Re: [PATCH 4/5] ceph: flush the mdlog before waiting on unsafe reqs

[Xiubo Li <xiubli@redhat.com>]

On 6/30/21 8:13 PM, Jeff Layton wrote:
> On Wed, 2021-06-30 at 09:26 +0800, Xiubo Li wrote:
>> On 6/29/21 9:25 PM, Jeff Layton wrote:
>>> On Tue, 2021-06-29 at 12:42 +0800, xiubli@redhat.com wrote:
>>>> From: Xiubo Li <xiubli@redhat.com>
>>>>
>>>> For the client requests who will have unsafe and safe replies from
>>>> MDS daemons, in the MDS side the MDS daemons won't flush the mdlog
>>>> (journal log) immediatelly, because they think it's unnecessary.
>>>> That's true for most cases but not all, likes the fsync request.
>>>> The fsync will wait until all the unsafe replied requests to be
>>>> safely replied.
>>>>
>>>> Normally if there have multiple threads or clients are running, the
>>>> whole mdlog in MDS daemons could be flushed in time if any request
>>>> will trigger the mdlog submit thread. So usually we won't experience
>>>> the normal operations will stuck for a long time. But in case there
>>>> has only one client with only thread is running, the stuck phenomenon
>>>> maybe obvious and the worst case it must wait at most 5 seconds to
>>>> wait the mdlog to be flushed by the MDS's tick thread periodically.
>>>>
>>>> This patch will trigger to flush the mdlog in all the MDSes manually
>>>> just before waiting the unsafe requests to finish.
>>>>
>>>> Signed-off-by: Xiubo Li <xiubli@redhat.com>
>>>> ---
>>>>    fs/ceph/caps.c | 9 +++++++++
>>>>    1 file changed, 9 insertions(+)
>>>>
>>>> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
>>>> index c6a3352a4d52..6e80e4649c7a 100644
>>>> --- a/fs/ceph/caps.c
>>>> +++ b/fs/ceph/caps.c
>>>> @@ -2286,6 +2286,7 @@ static int caps_are_flushed(struct inode *inode, u64 flush_tid)
>>>>     */
>>>>    static int unsafe_request_wait(struct inode *inode)
>>>>    {
>>>> +	struct ceph_mds_client *mdsc = ceph_sb_to_client(inode->i_sb)->mdsc;
>>>>    	struct ceph_inode_info *ci = ceph_inode(inode);
>>>>    	struct ceph_mds_request *req1 = NULL, *req2 = NULL;
>>>>    	int ret, err = 0;
>>>> @@ -2305,6 +2306,14 @@ static int unsafe_request_wait(struct inode *inode)
>>>>    	}
>>>>    	spin_unlock(&ci->i_unsafe_lock);
>>>>    
>>>> +	/*
>>>> +	 * Trigger to flush the journal logs in all the MDSes manually,
>>>> +	 * or in the worst case we must wait at most 5 seconds to wait
>>>> +	 * the journal logs to be flushed by the MDSes periodically.
>>>> +	 */
>>>> +	if (req1 || req2)
>>>> +		flush_mdlog(mdsc);
>>>> +
>>> So this is called on fsync(). Do we really need to flush all of the mds
>>> logs on every fsync? That sounds like it might have some performance
>>> impact. Would it be possible to just flush the mdslog on the MDS that's
>>> authoritative for this inode?
>> I hit one case before, the mds.0 is the auth mds, but the client just
>> sent the request to mds.2, then when the mds.2 tried to gather the
>> rdlocks then it was stuck for waiting for the mds.0 to flush the mdlog.
>> I think it also will happen that if the mds.0 could also be stuck just
>> like this even its the auth MDS.
>>
> It sounds like mds.0 should flush its own mdlog in this situation once
> mds.2 started requesting locks that mds.0 was holding. Shouldn't it?

Yeah, it is. I have fixed this case. I am not sure whether there has 
some other situations just like this.

But they should be bugs in MDS.


>
>> Normally the mdlog submit thread will be triggered per MDS's tick,
>> that's 5 seconds. But this is not always true mostly because any other
>> client request could trigger the mdlog submit thread to run at any time.
>> Since the fsync is not running all the time, so IMO the performance
>> impact should be okay.
>>
>>
> I'm not sure I'm convinced.
>
> Consider a situation where we have a large(ish) ceph cluster with
> several MDSs. One client is writing to a file that is on mds.0 and there
> is little other activity there. Several other clients are doing heavy
> I/O on other inodes (of which mds.1 is auth).
>
> The first client then calls fsync, and now the other clients stall for a
> bit while mds.1 unnecessarily flushes its mdlog. I think we need to take
> care to only flush the mdlog for mds's that we care about here.

Okay, except the above case I mentioned I didn't find any case that 
could prevent us doing this.

Let me test more about it by just flushing the mdlog in auth MDS.

>
>
>>>>    	dout("unsafe_request_wait %p wait on tid %llu %llu\n",
>>>>    	     inode, req1 ? req1->r_tid : 0ULL, req2 ? req2->r_tid : 0ULL);
>>>>    	if (req1) {