Date:   Sat, 21 May 2022 19:11:49 +0000
From:   Chuck Lever III <chuck.lever@...cle.com>
To:     Trond Myklebust <trondmy@...merspace.com>
CC:     "jlayton@...nel.org" <jlayton@...nel.org>,
        "linux@...m.de" <linux@...m.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>
Subject: Re: 5.4.188 and later: massive performance regression with nfsd



> On May 21, 2022, at 2:10 PM, Trond Myklebust <trondmy@...merspace.com> wrote:
> 
> On Sat, 2022-05-21 at 17:22 +0000, Chuck Lever III wrote:
>> 
>> 
>>> On May 20, 2022, at 7:43 PM, Chuck Lever III
>>> <chuck.lever@...cle.com> wrote:
>>> 
>>> 
>>> 
>>>> On May 20, 2022, at 6:24 PM, Trond Myklebust
>>>> <trondmy@...merspace.com> wrote:
>>>> 
>>>> On Fri, 2022-05-20 at 21:52 +0000, Chuck Lever III wrote:
>>>>> 
>>>>> 
>>>>>> On May 20, 2022, at 12:40 PM, Trond Myklebust
>>>>>> <trondmy@...merspace.com> wrote:
>>>>>> 
>>>>>> On Fri, 2022-05-20 at 15:36 +0000, Chuck Lever III wrote:
>>>>>>> 
>>>>>>> 
>>>>>>>> On May 11, 2022, at 10:36 AM, Chuck Lever III
>>>>>>>> <chuck.lever@...cle.com> wrote:
>>>>>>>> 
>>>>>>>> 
>>>>>>>> 
>>>>>>>>> On May 11, 2022, at 10:23 AM, Greg KH
>>>>>>>>> <gregkh@...uxfoundation.org> wrote:
>>>>>>>>> 
>>>>>>>>> On Wed, May 11, 2022 at 02:16:19PM +0000, Chuck Lever
>>>>>>>>> III
>>>>>>>>> wrote:
>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>>> On May 11, 2022, at 8:38 AM, Greg KH
>>>>>>>>>>> <gregkh@...uxfoundation.org> wrote:
>>>>>>>>>>> 
>>>>>>>>>>> On Wed, May 11, 2022 at 12:03:13PM +0200, Wolfgang
>>>>>>>>>>> Walter
>>>>>>>>>>> wrote:
>>>>>>>>>>>> Hi,
>>>>>>>>>>>> 
>>>>>>>>>>>> starting with 5.4.188 wie see a massive
>>>>>>>>>>>> performance
>>>>>>>>>>>> regression on our
>>>>>>>>>>>> nfs-server. It basically is serving requests very
>>>>>>>>>>>> very
>>>>>>>>>>>> slowly with cpu
>>>>>>>>>>>> utilization of 100% (with 5.4.187 and earlier it
>>>>>>>>>>>> is
>>>>>>>>>>>> 10%) so
>>>>>>>>>>>> that it is
>>>>>>>>>>>> unusable as a fileserver.
>>>>>>>>>>>> 
>>>>>>>>>>>> The culprit are commits (or one of it):
>>>>>>>>>>>> 
>>>>>>>>>>>> c32f1041382a88b17da5736886da4a492353a1bb "nfsd:
>>>>>>>>>>>> cleanup
>>>>>>>>>>>> nfsd_file_lru_dispose()"
>>>>>>>>>>>> 628adfa21815f74c04724abc85847f24b5dd1645 "nfsd:
>>>>>>>>>>>> Containerise filecache
>>>>>>>>>>>> laundrette"
>>>>>>>>>>>> 
>>>>>>>>>>>> (upstream
>>>>>>>>>>>> 36ebbdb96b694dd9c6b25ad98f2bbd263d022b63 and
>>>>>>>>>>>> 9542e6a643fc69d528dfb3303f145719c61d3050)
>>>>>>>>>>>> 
>>>>>>>>>>>> If I revert them in v5.4.192 the kernel works as
>>>>>>>>>>>> before
>>>>>>>>>>>> and
>>>>>>>>>>>> performance is
>>>>>>>>>>>> ok again.
>>>>>>>>>>>> 
>>>>>>>>>>>> I did not try to revert them one by one as any
>>>>>>>>>>>> disruption
>>>>>>>>>>>> of our nfs-server
>>>>>>>>>>>> is a severe problem for us and I'm not sure if
>>>>>>>>>>>> they are
>>>>>>>>>>>> related.
>>>>>>>>>>>> 
>>>>>>>>>>>> 5.10 and 5.15 both always performed very badly on
>>>>>>>>>>>> our
>>>>>>>>>>>> nfs-
>>>>>>>>>>>> server in a
>>>>>>>>>>>> similar way so we were stuck with 5.4.
>>>>>>>>>>>> 
>>>>>>>>>>>> I now think this is because of
>>>>>>>>>>>> 36ebbdb96b694dd9c6b25ad98f2bbd263d022b63
>>>>>>>>>>>> and/or 9542e6a643fc69d528dfb3303f145719c61d3050
>>>>>>>>>>>> though
>>>>>>>>>>>> I
>>>>>>>>>>>> didn't tried to
>>>>>>>>>>>> revert them in 5.15 yet.
>>>>>>>>>>> 
>>>>>>>>>>> Odds are 5.18-rc6 is also a problem?
>>>>>>>>>> 
>>>>>>>>>> We believe that
>>>>>>>>>> 
>>>>>>>>>> 6b8a94332ee4 ("nfsd: Fix a write performance
>>>>>>>>>> regression")
>>>>>>>>>> 
>>>>>>>>>> addresses the performance regression. It was merged
>>>>>>>>>> into
>>>>>>>>>> 5.18-
>>>>>>>>>> rc.
>>>>>>>>> 
>>>>>>>>> And into 5.17.4 if someone wants to try that release.
>>>>>>>> 
>>>>>>>> I don't have a lot of time to backport this one myself,
>>>>>>>> so
>>>>>>>> I welcome anyone who wants to apply that commit to their
>>>>>>>> favorite LTS kernel and test it for us.
>>>>>>>> 
>>>>>>>> 
>>>>>>>>>>> If so, I'll just wait for the fix to get into
>>>>>>>>>>> Linus's
>>>>>>>>>>> tree as
>>>>>>>>>>> this does
>>>>>>>>>>> not seem to be a stable-tree-only issue.
>>>>>>>>>> 
>>>>>>>>>> Unfortunately I've received a recent report that the
>>>>>>>>>> fix
>>>>>>>>>> introduces
>>>>>>>>>> a "sleep while spinlock is held" for NFSv4.0 in rare
>>>>>>>>>> cases.
>>>>>>>>> 
>>>>>>>>> Ick, not good, any potential fixes for that?
>>>>>>>> 
>>>>>>>> Not yet. I was at LSF last week, so I've just started
>>>>>>>> digging
>>>>>>>> into this one. I've confirmed that the report is a real
>>>>>>>> bug,
>>>>>>>> but we still don't know how hard it is to hit it with
>>>>>>>> real
>>>>>>>> workloads.
>>>>>>> 
>>>>>>> We believe the following, which should be part of the first
>>>>>>> NFSD pull request for 5.19, will properly address the
>>>>>>> splat.
>>>>>>> 
>>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/commit/?h=for-next&id=556082f5e5d7ecfd0ee45c3641e2b364bff9ee44
>>>>>>> 
>>>>>>> 
>>>>>> Uh... What happens if you have 2 simultaneous calls to
>>>>>> nfsd4_release_lockowner() for the same file? i.e. 2 separate
>>>>>> processes
>>>>>> owned by the same user, both locking the same file.
>>>>>> 
>>>>>> Can't that cause the 'putlist' to get corrupted when both
>>>>>> callers
>>>>>> add
>>>>>> the same nf->nf_putfile to two separate lists?
>>>>> 
>>>>> IIUC, cl_lock serializes the two RELEASE_LOCKOWNER calls.
>>>>> 
>>>>> The first call finds the lockowner in cl_ownerstr_hashtbl and
>>>>> unhashes it before releasing cl_lock.
>>>>> 
>>>>> Then the second cannot find that lockowner, thus it can't
>>>>> requeue it for bulk_put.
>>>>> 
>>>>> Am I missing something?
>>>> 
>>>> In the example I quoted, there are 2 separate processes running
>>>> on the
>>>> client. Those processes could share the same open owner + open
>>>> stateid,
>>>> and hence the same struct nfs4_file, since that depends only on
>>>> the
>>>> process credentials matching. However they will not normally
>>>> share a
>>>> lock owner, since POSIX does not expect different processes to
>>>> share
>>>> locks.
>>>> 
>>>> IOW: The point is that one can relatively easily create 2
>>>> different
>>>> lock owners with different lock stateids that share the same
>>>> underlying
>>>> struct nfs4_file.
>>> 
>>> Is there a similar exposure if two different clients are locking
>>> the same file? If so, then we can't use a per-nfs4_client semaphore
>>> to serialize access to the nf_putfile field.
>> 
>> I had a thought about an alternate approach.
>> 
>> Create a second nfsd_file_put API that is not allowed to sleep.
>> Let's call it "nfsd_file_put_async()". Teach check_for_locked()
>> to use that instead of nfsd_file_put().
>> 
>> Here's where I'm a little fuzzy: nfsd_file_put_async() could do
>> something like:
>> 
>> void nfsd_file_put_async(struct nfsd_file *nf)
>> {
>>         if (refcount_dec_and_test(&nf->nf_ref))
>>                 nfsd_file_close_inode(nf->nf_inode);
>> }
>> 
>> 
> 
> That approach moves the sync to the garbage collector, which was
> exactly what we're trying to avoid in the first place.

Totally understood.

My thought was that "put" for RELEASE_LOCKOWNER/FREE_STATEID
would be unlikely to have any data to sync -- callers that
actually have data to flush are elsewhere, and those would
continue to use the synchronous nfsd_file_put() API.

Do you have a workload where we can test this assumption?


> Why not just do this "check_for_locks()" thing differently?
> 
> It really shouldn't be too hard to add something to
> nfsd4_lm_get_owner()/nfsd4_lm_put_owner() that bumps a counter in the
> lockowner in order to tell you whether or not locks are still held
> instead of doing this bone headed walk through the list of locks.

I thought of that a couple weeks ago. That doesn't work
because you can lock or unlock by range. That means the
symmetry of LOCK and LOCKU is not guaranteed, and I don't
believe these calls are used that way anyway. So I
abandoned the idea of using get_owner / put_owner.

But maybe we can provide some other mechanism to record
whether a lockowner is associated with file locks.


--
Chuck Lever

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