Commit 0c14398b authored by Mike Rapoport's avatar Mike Rapoport Committed by Jonathan Corbet

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>
parent acc9f3a3
.. _slub:
==========================
Short users guide for SLUB Short users guide for SLUB
-------------------------- ==========================
The basic philosophy of SLUB is very different from SLAB. SLAB The basic philosophy of SLUB is very different from SLAB. SLAB
requires rebuilding the kernel to activate debug options for all 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 ...@@ -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 an impact on overall system performance which may make a bug more
difficult to find. 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 to the kernel command line. That will enable full debugging for
all slabs. all slabs.
Typically one would then use the "slabinfo" command to get statistical Typically one would then use the ``slabinfo`` command to get statistical
data and perform operation on the slabs. By default slabinfo only lists 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 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 be enabled on the command line. F.e. no tracking information will be
available without debugging on and validation can only partially available without debugging on and validation can only partially
be performed if debugging was not switched on. be performed if debugging was not switched on.
...@@ -27,14 +31,17 @@ 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: 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: 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> 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 F Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS
Sorry SLAB legacy issues) Sorry SLAB legacy issues)
Z Red zoning Z Red zoning
...@@ -47,18 +54,18 @@ Possible debug options are ...@@ -47,18 +54,18 @@ Possible debug options are
- Switch all debugging off (useful if the kernel is - Switch all debugging off (useful if the kernel is
configured with CONFIG_SLUB_DEBUG_ON) 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 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 slub_debug=,dentry
to only enable debugging on the dentry cache. to only enable debugging on the dentry cache.
Red zoning and tracking may realign the slab. We can just apply sanity checks 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 slub_debug=F,dentry
...@@ -66,15 +73,15 @@ Debugging options may require the minimum possible slab order to increase as ...@@ -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 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 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 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 slub_debug=O
In case you forgot to enable debugging on the kernel command line: It is 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 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 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 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 ...@@ -86,98 +93,103 @@ Careful with tracing: It may spew out lots of information and never stop if
used on the wrong slab. used on the wrong slab.
Slab merging Slab merging
------------ ============
If no debug options are specified then SLUB may merge similar slabs together If no debug options are specified then SLUB may merge similar slabs together
in order to reduce overhead and increase cache hotness of objects. 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 Slab validation
--------------- ===============
SLUB can validate all object if the kernel was booted with slub_debug. In 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. 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. 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 these are in the cpu slabs and the partial slabs. Full slabs are not
tracked by SLUB in a non debug situation. tracked by SLUB in a non debug situation.
Getting more performance Getting more performance
------------------------ ========================
To some degree SLUB's performance is limited by the need to take the 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 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 governed by the order of the allocation for each slab. The allocations
can be influenced by kernel parameters: can be influenced by kernel parameters:
slub_min_objects=x (default 4) .. slub_min_objects=x (default 4)
slub_min_order=x (default 0) .. slub_min_order=x (default 0)
slub_max_order=x (default 3 (PAGE_ALLOC_COSTLY_ORDER)) .. slub_max_order=x (default 3 (PAGE_ALLOC_COSTLY_ORDER))
slub_min_objects allows to specify how many objects must at least fit ``slub_min_objects``
into one slab in order for the allocation order to be acceptable. allows to specify how many objects must at least fit into one
In general slub will be able to perform this number of allocations slab in order for the allocation order to be acceptable. In
on a slab without consulting centralized resources (list_lock) where general slub will be able to perform this number of
contention may occur. 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_min_order``
specifies a minim order of slabs. A similar effect like
slub_max_order specified the order at which slub_min_objects should no ``slub_min_objects``.
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 ``slub_max_order``
large object sizes into one high order page. Setting command line specified the order at which ``slub_min_objects`` should no
parameter debug_guardpage_minorder=N (N > 0), forces setting longer be checked. This is useful to avoid SLUB trying to
slub_max_order to 0, what cause minimum possible order of slabs generate super large order pages to fit ``slub_min_objects``
allocation. 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 SLUB Debug output
----------------- =================
Here is a sample of slub debug output: Here is a sample of slub debug output::
==================================================================== ====================================================================
BUG kmalloc-8: Redzone overwritten BUG kmalloc-8: Redzone overwritten
-------------------------------------------------------------------- --------------------------------------------------------------------
INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58 INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58 INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554 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 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 Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005
Redzone 0xc90f6d28: 00 cc cc cc . Redzone 0xc90f6d28: 00 cc cc cc .
Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
[<c010523d>] dump_trace+0x63/0x1eb [<c010523d>] dump_trace+0x63/0x1eb
[<c01053df>] show_trace_log_lvl+0x1a/0x2f [<c01053df>] show_trace_log_lvl+0x1a/0x2f
[<c010601d>] show_trace+0x12/0x14 [<c010601d>] show_trace+0x12/0x14
[<c0106035>] dump_stack+0x16/0x18 [<c0106035>] dump_stack+0x16/0x18
[<c017e0fa>] object_err+0x143/0x14b [<c017e0fa>] object_err+0x143/0x14b
[<c017e2cc>] check_object+0x66/0x234 [<c017e2cc>] check_object+0x66/0x234
[<c017eb43>] __slab_free+0x239/0x384 [<c017eb43>] __slab_free+0x239/0x384
[<c017f446>] kfree+0xa6/0xc6 [<c017f446>] kfree+0xa6/0xc6
[<c02e2335>] get_modalias+0xb9/0xf5 [<c02e2335>] get_modalias+0xb9/0xf5
[<c02e23b7>] dmi_dev_uevent+0x27/0x3c [<c02e23b7>] dmi_dev_uevent+0x27/0x3c
[<c027866a>] dev_uevent+0x1ad/0x1da [<c027866a>] dev_uevent+0x1ad/0x1da
[<c0205024>] kobject_uevent_env+0x20a/0x45b [<c0205024>] kobject_uevent_env+0x20a/0x45b
[<c020527f>] kobject_uevent+0xa/0xf [<c020527f>] kobject_uevent+0xa/0xf
[<c02779f1>] store_uevent+0x4f/0x58 [<c02779f1>] store_uevent+0x4f/0x58
[<c027758e>] dev_attr_store+0x29/0x2f [<c027758e>] dev_attr_store+0x29/0x2f
[<c01bec4f>] sysfs_write_file+0x16e/0x19c [<c01bec4f>] sysfs_write_file+0x16e/0x19c
[<c0183ba7>] vfs_write+0xd1/0x15a [<c0183ba7>] vfs_write+0xd1/0x15a
[<c01841d7>] sys_write+0x3d/0x72 [<c01841d7>] sys_write+0x3d/0x72
[<c0104112>] sysenter_past_esp+0x5f/0x99 [<c0104112>] sysenter_past_esp+0x5f/0x99
[<b7f7b410>] 0xb7f7b410 [<b7f7b410>] 0xb7f7b410
======================= =======================
FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
If SLUB encounters a corrupted object (full detection requires the kernel If SLUB encounters a corrupted object (full detection requires the kernel
to be booted with slub_debug) then the following output will be dumped to be booted with slub_debug) then the following output will be dumped
...@@ -185,38 +197,38 @@ into the syslog: ...@@ -185,38 +197,38 @@ into the syslog:
1. Description of the problem encountered 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: <corruption start>-<corruption_end> <more info>
INFO: Slab <address> <slab information> INFO: Slab <address> <slab information>
INFO: Object <address> <object information> INFO: Object <address> <object information>
INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
cpu> pid=<pid of the process> cpu> pid=<pid of the process>
INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu> INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
pid=<pid of the process> pid=<pid of the process>
(Object allocation / free information is only available if SLAB_STORE_USER is (Object allocation / free information is only available if SLAB_STORE_USER is
set for the slab. slub_debug sets that option) set for the slab. slub_debug sets that option)
2. The object contents if an object was involved. 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. 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 Can be useful if the corruption does not stop with the start of the
object. object.
Object <address> : <bytes> Object <address> : <bytes>
The bytes of the object. If the object is inactive then the bytes The bytes of the object. If the object is inactive then the bytes
typically contain poison values. Any non-poison value shows a typically contain poison values. Any non-poison value shows a
corruption by a write after free. corruption by a write after free.
Redzone <address> : <bytes> Redzone <address> : <bytes>
The Redzone following the object. The Redzone is used to detect The Redzone following the object. The Redzone is used to detect
writes after the object. All bytes should always have the same 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 value. If there is any deviation then it is due to a write after
...@@ -225,7 +237,7 @@ Redzone <address> : <bytes> ...@@ -225,7 +237,7 @@ Redzone <address> : <bytes>
(Redzone information is only available if SLAB_RED_ZONE is set. (Redzone information is only available if SLAB_RED_ZONE is set.
slub_debug sets that option) 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 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 properly aligned. In the debug case we make sure that there are
at least 4 bytes of padding. This allows the detection of writes at least 4 bytes of padding. This allows the detection of writes
...@@ -233,29 +245,29 @@ Padding <address> : <bytes> ...@@ -233,29 +245,29 @@ Padding <address> : <bytes>
3. A stackdump 3. A stackdump
The stackdump describes the location where the error was detected. The cause 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 of the corruption is may be more likely found by looking at the function that
allocated or freed the object. allocated or freed the object.
4. Report on how the problem was dealt with in order to ensure the continued 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 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 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 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. 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 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 tells us that SLUB has restored the Redzone to its proper value and then
system operations continue. 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 slub_debug=F
...@@ -270,73 +282,80 @@ No guarantees. The kernel component still needs to be fixed. Performance ...@@ -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 may be optimized further by locating the slab that experiences corruption
and enabling debugging only for that cache and enabling debugging only for that cache
I.e. I.e.::
slub_debug=F,dentry slub_debug=F,dentry
If the corruption occurs by writing after the end of the object then it 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 may be advisable to enable a Redzone to avoid corrupting the beginning
of other objects. of other objects::
slub_debug=FZ,dentry slub_debug=FZ,dentry
Extended slabinfo mode and plotting 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 - Slabcache Totals
- Slabs sorted by size (up to -N <num> slabs, default 1) - Slabs sorted by size (up to -N <num> slabs, default 1)
- Slabs sorted by loss (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) Additionally, in this mode ``slabinfo`` does not dynamically scale
and reports everything in bytes (this functionality is also available to sizes (G/M/K) and reports everything in bytes (this functionality is
other slabinfo modes via '-B' option) which makes reporting more precise and also available to other slabinfo modes via '-B' option) which makes
accurate. Moreover, in some sense the `-X' mode also simplifies the analysis reporting more precise and accurate. Moreover, in some sense the `-X'
of slabs' behaviour, because its output can be plotted using the mode also simplifies the analysis of slabs' behaviour, because its
slabinfo-gnuplot.sh script. So it pushes the analysis from looking through output can be plotted using the ``slabinfo-gnuplot.sh`` script. So it
the numbers (tons of numbers) to something easier -- visual analysis. pushes the analysis from looking through the numbers (tons of numbers)
to something easier -- visual analysis.
To generate plots: To generate plots:
a) collect slabinfo extended records, for example:
a) collect slabinfo extended records, for example::
while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done
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] b) pass stats file(-s) to ``slabinfo-gnuplot.sh`` script::
The slabinfo-gnuplot.sh script will pre-processes the collected records slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]
and generates 3 png files (and 3 pre-processing cache files) per STATS
file: The ``slabinfo-gnuplot.sh`` script will pre-processes the collected records
- Slabcache Totals: FOO_STATS-totals.png and generates 3 png files (and 3 pre-processing cache files) per STATS
- Slabs sorted by size: FOO_STATS-slabs-by-size.png file:
- Slabs sorted by loss: FOO_STATS-slabs-by-loss.png - Slabcache Totals: FOO_STATS-totals.png
- Slabs sorted by size: FOO_STATS-slabs-by-size.png
Another use case, when slabinfo-gnuplot can be useful, is when you need - Slabs sorted by loss: FOO_STATS-slabs-by-loss.png
to compare slabs' behaviour "prior to" and "after" some code modification.
To help you out there, slabinfo-gnuplot.sh script can 'merge' the Another use case, when ``slabinfo-gnuplot.sh`` can be useful, is when you
`Slabcache Totals` sections from different measurements. To visually need to compare slabs' behaviour "prior to" and "after" some code
compare N plots: modification. To help you out there, ``slabinfo-gnuplot.sh`` script
can 'merge' the `Slabcache Totals` sections from different
a) Collect as many STATS1, STATS2, .. STATSN files as you need measurements. To visually compare N plots:
while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
a) Collect as many STATS1, STATS2, .. STATSN files as you need::
b) Pre-process those STATS files
slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
c) Execute slabinfo-gnuplot.sh in '-t' mode, passing all of the b) Pre-process those STATS files::
generated pre-processed *-totals
slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN
This will produce a single plot (png file). c) Execute ``slabinfo-gnuplot.sh`` in '-t' mode, passing all of the
generated pre-processed \*-totals::
Plots, expectedly, can be large so some fluctuations or small spikes
can go unnoticed. To deal with that, `slabinfo-gnuplot.sh' has two slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals
options to 'zoom-in'/'zoom-out':
a) -s %d,%d overwrites the default image width and heigh This will produce a single plot (png file).
b) -r %d,%d specifies a range of samples to use (for example,
in `slabinfo -X >> FOO_STATS; sleep 1;' case, using Plots, expectedly, can be large so some fluctuations or small spikes
a "-r 40,60" range will plot only samples collected can go unnoticed. To deal with that, ``slabinfo-gnuplot.sh`` has two
between 40th and 60th seconds). 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 Christoph Lameter, May 30, 2007
Sergey Senozhatsky, October 23, 2015 Sergey Senozhatsky, October 23, 2015
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment