOL_GPL(rpc_malloc);
>> diff --git a/net/sunrpc/xprtrdma/transport.c b/net/sunrpc/xprtrdma/transport.c
>> index ebf14ba..136caf3 100644
>> --- a/net/sunrpc/xprtrdma/transport.c
>> +++ b/net/sunrpc/xprtrdma/transport.c
>> @@ -524,6 +524,7 @@ out:
>> dprintk("RPC: %s: size %zd, request 0x%p\n", __func__, size, req);
>> req->rl_connect_cookie = 0; /* our reserved value */
>> rqst->rq_buffer = req->rl_sendbuf->rg_base;
>> + rqst->rq_rbuffer = (char *)rqst->rq_buffer + rqst->rq_rcvsize;
>> return 0;
>>
>> out_rdmabuf:
>>
>> --
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>
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--
Chuck Lever
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