389 строки
14 KiB
ReStructuredText
389 строки
14 KiB
ReStructuredText
.. _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
|
|
slab caches. SLUB always includes full debugging but it is off by default.
|
|
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``
|
|
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
|
|
slabs that have data in them. See "slabinfo -h" for more options when
|
|
running the command. ``slabinfo`` can be compiled with
|
|
::
|
|
|
|
gcc -o slabinfo tools/vm/slabinfo.c
|
|
|
|
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.
|
|
|
|
Some more sophisticated uses of slub_debug:
|
|
-------------------------------------------
|
|
|
|
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>,<slab name1>,<slab name2>,...
|
|
Enable options only for select slabs (no spaces
|
|
after a comma)
|
|
|
|
Multiple blocks of options for all slabs or selected slabs can be given, with
|
|
blocks of options delimited by ';'. The last of "all slabs" blocks is applied
|
|
to all slabs except those that match one of the "select slabs" block. Options
|
|
of the first "select slabs" blocks that matches the slab's name are applied.
|
|
|
|
Possible debug options are::
|
|
|
|
F Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS
|
|
Sorry SLAB legacy issues)
|
|
Z Red zoning
|
|
P Poisoning (object and padding)
|
|
U User tracking (free and alloc)
|
|
T Trace (please only use on single slabs)
|
|
A Enable failslab filter mark for the cache
|
|
O Switch debugging off for caches that would have
|
|
caused higher minimum slab orders
|
|
- 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::
|
|
|
|
slub_debug=FZ
|
|
|
|
Trying to find an issue in the dentry cache? Try::
|
|
|
|
slub_debug=,dentry
|
|
|
|
to only enable debugging on the dentry cache. You may use an asterisk at the
|
|
end of the slab name, in order to cover all slabs with the same prefix. For
|
|
example, here's how you can poison the dentry cache as well as all kmalloc
|
|
slabs::
|
|
|
|
slub_debug=P,kmalloc-*,dentry
|
|
|
|
Red zoning and tracking may realign the slab. We can just apply sanity checks
|
|
to the dentry cache with::
|
|
|
|
slub_debug=F,dentry
|
|
|
|
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::
|
|
|
|
slub_debug=O
|
|
|
|
You can apply different options to different list of slab names, using blocks
|
|
of options. This will enable red zoning for dentry and user tracking for
|
|
kmalloc. All other slabs will not get any debugging enabled::
|
|
|
|
slub_debug=Z,dentry;U,kmalloc-*
|
|
|
|
You can also enable options (e.g. sanity checks and poisoning) for all caches
|
|
except some that are deemed too performance critical and don't need to be
|
|
debugged by specifying global debug options followed by a list of slab names
|
|
with "-" as options::
|
|
|
|
slub_debug=FZ;-,zs_handle,zspage
|
|
|
|
The state of each debug option for a slab can be found in the respective files
|
|
under::
|
|
|
|
/sys/kernel/slab/<slab name>/
|
|
|
|
If the file contains 1, the option is enabled, 0 means disabled. The debug
|
|
options from the ``slub_debug`` parameter translate to the following files::
|
|
|
|
F sanity_checks
|
|
Z red_zone
|
|
P poison
|
|
U store_user
|
|
T trace
|
|
A failslab
|
|
|
|
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.
|
|
|
|
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
|
|
::
|
|
|
|
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
|
|
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 minimum 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: Right 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
|
|
into the syslog:
|
|
|
|
1. Description of the problem encountered
|
|
|
|
This will be a message in the system log starting with::
|
|
|
|
===============================================
|
|
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
|
|
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)
|
|
|
|
2. The object contents if an object was involved.
|
|
|
|
Various types of lines can follow the BUG SLUB line:
|
|
|
|
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>
|
|
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>
|
|
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
|
|
the object boundary.
|
|
|
|
(Redzone information is only available if SLAB_RED_ZONE is set.
|
|
slub_debug sets that option)
|
|
|
|
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
|
|
before the object.
|
|
|
|
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.
|
|
|
|
4. Report on how the problem was dealt with in order to ensure the continued
|
|
operation of the system.
|
|
|
|
These are messages in the system log beginning with::
|
|
|
|
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.
|
|
|
|
Emergency operations
|
|
====================
|
|
|
|
Minimal debugging (sanity checks alone) can be enabled by booting with::
|
|
|
|
slub_debug=F
|
|
|
|
This will be generally be enough to enable the resiliency features of slub
|
|
which will keep the system running even if a bad kernel component will
|
|
keep corrupting objects. This may be important for production systems.
|
|
Performance will be impacted by the sanity checks and there will be a
|
|
continual stream of error messages to the syslog but no additional memory
|
|
will be used (unlike full debugging).
|
|
|
|
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.::
|
|
|
|
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::
|
|
|
|
slub_debug=FZ,dentry
|
|
|
|
Extended slabinfo mode and plotting
|
|
===================================
|
|
|
|
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.
|
|
|
|
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.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 height
|
|
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
|