Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot()

Chuck Lever
2019-01-14T17:24:34Z

> On Jan 11, 2019, at 7:56 PM, Bruce Fields <bfields@fieldses.org> wrote:
>
> On Fri, Jan 11, 2019 at 05:27:30PM -0500, Chuck Lever wrote:
>>
>>
>>> On Jan 11, 2019, at 5:10 PM, Bruce Fields <bfields@fieldses.org> wrote:
>>>
>>> On Fri, Jan 11, 2019 at 04:54:01PM -0500, Chuck Lever wrote:
>>>>> On Jan 11, 2019, at 4:52 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
>>>>>> So, I think we need your patch plus something like this.
>>>>>>
>>>>>> Chuck, maybe you could help me with the "XXX: Chuck:" parts?
>>>>>
>>>>> I haven't been following. Why do you think those are necessary?
>>>
>>> I'm worried something like this could happen:
>>>
>>> CPU 1 CPU 2
>>> ----- -----
>>>
>>> set XPT_DATA dec xpt_nr_rqsts
>>>
>>> svc_xprt_enqueue svc_xprt_enqueue
>>>
>>> And both decide nothing should be done if neither sees the change that
>>> the other made.
>>>
>>> Maybe I'm still missing some reason that couldn't happen.
>>>
>>> Even if it can happen, it's an unlikely race that will likely be fixed
>>> when another event comes along a little later, which would explain why
>>> we've never seen any reports.
>>>
>>>>> We've had set_bit and atomic_{inc,dec} in this code for ages,
>>>>> and I've never noticed a problem.
>>>>>
>>>>> Rather than adding another CPU pipeline bubble in the RDMA code,
>>>>> though, could you simply move the set_bit() call site inside the
>>>>> critical sections?
>>>>
>>>> er, inside the preceding critical section. Just reverse the order
>>>> of the spin_unlock and the set_bit.
>>>
>>> That'd do it, thanks!
>>
>> I can try that here and see if it results in a performance regression.
>
> Thanks, I've got a version with a typo fixed at
>
> git://linux-nfs.org/~bfields/linux.git nfsd-next

Applied all four patches here. I don't see any performance regressions,
but my server has only a single last-level CPU cache.

--
Chuck Lever