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Subject: Re: [PATCH] ceph: show max caps in debugfs caps file
From:   Jeff Layton <jlayton@kernel.org>
To:     Yanhu Cao <gmayyyha@gmail.com>
Cc:     Ilya Dryomov <idryomov@gmail.com>,
        ceph-devel <ceph-devel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Date:   Thu, 21 May 2020 08:51:54 -0400
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On Thu, 2020-05-21 at 20:19 +0800, Yanhu Cao wrote:
> On Thu, May 21, 2020 at 7:09 PM Jeff Layton <jlayton@kernel.org> wrote:
> > On Thu, 2020-05-21 at 17:38 +0800, Yanhu Cao wrote:
> > >         before
> > >         ------
> > >         total           1026
> > >         avail           1024
> > >         used            2
> > >         reserved        0
> > >         min             1024
> > > 
> > >         after
> > >         ------
> > >         total           1026
> > >         avail           1024
> > >         used            2
> > >         max             2048
> > >         reserved        0
> > >         min             1024
> > > 
> > > Signed-off-by: Yanhu Cao <gmayyyha@gmail.com>
> > > ---
> > >  fs/ceph/caps.c    | 6 ++++--
> > >  fs/ceph/debugfs.c | 7 ++++---
> > >  fs/ceph/super.h   | 2 +-
> > >  3 files changed, 9 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> > > index 5f3aa4d607de..e2c759a2ef35 100644
> > > --- a/fs/ceph/caps.c
> > > +++ b/fs/ceph/caps.c
> > > @@ -404,8 +404,8 @@ void ceph_put_cap(struct ceph_mds_client *mdsc, struct ceph_cap *cap)
> > >  }
> > > 
> > >  void ceph_reservation_status(struct ceph_fs_client *fsc,
> > > -                          int *total, int *avail, int *used, int *reserved,
> > > -                          int *min)
> > > +                          int *total, int *avail, int *used, int *max,
> > > +                          int *reserved, int *min)
> > >  {
> > >       struct ceph_mds_client *mdsc = fsc->mdsc;
> > > 
> > > @@ -417,6 +417,8 @@ void ceph_reservation_status(struct ceph_fs_client *fsc,
> > >               *avail = mdsc->caps_avail_count;
> > >       if (used)
> > >               *used = mdsc->caps_use_count;
> > > +     if (max)
> > > +             *max = mdsc->caps_use_max;
> > 
> > Can you lay out what value this will provide? I'm not convinced that
> > this information is really that helpful:
> > 
> > mdsc->caps_use_max is just set to the value of the "caps_max" mount
> > option, and that information is displayed in /proc/mounts if it's not
> > set to the default.
> > 
> > What might be more interesting is to track the most recent "max_caps"
> > value sent by the MDS (see the CEPH_SESSION_RECALL_STATE message
> > handling). Tracking that would give us a more dynamic view of the
> > current maximum requested by the MDS, which is often going to be less
> > than what "caps_max" was set to at mount time.
> 
> Do you mean the 'mds_recall_max_caps'? which can be set by the MDS.
> Clients use this value every time to trim caps.
> 
> There is an option mds_max_caps_per_client which is a soft limit,
> which is determined by the behavior of the client.
> and we recently encounter a warning '1 MDSs report oversized
> cache'(ceph-v12.2.12: mds_cache_memory_limit=64G, used=100G),
> Therefore, the effect is not good.
> 
> So we want to know whether the caps held by the client exceed caps_max
> through the debugfs caps file (default or mount option).
> 
> 

Ok, I doubt this patch is going to tell you what you want to know then. 
There are two limits involved here:

1/ the limit set by the caps_max mount option for the client
2/ a dynamic limit that is managed by the MDS issuing
   CEPH_SESSION_RECALL_STATE messages to the clients (max_caps).

This patch is only going to tell you about the first one, but the second
one is more interesting to monitor on a long-term basis.

When the mds exceeds its memory limits, it can issue
CEPH_SESSION_RECALL_STATE messages to the clients, to tell them to
reduce their own caches to a particular size. Currently we just take
that value and feed it into ceph_trim_caps and forget about it.

It would probably be useful though to keep track of the most recent
value issued by the MDS, and print this value as min(mount_option_max,
max_caps_from_mds). Bonus points if you can help untangle the confusing
naming of all these values in the process.

> > >       if (reserved)
> > >               *reserved = mdsc->caps_reserve_count;
> > >       if (min)
> > > diff --git a/fs/ceph/debugfs.c b/fs/ceph/debugfs.c
> > > index 481ac97b4d25..942004376588 100644
> > > --- a/fs/ceph/debugfs.c
> > > +++ b/fs/ceph/debugfs.c
> > > @@ -138,16 +138,17 @@ static int caps_show(struct seq_file *s, void *p)
> > >  {
> > >       struct ceph_fs_client *fsc = s->private;
> > >       struct ceph_mds_client *mdsc = fsc->mdsc;
> > > -     int total, avail, used, reserved, min, i;
> > > +     int total, avail, used, max, reserved, min, i;
> > >       struct cap_wait *cw;
> > > 
> > > -     ceph_reservation_status(fsc, &total, &avail, &used, &reserved, &min);
> > > +     ceph_reservation_status(fsc, &total, &avail, &used, &max,
> > > +                             &reserved, &min);
> > >       seq_printf(s, "total\t\t%d\n"
> > >                  "avail\t\t%d\n"
> > >                  "used\t\t%d\n"
> > >                  "reserved\t%d\n"
> > >                  "min\t\t%d\n\n",
> > > -                total, avail, used, reserved, min);
> > > +                total, avail, used, max, reserved, min);
> > >       seq_printf(s, "ino                issued           implemented\n");
> > >       seq_printf(s, "-----------------------------------------------\n");
> > > 
> > > diff --git a/fs/ceph/super.h b/fs/ceph/super.h
> > > index 60aac3aee055..79aa42d9336c 100644
> > > --- a/fs/ceph/super.h
> > > +++ b/fs/ceph/super.h
> > > @@ -700,7 +700,7 @@ extern void ceph_unreserve_caps(struct ceph_mds_client *mdsc,
> > >                              struct ceph_cap_reservation *ctx);
> > >  extern void ceph_reservation_status(struct ceph_fs_client *client,
> > >                                   int *total, int *avail, int *used,
> > > -                                 int *reserved, int *min);
> > > +                                 int *max, int *reserved, int *min);
> > > 
> > > 
> > > 
> > 
> > --
> > Jeff Layton <jlayton@kernel.org>
> > 

-- 
Jeff Layton <jlayton@kernel.org>