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Subject: Re: [RFC PATCH 5/5] SUNRPC: Reduce the priority of the xprtiod queue
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <2fd3177890a8c8fba9b40468df213bafa30b5481.camel@hammerspace.com>
Date:   Tue, 28 May 2019 15:52:58 -0400
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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To:     Trond Myklebust <trondmy@hammerspace.com>
Sender: linux-nfs-owner@vger.kernel.org
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> On May 28, 2019, at 3:33 PM, Trond Myklebust <trondmy@hammerspace.com> =
wrote:
>=20
> On Tue, 2019-05-28 at 15:03 -0400, Chuck Lever wrote:
>> Following up on this. Now with even more data!
>>=20
>>> On May 6, 2019, at 4:41 PM, Chuck Lever <chuck.lever@oracle.com>
>>> wrote:
>>>=20
>>>=20
>>>> On May 3, 2019, at 7:18 AM, Trond Myklebust <trondmy@gmail.com>
>>>> wrote:
>>>>=20
>>>> Allow more time for softirqd
>>>=20
>>> Have you thought about performance tests for this one?
>>=20
>> I tested this series on my 12-core two-socket client using a variety
>> of tests including iozone, fio, and fstests. The network under test
>> is 56Gb InfiniBand (TCP uses IPoIB). I tested both TCP and RDMA.
>>=20
>> With lock debugging and memory leak testing enabled, I did not see
>> any functional regressions or new leaks or crashes. Thus IMO this
>> series is "safe to apply."
>>=20
>> With TCP, I saw no change in performance between a "stock" kernel
>> and one with all five patches in this series applied, as, IIRC,
>> you predicted.
>>=20
>> The following discussion is based on testing with NFS/RDMA.
>>=20
>> With RDMA, I saw an improvement of 5-10% in IOPS rate between the
>> "stock" kernel and a kernel with the first four patches applied. When
>> the fifth patch is applied, I saw IOPS throughput significantly worse
>> than "stock" -- like 20% worse.
>>=20
>> I also studied average RPC execution time (the "execute" metric) with
>> the "stock" kernel, the one with four patches applied, and with the
>> one where all five are applied. The workload is 100% 4KB READs with
>> an iodepth of 1024 in order to saturate the transmit queue.
>>=20
>> With four patches, the execute time is about 2.5 msec faster (average
>> execution time is around 75 msec due to the large backlog this test
>> generates). With five patches, it's slower than "stock" by 12 msec.
>>=20
>> I also saw a 30 usec improvement in the average latency of
>> xprt_complete_rqst with the four patch series.
>>=20
>> As far as I can tell, the benefit of this series comes mostly from
>> the third patch, which changes spin_lock_bh(&xprt->transport_lock) to
>> spin_lock(&xprt->transport_lock). When the xprtiod work queue is
>> lowered in priority in 5/5, that benefit vanishes.
>>=20
>> I am still confused about why 5/5 is needed. I did not see any soft
>> lockups without this patch applied when using RDMA. Is the issue
>> with xprtsock's use of xprtiod for handling incoming TCP receives?
>>=20
>> I still have some things I'd like to look at. One thing I haven't
>> yet tried is looking at lock_stat, which would confirm or refute
>> my theory that this is all about the transport_lock, for instance.
>>=20
>=20
> OK. I can drop 5/5.
>=20
> The issue there was not about soft lockups. However since we were
> previously running most soft irqs as part of spin_unlock_bh(), the
> question was whether or not we would see more of them needing to move
> to softirqd. As far as I can see, your answer to that question is 'no'
> (at least for your system).

The top contended lock now is the work queue lock. I believe that's a
full irqsave lock. Someone should try testing on a single core system.

I also plan to try this series on my mlx5_en system. The mlx5 Ethernet
driver does a lot more work in soft IRQ than mlx4/IB does.


--
Chuck Lever