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Commit 0c14398b authored by Mike Rapoport's avatar Mike Rapoport Committed by Jonathan Corbet
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docs/vm: slub.txt: convert to ReST format 

Signed-off-by: default avatarMike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
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Documentation/vm/slub.txt
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.. _slub:
==========================
Short users guide for SLUB
--------------------------
==========================
The basic philosophy of SLUB is very different from SLAB. SLAB
requires rebuilding the kernel to activate debug options for all
......@@ -8,18 +11,19 @@ SLUB can enable debugging only for selected slabs in order to avoid
an impact on overall system performance which may make a bug more
difficult to find.
In order to switch debugging on one can add an option "slub_debug"
In order to switch debugging on one can add an option ``slub_debug``
to the kernel command line. That will enable full debugging for
all slabs.
Typically one would then use the "slabinfo" command to get statistical
data and perform operation on the slabs. By default slabinfo only lists
Typically one would then use the ``slabinfo`` command to get statistical
data and perform operation on the slabs. By default ``slabinfo`` only lists
slabs that have data in them. See "slabinfo -h" for more options when
running the command. slabinfo can be compiled with
running the command. ``slabinfo`` can be compiled with
::
gcc -o slabinfo tools/vm/slabinfo.c
gcc -o slabinfo tools/vm/slabinfo.c
Some of the modes of operation of slabinfo require that slub debugging
Some of the modes of operation of ``slabinfo`` require that slub debugging
be enabled on the command line. F.e. no tracking information will be
available without debugging on and validation can only partially
be performed if debugging was not switched on.
......@@ -27,14 +31,17 @@ be performed if debugging was not switched on.
Some more sophisticated uses of slub_debug:
-------------------------------------------
Parameters may be given to slub_debug. If none is specified then full
Parameters may be given to ``slub_debug``. If none is specified then full
debugging is enabled. Format:
slub_debug=<Debug-Options> Enable options for all slabs
slub_debug=<Debug-Options>
Enable options for all slabs
slub_debug=<Debug-Options>,<slab name>
Enable options only for select slabs
Enable options only for select slabs
Possible debug options are::
Possible debug options are
F Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS
Sorry SLAB legacy issues)
Z Red zoning
......@@ -47,18 +54,18 @@ Possible debug options are
- Switch all debugging off (useful if the kernel is
configured with CONFIG_SLUB_DEBUG_ON)
F.e. in order to boot just with sanity checks and red zoning one would specify:
F.e. in order to boot just with sanity checks and red zoning one would specify::
slub_debug=FZ
Trying to find an issue in the dentry cache? Try
Trying to find an issue in the dentry cache? Try::
slub_debug=,dentry
to only enable debugging on the dentry cache.
Red zoning and tracking may realign the slab. We can just apply sanity checks
to the dentry cache with
to the dentry cache with::
slub_debug=F,dentry
......@@ -66,15 +73,15 @@ Debugging options may require the minimum possible slab order to increase as
a result of storing the metadata (for example, caches with PAGE_SIZE object
sizes). This has a higher liklihood of resulting in slab allocation errors
in low memory situations or if there's high fragmentation of memory. To
switch off debugging for such caches by default, use
switch off debugging for such caches by default, use::
slub_debug=O
In case you forgot to enable debugging on the kernel command line: It is
possible to enable debugging manually when the kernel is up. Look at the
contents of:
contents of::
/sys/kernel/slab/<slab name>/
/sys/kernel/slab/<slab name>/
Look at the writable files. Writing 1 to them will enable the
corresponding debug option. All options can be set on a slab that does
......@@ -86,98 +93,103 @@ Careful with tracing: It may spew out lots of information and never stop if
used on the wrong slab.
Slab merging
------------
============
If no debug options are specified then SLUB may merge similar slabs together
in order to reduce overhead and increase cache hotness of objects.
slabinfo -a displays which slabs were merged together.
``slabinfo -a`` displays which slabs were merged together.
Slab validation
---------------
===============
SLUB can validate all object if the kernel was booted with slub_debug. In
order to do so you must have the slabinfo tool. Then you can do
order to do so you must have the ``slabinfo`` tool. Then you can do
::
slabinfo -v
slabinfo -v
which will test all objects. Output will be generated to the syslog.
This also works in a more limited way if boot was without slab debug.
In that case slabinfo -v simply tests all reachable objects. Usually
In that case ``slabinfo -v`` simply tests all reachable objects. Usually
these are in the cpu slabs and the partial slabs. Full slabs are not
tracked by SLUB in a non debug situation.
Getting more performance
------------------------
========================
To some degree SLUB's performance is limited by the need to take the
list_lock once in a while to deal with partial slabs. That overhead is
governed by the order of the allocation for each slab. The allocations
can be influenced by kernel parameters:
slub_min_objects=x (default 4)
slub_min_order=x (default 0)
slub_max_order=x (default 3 (PAGE_ALLOC_COSTLY_ORDER))
slub_min_objects allows to specify how many objects must at least fit
into one slab in order for the allocation order to be acceptable.
In general slub will be able to perform this number of allocations
on a slab without consulting centralized resources (list_lock) where
contention may occur.
slub_min_order specifies a minim order of slabs. A similar effect like
slub_min_objects.
slub_max_order specified the order at which slub_min_objects should no
longer be checked. This is useful to avoid SLUB trying to generate
super large order pages to fit slub_min_objects of a slab cache with
large object sizes into one high order page. Setting command line
parameter debug_guardpage_minorder=N (N > 0), forces setting
slub_max_order to 0, what cause minimum possible order of slabs
allocation.
.. slub_min_objects=x (default 4)
.. slub_min_order=x (default 0)
.. slub_max_order=x (default 3 (PAGE_ALLOC_COSTLY_ORDER))
``slub_min_objects``
allows to specify how many objects must at least fit into one
slab in order for the allocation order to be acceptable. In
general slub will be able to perform this number of
allocations on a slab without consulting centralized resources
(list_lock) where contention may occur.
``slub_min_order``
specifies a minim order of slabs. A similar effect like
``slub_min_objects``.
``slub_max_order``
specified the order at which ``slub_min_objects`` should no
longer be checked. This is useful to avoid SLUB trying to
generate super large order pages to fit ``slub_min_objects``
of a slab cache with large object sizes into one high order
page. Setting command line parameter
``debug_guardpage_minorder=N`` (N > 0), forces setting
``slub_max_order`` to 0, what cause minimum possible order of
slabs allocation.
SLUB Debug output
-----------------
Here is a sample of slub debug output:
====================================================================
BUG kmalloc-8: Redzone overwritten
--------------------------------------------------------------------
INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005
Redzone 0xc90f6d28: 00 cc cc cc .
Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
[<c010523d>] dump_trace+0x63/0x1eb
[<c01053df>] show_trace_log_lvl+0x1a/0x2f
[<c010601d>] show_trace+0x12/0x14
[<c0106035>] dump_stack+0x16/0x18
[<c017e0fa>] object_err+0x143/0x14b
[<c017e2cc>] check_object+0x66/0x234
[<c017eb43>] __slab_free+0x239/0x384
[<c017f446>] kfree+0xa6/0xc6
[<c02e2335>] get_modalias+0xb9/0xf5
[<c02e23b7>] dmi_dev_uevent+0x27/0x3c
[<c027866a>] dev_uevent+0x1ad/0x1da
[<c0205024>] kobject_uevent_env+0x20a/0x45b
[<c020527f>] kobject_uevent+0xa/0xf
[<c02779f1>] store_uevent+0x4f/0x58
[<c027758e>] dev_attr_store+0x29/0x2f
[<c01bec4f>] sysfs_write_file+0x16e/0x19c
[<c0183ba7>] vfs_write+0xd1/0x15a
[<c01841d7>] sys_write+0x3d/0x72
[<c0104112>] sysenter_past_esp+0x5f/0x99
[<b7f7b410>] 0xb7f7b410
=======================
FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
=================
Here is a sample of slub debug output::
====================================================================
BUG kmalloc-8: Redzone overwritten
--------------------------------------------------------------------
INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005
Redzone 0xc90f6d28: 00 cc cc cc .
Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
[<c010523d>] dump_trace+0x63/0x1eb
[<c01053df>] show_trace_log_lvl+0x1a/0x2f
[<c010601d>] show_trace+0x12/0x14
[<c0106035>] dump_stack+0x16/0x18
[<c017e0fa>] object_err+0x143/0x14b
[<c017e2cc>] check_object+0x66/0x234
[<c017eb43>] __slab_free+0x239/0x384
[<c017f446>] kfree+0xa6/0xc6
[<c02e2335>] get_modalias+0xb9/0xf5
[<c02e23b7>] dmi_dev_uevent+0x27/0x3c
[<c027866a>] dev_uevent+0x1ad/0x1da
[<c0205024>] kobject_uevent_env+0x20a/0x45b
[<c020527f>] kobject_uevent+0xa/0xf
[<c02779f1>] store_uevent+0x4f/0x58
[<c027758e>] dev_attr_store+0x29/0x2f
[<c01bec4f>] sysfs_write_file+0x16e/0x19c
[<c0183ba7>] vfs_write+0xd1/0x15a
[<c01841d7>] sys_write+0x3d/0x72
[<c0104112>] sysenter_past_esp+0x5f/0x99
[<b7f7b410>] 0xb7f7b410
=======================
FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
If SLUB encounters a corrupted object (full detection requires the kernel
to be booted with slub_debug) then the following output will be dumped
......@@ -185,38 +197,38 @@ into the syslog:
1. Description of the problem encountered
This will be a message in the system log starting with
This will be a message in the system log starting with::
===============================================
BUG <slab cache affected>: <What went wrong>
-----------------------------------------------
===============================================
BUG <slab cache affected>: <What went wrong>
-----------------------------------------------
INFO: <corruption start>-<corruption_end> <more info>
INFO: Slab <address> <slab information>
INFO: Object <address> <object information>
INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
INFO: <corruption start>-<corruption_end> <more info>
INFO: Slab <address> <slab information>
INFO: Object <address> <object information>
INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
cpu> pid=<pid of the process>
INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
pid=<pid of the process>
INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
pid=<pid of the process>
(Object allocation / free information is only available if SLAB_STORE_USER is
set for the slab. slub_debug sets that option)
(Object allocation / free information is only available if SLAB_STORE_USER is
set for the slab. slub_debug sets that option)
2. The object contents if an object was involved.
Various types of lines can follow the BUG SLUB line:
Various types of lines can follow the BUG SLUB line:
Bytes b4 <address> : <bytes>
Bytes b4 <address> : <bytes>
Shows a few bytes before the object where the problem was detected.
Can be useful if the corruption does not stop with the start of the
object.
Object <address> : <bytes>
Object <address> : <bytes>
The bytes of the object. If the object is inactive then the bytes
typically contain poison values. Any non-poison value shows a
corruption by a write after free.
Redzone <address> : <bytes>
Redzone <address> : <bytes>
The Redzone following the object. The Redzone is used to detect
writes after the object. All bytes should always have the same
value. If there is any deviation then it is due to a write after
......@@ -225,7 +237,7 @@ Redzone <address> : <bytes>
(Redzone information is only available if SLAB_RED_ZONE is set.
slub_debug sets that option)
Padding <address> : <bytes>
Padding <address> : <bytes>
Unused data to fill up the space in order to get the next object
properly aligned. In the debug case we make sure that there are
at least 4 bytes of padding. This allows the detection of writes
......@@ -233,29 +245,29 @@ Padding <address> : <bytes>
3. A stackdump
The stackdump describes the location where the error was detected. The cause
of the corruption is may be more likely found by looking at the function that
allocated or freed the object.
The stackdump describes the location where the error was detected. The cause
of the corruption is may be more likely found by looking at the function that
allocated or freed the object.
4. Report on how the problem was dealt with in order to ensure the continued
operation of the system.
operation of the system.
These are messages in the system log beginning with
These are messages in the system log beginning with::
FIX <slab cache affected>: <corrective action taken>
FIX <slab cache affected>: <corrective action taken>
In the above sample SLUB found that the Redzone of an active object has
been overwritten. Here a string of 8 characters was written into a slab that
has the length of 8 characters. However, a 8 character string needs a
terminating 0. That zero has overwritten the first byte of the Redzone field.
After reporting the details of the issue encountered the FIX SLUB message
tells us that SLUB has restored the Redzone to its proper value and then
system operations continue.
In the above sample SLUB found that the Redzone of an active object has
been overwritten. Here a string of 8 characters was written into a slab that
has the length of 8 characters. However, a 8 character string needs a
terminating 0. That zero has overwritten the first byte of the Redzone field.
After reporting the details of the issue encountered the FIX SLUB message
tells us that SLUB has restored the Redzone to its proper value and then
system operations continue.
Emergency operations:
---------------------
Emergency operations
====================
Minimal debugging (sanity checks alone) can be enabled by booting with
Minimal debugging (sanity checks alone) can be enabled by booting with::
slub_debug=F
......@@ -270,73 +282,80 @@ No guarantees. The kernel component still needs to be fixed. Performance
may be optimized further by locating the slab that experiences corruption
and enabling debugging only for that cache
I.e.
I.e.::
slub_debug=F,dentry
If the corruption occurs by writing after the end of the object then it
may be advisable to enable a Redzone to avoid corrupting the beginning
of other objects.
of other objects::
slub_debug=FZ,dentry
Extended slabinfo mode and plotting
-----------------------------------
===================================
The slabinfo tool has a special 'extended' ('-X') mode that includes:
The ``slabinfo`` tool has a special 'extended' ('-X') mode that includes:
- Slabcache Totals
- Slabs sorted by size (up to -N <num> slabs, default 1)
- Slabs sorted by loss (up to -N <num> slabs, default 1)
Additionally, in this mode slabinfo does not dynamically scale sizes (G/M/K)
and reports everything in bytes (this functionality is also available to
other slabinfo modes via '-B' option) which makes reporting more precise and
accurate. Moreover, in some sense the `-X' mode also simplifies the analysis
of slabs' behaviour, because its output can be plotted using the
slabinfo-gnuplot.sh script. So it pushes the analysis from looking through
the numbers (tons of numbers) to something easier -- visual analysis.
Additionally, in this mode ``slabinfo`` does not dynamically scale
sizes (G/M/K) and reports everything in bytes (this functionality is
also available to other slabinfo modes via '-B' option) which makes
reporting more precise and accurate. Moreover, in some sense the `-X'
mode also simplifies the analysis of slabs' behaviour, because its
output can be plotted using the ``slabinfo-gnuplot.sh`` script. So it
pushes the analysis from looking through the numbers (tons of numbers)
to something easier -- visual analysis.
To generate plots:
a) collect slabinfo extended records, for example:
while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done
b) pass stats file(-s) to slabinfo-gnuplot.sh script:
slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]
The slabinfo-gnuplot.sh script will pre-processes the collected records
and generates 3 png files (and 3 pre-processing cache files) per STATS
file:
- Slabcache Totals: FOO_STATS-totals.png
- Slabs sorted by size: FOO_STATS-slabs-by-size.png
- Slabs sorted by loss: FOO_STATS-slabs-by-loss.png
Another use case, when slabinfo-gnuplot can be useful, is when you need
to compare slabs' behaviour "prior to" and "after" some code modification.
To help you out there, slabinfo-gnuplot.sh script can 'merge' the
`Slabcache Totals` sections from different measurements. To visually
compare N plots:
a) Collect as many STATS1, STATS2, .. STATSN files as you need
while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
b) Pre-process those STATS files
slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN
c) Execute slabinfo-gnuplot.sh in '-t' mode, passing all of the
generated pre-processed *-totals
slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals
This will produce a single plot (png file).
Plots, expectedly, can be large so some fluctuations or small spikes
can go unnoticed. To deal with that, `slabinfo-gnuplot.sh' has two
options to 'zoom-in'/'zoom-out':
a) -s %d,%d overwrites the default image width and heigh
b) -r %d,%d specifies a range of samples to use (for example,
in `slabinfo -X >> FOO_STATS; sleep 1;' case, using
a "-r 40,60" range will plot only samples collected
between 40th and 60th seconds).
a) collect slabinfo extended records, for example::
while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done
b) pass stats file(-s) to ``slabinfo-gnuplot.sh`` script::
slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]
The ``slabinfo-gnuplot.sh`` script will pre-processes the collected records
and generates 3 png files (and 3 pre-processing cache files) per STATS
file:
- Slabcache Totals: FOO_STATS-totals.png
- Slabs sorted by size: FOO_STATS-slabs-by-size.png
- Slabs sorted by loss: FOO_STATS-slabs-by-loss.png
Another use case, when ``slabinfo-gnuplot.sh`` can be useful, is when you
need to compare slabs' behaviour "prior to" and "after" some code
modification. To help you out there, ``slabinfo-gnuplot.sh`` script
can 'merge' the `Slabcache Totals` sections from different
measurements. To visually compare N plots:
a) Collect as many STATS1, STATS2, .. STATSN files as you need::
while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
b) Pre-process those STATS files::
slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN
c) Execute ``slabinfo-gnuplot.sh`` in '-t' mode, passing all of the
generated pre-processed \*-totals::
slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals
This will produce a single plot (png file).
Plots, expectedly, can be large so some fluctuations or small spikes
can go unnoticed. To deal with that, ``slabinfo-gnuplot.sh`` has two
options to 'zoom-in'/'zoom-out':
a) ``-s %d,%d`` -- overwrites the default image width and heigh
b) ``-r %d,%d`` -- specifies a range of samples to use (for example,
in ``slabinfo -X >> FOO_STATS; sleep 1;`` case, using a ``-r
40,60`` range will plot only samples collected between 40th and
60th seconds).
Christoph Lameter, May 30, 2007
Sergey Senozhatsky, October 23, 2015
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