summaryrefslogtreecommitdiff
path: root/Documentation/vm/slub.txt
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/vm/slub.txt')
-rw-r--r--Documentation/vm/slub.txt342
1 files changed, 0 insertions, 342 deletions
diff --git a/Documentation/vm/slub.txt b/Documentation/vm/slub.txt
deleted file mode 100644
index 84652419bff2..000000000000
--- a/Documentation/vm/slub.txt
+++ /dev/null
@@ -1,342 +0,0 @@
-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 name>
- Enable options only for select slabs
-
-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 Toggle 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.
-
-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
-
-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:
-
-/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
-not contain objects. If the slab already contains objects then sanity checks
-and tracing may only be enabled. The other options may cause the realignment
-of objects.
-
-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 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
-
-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 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