Subject: Re: [PATCH] SIW: Object management

From: Steve Wise <[email-masked]>
Date: Tue, 05 Oct 2010 10:37:49 -0500

On 10/05/2010 10:25 AM, Bernard Metzler wrote:
> Steve Wise<[email-masked]>  wrote on 10/05/2010 05:02:37 PM:
>
>    
>> Steve Wise<[email-masked]>
>> 10/05/2010 05:02 PM
>>
>> To
>>
>> Bernard Metzler<[email-masked]>
>>
>> cc
>>
>> [email-masked], [email-masked]
>>
>> Subject
>>
>> Re: [PATCH] SIW: Object management
>>
>> On 10/05/2010 09:56 AM, Bernard Metzler wrote:
>>      
>>> Steve Wise<[email-masked]>   wrote on 10/05/2010 04:26:48
>>>        
> PM:
>    
>>>
>>>        
>>>> Steve Wise<[email-masked]>
>>>> 10/05/2010 04:26 PM
>>>>
>>>> To
>>>>
>>>> Bernard Metzler<[email-masked]>
>>>>
>>>> cc
>>>>
>>>> [email-masked], [email-masked]
>>>>
>>>> Subject
>>>>
>>>> Re: [PATCH] SIW: Object management
>>>>
>>>> On 10/05/2010 01:54 AM, Bernard Metzler wrote:
>>>>
>>>> <snip>+
>>>>
>>>>          
>>>>> +
>>>>> +/***** routines for WQE handling ***/
>>>>> +
>>>>> +/*
>>>>> + * siw_wqe_get()
>>>>> + *
>>>>> + * Get new WQE. For READ RESPONSE, take it from the free list which
>>>>> + * has a maximum size of maximum inbound READs. All other WQE are
>>>>> + * malloc'ed which creates some overhead. Consider change to
>>>>> + *
>>>>> + * 1. malloc WR only if it cannot be synchonously completed, or
>>>>> + * 2. operate own cache of reuseable WQE's.
>>>>> + *
>>>>> + * Current code trusts on malloc efficiency.
>>>>> + */
>>>>> +inline struct siw_wqe *siw_wqe_get(struct siw_qp *qp, enum
>>>>>
>>>>>            
>>>> siw_wr_opcode op)
>>>>
>>>>          
>>>>> +{
>>>>> +   struct siw_wqe *wqe;
>>>>> +
>>>>> +   if (op == SIW_WR_RDMA_READ_RESP) {
>>>>> +      spin_lock(&qp->freelist_lock);
>>>>> +      if (!(list_empty(&qp->wqe_freelist))) {
>>>>> +         wqe = list_entry(qp->wqe_freelist.next,
>>>>> +                struct siw_wqe, list);
>>>>> +         list_del(&wqe->list);
>>>>> +         spin_unlock(&qp->freelist_lock);
>>>>> +         wqe->processed = 0;
>>>>> +         dprint(DBG_OBJ|DBG_WR,
>>>>> +            "(QP%d): WQE from FreeList p: %p\n",
>>>>> +            QP_ID(qp), wqe);
>>>>> +      } else {
>>>>> +         spin_unlock(&qp->freelist_lock);
>>>>> +         wqe = NULL;
>>>>> +         dprint(DBG_ON|DBG_OBJ|DBG_WR,
>>>>> +            "(QP%d): FreeList empty!\n", QP_ID(qp));
>>>>> +      }
>>>>> +   } else {
>>>>> +      wqe = kzalloc(sizeof(struct siw_wqe), GFP_KERNEL);
>>>>> +      dprint(DBG_OBJ|DBG_WR, "(QP%d): New WQE p: %p\n",
>>>>> +         QP_ID(qp), wqe);
>>>>> +   }
>>>>>
>>>>>
>>>>>            
>>>> I think you can't allocate at GFP_KERNEL here if this is called from
>>>>          
> the
>    
>>>>          
>>>        
>>>> post_ functions.  I think you might want to pre-allocate these when
>>>>          
> you
>    
>>>> create the QP...
>>>>
>>>>
>>>>          
>>> the idea was to keep the memory footprint small and flexible
>>> while using the linux/list.h routines to manipulate all queues
>>> (no ring buffers etc, just lists). at the same time we
>>> decided to take the provided uverbs_cmd-syscall path down to
>>> the driver even for the post_-functions (since we would have to ring a
>>> doorbell on the send path anyway, which in software, is a syscall).
>>> in that path, even ib_uverbs_post_send() does one kmalloc() per wr
>>> (it would be helpful if the provider could keep and reuse that wr of
>>> known size, freeing it later at its own premises. that would avoid
>>> the second kmalloc here.)
>>>
>>> currently only work queue elements which are needed to satisfy
>>> inbound read requests are pre-allocated (amount corresponding
>>> to inbound read queue depth), since the read response is
>>> scheduled in network softirq context which must not sleep.
>>>
>>> that discussion may relate to the spinlock at the entrance to the
>>> post_ verbs. going down the uverbs_cmd path may sleep anyway...?
>>>
>>>
>>>        
>>
>> The uverb calls may sleep, but certain kernel verbs must not.  Remember,
>>      
>    
>> the post_send/recv and other functions in your driver are called
>> directly (almost) by kernel users like NFSRDMA.  These users may be
>> calling in an interrupt context and thus you cannot block/sleep.
>>
>>      
> OK, very convincing. not a big change since siw_wqe_get/_put()
> already maintain a list of pre-allocated wqe's (currently for
> the read.responses).
> but, would it be ok if the code distinguishes between user
> land and in-kernel consumers? i would be very happy if we could
> keep the pre-allocations per user land connection to its very
> minimum...
>
>    

I think that's ok, but its bending the core locking rules a little I 
guess.  But the intent is that kernel users can definitely 
send/recv/poll in interrupt context, so possibly blocking for user mode 
QPs in on-kernel-bypass operations is probably ok...

What do you think Roland?

Steve.