by Chuck Lever III

[permalink] [raw]

Subject: Re: 5.4.188 and later: massive performance regression with nfsd

> On May 20, 2022, at 12:40 PM, Trond Myklebust <[email protected]> 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
>>> <[email protected]> wrote:
>>>
>>>
>>>
>>>> On May 11, 2022, at 10:23 AM, Greg KH
>>>> <[email protected]> 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
>>>>>> <[email protected]> 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?

--
Chuck Lever

2022-05-23 06:40:06

by Chuck Lever III

[permalink] [raw]

Subject: Re: 5.4.188 and later: massive performance regression with nfsd

> On May 20, 2022, at 7:43 PM, Chuck Lever III <[email protected]> wrote:
>
>
>
>> On May 20, 2022, at 6:24 PM, Trond Myklebust <[email protected]> wrote:
>>
>> On Fri, 2022-05-20 at 21:52 +0000, Chuck Lever III wrote:
>>>
>>>
>>>> On May 20, 2022, at 12:40 PM, Trond Myklebust
>>>> <[email protected]> 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
>>>>>> <[email protected]> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>> On May 11, 2022, at 10:23 AM, Greg KH
>>>>>>> <[email protected]> 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
>>>>>>>>> <[email protected]> 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);
}

--
Chuck Lever