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Diffstat (limited to 'Documentation/vm/slub.txt')
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diff --git a/Documentation/vm/slub.txt b/Documentation/vm/slub.txt deleted file mode 100644 index 84652419bff25..0000000000000 --- 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 |