On May 21, 2022, at 2:10 PM, Trond Myklebust trondmy@hammerspace.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@oracle.com wrote:
On May 20, 2022, at 6:24 PM, Trond Myklebust trondmy@hammerspace.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@hammerspace.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@oracle.com wrote: >> >> >> >>> On May 11, 2022, at 10:23 AM, Greg KH >>> gregkh@linuxfoundation.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@linuxfoundation.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-... > > 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