Re: [PATCH 1/2] svcrdma: Release write chunk resources without re-queuing

[Chuck Lever <cel@kernel.org>]

On 5/7/26 4:46 PM, Mike Snitzer wrote:
> On Wed, May 06, 2026 at 11:26:50AM -0400, Chuck Lever wrote:
>> From: Chuck Lever <chuck.lever@oracle.com>
>>
>> Each RDMA Send completion triggers a cascade of work items on the
>> svcrdma_wq unbound workqueue:
>>
>>   ib_cq_poll_work (on ib_comp_wq, per-CPU)
>>     -> svc_rdma_send_ctxt_put -> queue_work    [work item 1]
>>       -> svc_rdma_write_info_free -> queue_work [work item 2]
>>
>> Every transition through queue_work contends on the unbound
>> pool's spinlock. Profiling an 8KB NFSv3 read/write workload
>> over RDMA shows about 4% of total CPU cycles spent on this
>> lock, with the cascading re-queue of write_info release
>> contributing roughly 1%.
>>
>> The initial queue_work in svc_rdma_send_ctxt_put is needed to
>> move release work off the CQ completion context (which runs on
>> a per-CPU bound workqueue). However, once executing on
>> svcrdma_wq, there is no need to re-queue for each write_info
>> structure. svc_rdma_reply_chunk_release already calls
>> svc_rdma_cc_release inline from the same svcrdma_wq context,
>> and svc_rdma_recv_ctxt_put does the same from nfsd thread
>> context.
>>
>> Release write chunk resources inline in
>> svc_rdma_write_info_free, removing the intermediate
>> svc_rdma_write_info_free_async work item and the wi_work
>> field from struct svc_rdma_write_info.
>>
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> 
> You were correct: this patch alone eliminates the OOM (we tested with
> both 16K and then 4K read IO from 121 clients to 8 NFS servers, no
> measurable memory growth while testing).

Excellent news! Thanks to you both for testing.


> Feel free to add:
> 
> Reviewed-by: Mike Snitzer <snitzer@kernel.org>
> Tested-by: Jonathan Flynn <jonathan.flynn@hammerspace.com>
> 
> Thanks!
> Mike
> 
> ps.
> So you are aware, couldn't test your 2nd patch at the customer site
> because the baseline kernel there is based on 6.12-stable but your
> 2nd patch builds on your 7.1 svcrdma changes. I think your 2nd patch
> is ideal though, and will be able to pull it in to test in future, but
> won't have the ability to test at this customer's scale until we can
> role that newer kernel out there... might take a couple months.

Understood.


-- 
Chuck Lever