Re: [PATCH] ceph: request expedited service when flushing caps

[Luis Henriques <lhenriques@suse.com>]

On Tue, Mar 31, 2020 at 06:52:23AM -0400, Jeff Layton wrote:
> Jan noticed some long stalls when flushing caps using sync() after
> doing small file creates. For instance, running this:
> 
>     $ time for i in $(seq -w 11 30); do echo "Hello World" > hello-$i.txt; sync -f ./hello-$i.txt; done
> 
> Could take more than 90s in some cases. The sync() will flush out caps,
> but doesn't tell the MDS that it's waiting synchronously on the
> replies.
> 
> When ceph_check_caps finds that CHECK_CAPS_FLUSH is set, then set the
> CEPH_CLIENT_CAPS_SYNC bit in the cap update request. This clues the MDS
> into that fact and it can then expedite the reply.
> 
> URL: https://tracker.ceph.com/issues/44744
> Reported-and-Tested-by: Jan Fajerski <jfajerski@suse.com>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>

Feel free to add my Reviewed-by (and also tested).  Also, it may be worth
adding a stable tag for this patch, although it would require some
massaging to use __send_cap instead of __prep_cap.

Cheers,
--
Luis

> ---
>  fs/ceph/caps.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> index 61808793e0c0..6403178f2376 100644
> --- a/fs/ceph/caps.c
> +++ b/fs/ceph/caps.c
> @@ -2111,8 +2111,11 @@ void ceph_check_caps(struct ceph_inode_info *ci, int flags,
>  
>  		mds = cap->mds;  /* remember mds, so we don't repeat */
>  
> -		__prep_cap(&arg, cap, CEPH_CAP_OP_UPDATE, 0, cap_used, want,
> -			   retain, flushing, flush_tid, oldest_flush_tid);
> +		__prep_cap(&arg, cap, CEPH_CAP_OP_UPDATE,
> +			   (flags & CHECK_CAPS_FLUSH) ?
> +			    CEPH_CLIENT_CAPS_SYNC : 0,
> +			   cap_used, want, retain, flushing, flush_tid,
> +			   oldest_flush_tid);
>  		spin_unlock(&ci->i_ceph_lock);
>  
>  		__send_cap(mdsc, &arg, ci);
> -- 
> 2.25.1
>