> On Nov 8, 2016, at 3:20 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>
> On Tue, Nov 08, 2016 at 03:13:13PM -0500, Chuck Lever wrote:
>>
>>> On Nov 8, 2016, at 3:03 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>>>
>>> On Sat, Oct 29, 2016 at 12:21:03PM -0400, Chuck Lever wrote:
>>>> Hi Bruce-
>>>
>>> Sorry for the slow response!
>>>
>>> ...
>>>> In commit 39a9beab5acb83176e8b9a4f0778749a09341f1f ('rpc: share one xps between
>>>> all backchannels') you added:
>>>>
>>>> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
>>>> index f5572e3..4f01f63 100644
>>>> --- a/net/sunrpc/svc_xprt.c
>>>> +++ b/net/sunrpc/svc_xprt.c
>>>> @@ -136,6 +136,8 @@ static void svc_xprt_free(struct kref *kref)
>>>> /* See comment on corresponding get in xs_setup_bc_tcp(): */
>>>> if (xprt->xpt_bc_xprt)
>>>> xprt_put(xprt->xpt_bc_xprt);
>>>> + if (xprt->xpt_bc_xps)
>>>> + xprt_switch_put(xprt->xpt_bc_xps);
>>>> xprt->xpt_ops->xpo_free(xprt);
>>>> module_put(owner);
>>>> }
>>>>
>>>> svc_xprt_free() is invoked by svc_xprt_put(). svc_xprt_put() is called
>>>> from svc_rdma in soft IRQ context (eg. svc_rdma_wc_receive).
>>>
>>> Is that necessary? I wonder why the svcrdma code seems to be taking so
>>> many of its own references on svc_xprts.
>>
>> The idea is to keep the xprt around while Work Requests (I/O) are running,
>> so that the xprt is guaranteed to be there during work completions. The
>> completion handlers (where svc_xprt_put is often invoked) run in soft IRQ
>> context.
>>
>> It's simple to change completions to use a Work Queue instead, but testing
>> so far shows that will result in a performance loss. I'm still studying it.
>>
>> Is there another way to keep the xprt's ref count boosted while I/O is
>> going on?
>
> Why do you need the svc_xprt in the completion?
1. The svc_xprt contains the svcrdma_xprt, which contains the Send Queue
accounting mechanism. SQ accounting has to be updated for each completion:
the completion indicates that the SQ entry used by this Work Request is
now available, and that other callers waiting for enough SQEs can go ahead.
2. When handling a Receive completion, the incoming RPC message is enqueued
on the svc_xprt via svc_xprt_enqueue, unless RDMA Reads are needed.
3. When handling a Read completion, the incoming RPC message is enqueued on
the svc_xprt via svc_xprt_enqueue.
> Can the xpo_detach method wait for any pending completions?
So the completions would call wake_up on a waitqueue, or use a kernel
completion? That sounds more expensive than managing an atomic reference
count.
I suppose some other reference count could be used to trigger a work item
that would invoke xpo_detach.
But based on your comments, then, svc_xprt_put() was never intended to be
BH-safe.
> --b.
>
>>
>>
>>>> However, xprt_switch_put() takes a spin lock (xps_lock) which is locked
>>>> everywhere without disabling BHs.
>>>>
>>>> It looks to me like 39a9beab5acb makes svc_xprt_put() no longer BH-safe?
>>>> Not sure if svc_xprt_put() was intended to be BH-safe beforehand.
>>>>
>>>> Maybe xprt_switch_put() could be invoked in ->xpo_free, but that seems
>>>> like a temporary solution.
>>>
>>> Since xpo_free is also called from svc_xprt_put that doesn't sound like
>>> it would change anything. Or do we not trunk over RDMA for some reason?
>>
>> It's quite desirable to trunk NFS/RDMA on multiple connections, and it
>> should work just like it does for TCP, but so far it's not been tested.
--
Chuck Lever
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