Documentation / filesystems / gfs2-glocks.rst


Based on kernel version 5.10.1. Page generated on 2020-12-14 21:14 EST.

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.. SPDX-License-Identifier: GPL-2.0

============================
Glock internal locking rules
============================

This documents the basic principles of the glock state machine
internals. Each glock (struct gfs2_glock in fs/gfs2/incore.h)
has two main (internal) locks:

 1. A spinlock (gl_lockref.lock) which protects the internal state such
    as gl_state, gl_target and the list of holders (gl_holders)
 2. A non-blocking bit lock, GLF_LOCK, which is used to prevent other
    threads from making calls to the DLM, etc. at the same time. If a
    thread takes this lock, it must then call run_queue (usually via the
    workqueue) when it releases it in order to ensure any pending tasks
    are completed.

The gl_holders list contains all the queued lock requests (not
just the holders) associated with the glock. If there are any
held locks, then they will be contiguous entries at the head
of the list. Locks are granted in strictly the order that they
are queued, except for those marked LM_FLAG_PRIORITY which are
used only during recovery, and even then only for journal locks.

There are three lock states that users of the glock layer can request,
namely shared (SH), deferred (DF) and exclusive (EX). Those translate
to the following DLM lock modes:

==========	====== =====================================================
Glock mode      DLM    lock mode
==========	====== =====================================================
    UN          IV/NL  Unlocked (no DLM lock associated with glock) or NL
    SH          PR     (Protected read)
    DF          CW     (Concurrent write)
    EX          EX     (Exclusive)
==========	====== =====================================================

Thus DF is basically a shared mode which is incompatible with the "normal"
shared lock mode, SH. In GFS2 the DF mode is used exclusively for direct I/O
operations. The glocks are basically a lock plus some routines which deal
with cache management. The following rules apply for the cache:

==========      ==========   ==============   ==========   ==============
Glock mode      Cache data   Cache Metadata   Dirty Data   Dirty Metadata
==========      ==========   ==============   ==========   ==============
    UN             No              No             No            No
    SH             Yes             Yes            No            No
    DF             No              Yes            No            No
    EX             Yes             Yes            Yes           Yes
==========      ==========   ==============   ==========   ==============

These rules are implemented using the various glock operations which
are defined for each type of glock. Not all types of glocks use
all the modes. Only inode glocks use the DF mode for example.

Table of glock operations and per type constants:

=============      =============================================================
Field              Purpose
=============      =============================================================
go_xmote_th        Called before remote state change (e.g. to sync dirty data)
go_xmote_bh        Called after remote state change (e.g. to refill cache)
go_inval           Called if remote state change requires invalidating the cache
go_demote_ok       Returns boolean value of whether its ok to demote a glock
                   (e.g. checks timeout, and that there is no cached data)
go_lock            Called for the first local holder of a lock
go_unlock          Called on the final local unlock of a lock
go_dump            Called to print content of object for debugfs file, or on
                   error to dump glock to the log.
go_type            The type of the glock, ``LM_TYPE_*``
go_callback	   Called if the DLM sends a callback to drop this lock
go_flags	   GLOF_ASPACE is set, if the glock has an address space
                   associated with it
=============      =============================================================

The minimum hold time for each lock is the time after a remote lock
grant for which we ignore remote demote requests. This is in order to
prevent a situation where locks are being bounced around the cluster
from node to node with none of the nodes making any progress. This
tends to show up most with shared mmaped files which are being written
to by multiple nodes. By delaying the demotion in response to a
remote callback, that gives the userspace program time to make
some progress before the pages are unmapped.

There is a plan to try and remove the go_lock and go_unlock callbacks
if possible, in order to try and speed up the fast path though the locking.
Also, eventually we hope to make the glock "EX" mode locally shared
such that any local locking will be done with the i_mutex as required
rather than via the glock.

Locking rules for glock operations:

=============    ======================    =============================
Operation        GLF_LOCK bit lock held    gl_lockref.lock spinlock held
=============    ======================    =============================
go_xmote_th           Yes                       No
go_xmote_bh           Yes                       No
go_inval              Yes                       No
go_demote_ok          Sometimes                 Yes
go_lock               Yes                       No
go_unlock             Yes                       No
go_dump               Sometimes                 Yes
go_callback           Sometimes (N/A)           Yes
=============    ======================    =============================

.. Note::

   Operations must not drop either the bit lock or the spinlock
   if its held on entry. go_dump and do_demote_ok must never block.
   Note that go_dump will only be called if the glock's state
   indicates that it is caching uptodate data.

Glock locking order within GFS2:

 1. i_rwsem (if required)
 2. Rename glock (for rename only)
 3. Inode glock(s)
    (Parents before children, inodes at "same level" with same parent in
    lock number order)
 4. Rgrp glock(s) (for (de)allocation operations)
 5. Transaction glock (via gfs2_trans_begin) for non-read operations
 6. i_rw_mutex (if required)
 7. Page lock  (always last, very important!)

There are two glocks per inode. One deals with access to the inode
itself (locking order as above), and the other, known as the iopen
glock is used in conjunction with the i_nlink field in the inode to
determine the lifetime of the inode in question. Locking of inodes
is on a per-inode basis. Locking of rgrps is on a per rgrp basis.
In general we prefer to lock local locks prior to cluster locks.

Glock Statistics
----------------

The stats are divided into two sets: those relating to the
super block and those relating to an individual glock. The
super block stats are done on a per cpu basis in order to
try and reduce the overhead of gathering them. They are also
further divided by glock type. All timings are in nanoseconds.

In the case of both the super block and glock statistics,
the same information is gathered in each case. The super
block timing statistics are used to provide default values for
the glock timing statistics, so that newly created glocks
should have, as far as possible, a sensible starting point.
The per-glock counters are initialised to zero when the
glock is created. The per-glock statistics are lost when
the glock is ejected from memory.

The statistics are divided into three pairs of mean and
variance, plus two counters. The mean/variance pairs are
smoothed exponential estimates and the algorithm used is
one which will be very familiar to those used to calculation
of round trip times in network code. See "TCP/IP Illustrated,
Volume 1", W. Richard Stevens, sect 21.3, "Round-Trip Time Measurement",
p. 299 and onwards. Also, Volume 2, Sect. 25.10, p. 838 and onwards.
Unlike the TCP/IP Illustrated case, the mean and variance are
not scaled, but are in units of integer nanoseconds.

The three pairs of mean/variance measure the following
things:

 1. DLM lock time (non-blocking requests)
 2. DLM lock time (blocking requests)
 3. Inter-request time (again to the DLM)

A non-blocking request is one which will complete right
away, whatever the state of the DLM lock in question. That
currently means any requests when (a) the current state of
the lock is exclusive, i.e. a lock demotion (b) the requested
state is either null or unlocked (again, a demotion) or (c) the
"try lock" flag is set. A blocking request covers all the other
lock requests.

There are two counters. The first is there primarily to show
how many lock requests have been made, and thus how much data
has gone into the mean/variance calculations. The other counter
is counting queuing of holders at the top layer of the glock
code. Hopefully that number will be a lot larger than the number
of dlm lock requests issued.

So why gather these statistics? There are several reasons
we'd like to get a better idea of these timings:

1. To be able to better set the glock "min hold time"
2. To spot performance issues more easily
3. To improve the algorithm for selecting resource groups for
   allocation (to base it on lock wait time, rather than blindly
   using a "try lock")

Due to the smoothing action of the updates, a step change in
some input quantity being sampled will only fully be taken
into account after 8 samples (or 4 for the variance) and this
needs to be carefully considered when interpreting the
results.

Knowing both the time it takes a lock request to complete and
the average time between lock requests for a glock means we
can compute the total percentage of the time for which the
node is able to use a glock vs. time that the rest of the
cluster has its share. That will be very useful when setting
the lock min hold time.

Great care has been taken to ensure that we
measure exactly the quantities that we want, as accurately
as possible. There are always inaccuracies in any
measuring system, but I hope this is as accurate as we
can reasonably make it.

Per sb stats can be found here::

    /sys/kernel/debug/gfs2/<fsname>/sbstats

Per glock stats can be found here::

    /sys/kernel/debug/gfs2/<fsname>/glstats

Assuming that debugfs is mounted on /sys/kernel/debug and also
that <fsname> is replaced with the name of the gfs2 filesystem
in question.

The abbreviations used in the output as are follows:

=========  ================================================================
srtt       Smoothed round trip time for non blocking dlm requests
srttvar    Variance estimate for srtt
srttb      Smoothed round trip time for (potentially) blocking dlm requests
srttvarb   Variance estimate for srttb
sirt       Smoothed inter request time (for dlm requests)
sirtvar    Variance estimate for sirt
dlm        Number of dlm requests made (dcnt in glstats file)
queue      Number of glock requests queued (qcnt in glstats file)
=========  ================================================================

The sbstats file contains a set of these stats for each glock type (so 8 lines
for each type) and for each cpu (one column per cpu). The glstats file contains
a set of these stats for each glock in a similar format to the glocks file, but
using the format mean/variance for each of the timing stats.

The gfs2_glock_lock_time tracepoint prints out the current values of the stats
for the glock in question, along with some addition information on each dlm
reply that is received:

======   =======================================
status   The status of the dlm request
flags    The dlm request flags
tdiff    The time taken by this specific request
======   =======================================

(remaining fields as per above list)