- 27 Mar, 2006 40 commits
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Ingo Molnar authored
This patchset provides a new (written from scratch) implementation of robust futexes, called "lightweight robust futexes". We believe this new implementation is faster and simpler than the vma-based robust futex solutions presented before, and we'd like this patchset to be adopted in the upstream kernel. This is version 1 of the patchset. Background ---------- What are robust futexes? To answer that, we first need to understand what futexes are: normal futexes are special types of locks that in the noncontended case can be acquired/released from userspace without having to enter the kernel. A futex is in essence a user-space address, e.g. a 32-bit lock variable field. If userspace notices contention (the lock is already owned and someone else wants to grab it too) then the lock is marked with a value that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT) syscall is used to wait for the other guy to release it. The kernel creates a 'futex queue' internally, so that it can later on match up the waiter with the waker - without them having to know about each other. When the owner thread releases the futex, it notices (via the variable value) that there were waiter(s) pending, and does the sys_futex(FUTEX_WAKE) syscall to wake them up. Once all waiters have taken and released the lock, the futex is again back to 'uncontended' state, and there's no in-kernel state associated with it. The kernel completely forgets that there ever was a futex at that address. This method makes futexes very lightweight and scalable. "Robustness" is about dealing with crashes while holding a lock: if a process exits prematurely while holding a pthread_mutex_t lock that is also shared with some other process (e.g. yum segfaults while holding a pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need to be notified that the last owner of the lock exited in some irregular way. To solve such types of problems, "robust mutex" userspace APIs were created: pthread_mutex_lock() returns an error value if the owner exits prematurely - and the new owner can decide whether the data protected by the lock can be recovered safely. There is a big conceptual problem with futex based mutexes though: it is the kernel that destroys the owner task (e.g. due to a SEGFAULT), but the kernel cannot help with the cleanup: if there is no 'futex queue' (and in most cases there is none, futexes being fast lightweight locks) then the kernel has no information to clean up after the held lock! Userspace has no chance to clean up after the lock either - userspace is the one that crashes, so it has no opportunity to clean up. Catch-22. In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot is needed to release that futex based lock. This is one of the leading bugreports against yum. To solve this problem, 'Robust Futex' patches were created and presented on lkml: the one written by Todd Kneisel and David Singleton is the most advanced at the moment. These patches all tried to extend the futex abstraction by registering futex-based locks in the kernel - and thus give the kernel a chance to clean up. E.g. in David Singleton's robust-futex-6.patch, there are 3 new syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and FUTEX_RECOVER. The kernel attaches such robust futexes to vmas (via vma->vm_file->f_mapping->robust_head), and at do_exit() time, all vmas are searched to see whether they have a robust_head set. Lots of work went into the vma-based robust-futex patch, and recently it has improved significantly, but unfortunately it still has two fundamental problems left: - they have quite complex locking and race scenarios. The vma-based patches had been pending for years, but they are still not completely reliable. - they have to scan _every_ vma at sys_exit() time, per thread! The second disadvantage is a real killer: pthread_exit() takes around 1 microsecond on Linux, but with thousands (or tens of thousands) of vmas every pthread_exit() takes a millisecond or more, also totally destroying the CPU's L1 and L2 caches! This is very much noticeable even for normal process sys_exit_group() calls: the kernel has to do the vma scanning unconditionally! (this is because the kernel has no knowledge about how many robust futexes there are to be cleaned up, because a robust futex might have been registered in another task, and the futex variable might have been simply mmap()-ed into this process's address space). This huge overhead forced the creation of CONFIG_FUTEX_ROBUST, but worse than that: the overhead makes robust futexes impractical for any type of generic Linux distribution. So it became clear to us, something had to be done. Last week, when Thomas Gleixner tried to fix up the vma-based robust futex patch in the -rt tree, he found a handful of new races and we were talking about it and were analyzing the situation. At that point a fundamentally different solution occured to me. This patchset (written in the past couple of days) implements that new solution. Be warned though - the patchset does things we normally dont do in Linux, so some might find the approach disturbing. Parental advice recommended ;-) New approach to robust futexes ------------------------------ At the heart of this new approach there is a per-thread private list of robust locks that userspace is holding (maintained by glibc) - which userspace list is registered with the kernel via a new syscall [this registration happens at most once per thread lifetime]. At do_exit() time, the kernel checks this user-space list: are there any robust futex locks to be cleaned up? In the common case, at do_exit() time, there is no list registered, so the cost of robust futexes is just a simple current->robust_list != NULL comparison. If the thread has registered a list, then normally the list is empty. If the thread/process crashed or terminated in some incorrect way then the list might be non-empty: in this case the kernel carefully walks the list [not trusting it], and marks all locks that are owned by this thread with the FUTEX_OWNER_DEAD bit, and wakes up one waiter (if any). The list is guaranteed to be private and per-thread, so it's lockless. There is one race possible though: since adding to and removing from the list is done after the futex is acquired by glibc, there is a few instructions window for the thread (or process) to die there, leaving the futex hung. To protect against this possibility, userspace (glibc) also maintains a simple per-thread 'list_op_pending' field, to allow the kernel to clean up if the thread dies after acquiring the lock, but just before it could have added itself to the list. Glibc sets this list_op_pending field before it tries to acquire the futex, and clears it after the list-add (or list-remove) has finished. That's all that is needed - all the rest of robust-futex cleanup is done in userspace [just like with the previous patches]. Ulrich Drepper has implemented the necessary glibc support for this new mechanism, which fully enables robust mutexes. (Ulrich plans to commit these changes to glibc-HEAD later today.) Key differences of this userspace-list based approach, compared to the vma based method: - it's much, much faster: at thread exit time, there's no need to loop over every vma (!), which the VM-based method has to do. Only a very simple 'is the list empty' op is done. - no VM changes are needed - 'struct address_space' is left alone. - no registration of individual locks is needed: robust mutexes dont need any extra per-lock syscalls. Robust mutexes thus become a very lightweight primitive - so they dont force the application designer to do a hard choice between performance and robustness - robust mutexes are just as fast. - no per-lock kernel allocation happens. - no resource limits are needed. - no kernel-space recovery call (FUTEX_RECOVER) is needed. - the implementation and the locking is "obvious", and there are no interactions with the VM. Performance ----------- I have benchmarked the time needed for the kernel to process a list of 1 million (!) held locks, using the new method [on a 2GHz CPU]: - with FUTEX_WAIT set [contended mutex]: 130 msecs - without FUTEX_WAIT set [uncontended mutex]: 30 msecs I have also measured an approach where glibc does the lock notification [which it currently does for !pshared robust mutexes], and that took 256 msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls userspace had to do. (1 million held locks are unheard of - we expect at most a handful of locks to be held at a time. Nevertheless it's nice to know that this approach scales nicely.) Implementation details ---------------------- The patch adds two new syscalls: one to register the userspace list, and one to query the registered list pointer: asmlinkage long sys_set_robust_list(struct robust_list_head __user *head, size_t len); asmlinkage long sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr, size_t __user *len_ptr); List registration is very fast: the pointer is simply stored in current->robust_list. [Note that in the future, if robust futexes become widespread, we could extend sys_clone() to register a robust-list head for new threads, without the need of another syscall.] So there is virtually zero overhead for tasks not using robust futexes, and even for robust futex users, there is only one extra syscall per thread lifetime, and the cleanup operation, if it happens, is fast and straightforward. The kernel doesnt have any internal distinction between robust and normal futexes. If a futex is found to be held at exit time, the kernel sets the highest bit of the futex word: #define FUTEX_OWNER_DIED 0x40000000 and wakes up the next futex waiter (if any). User-space does the rest of the cleanup. Otherwise, robust futexes are acquired by glibc by putting the TID into the futex field atomically. Waiters set the FUTEX_WAITERS bit: #define FUTEX_WAITERS 0x80000000 and the remaining bits are for the TID. Testing, architecture support ----------------------------- I've tested the new syscalls on x86 and x86_64, and have made sure the parsing of the userspace list is robust [ ;-) ] even if the list is deliberately corrupted. i386 and x86_64 syscalls are wired up at the moment, and Ulrich has tested the new glibc code (on x86_64 and i386), and it works for his robust-mutex testcases. All other architectures should build just fine too - but they wont have the new syscalls yet. Architectures need to implement the new futex_atomic_cmpxchg_inuser() inline function before writing up the syscalls (that function returns -ENOSYS right now). This patch: Add placeholder futex_atomic_cmpxchg_inuser() implementations to every architecture that supports futexes. It returns -ENOSYS. Signed-off-by:Ingo Molnar <mingo@elte.hu> Signed-off-by:
Thomas Gleixner <tglx@linutronix.de> Signed-off-by:
Arjan van de Ven <arjan@infradead.org> Acked-by:
Ulrich Drepper <drepper@redhat.com> Signed-off-by:
Andrew Morton <akpm@osdl.org> Signed-off-by:
Linus Torvalds <torvalds@osdl.org>
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Ingo Molnar authored
Add ptr_to_compat() - needed by the new robust futex code. Signed-off-by:
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Ingo Molnar authored
Add ptr_to_compat() to parisc - needed by the new robust futex code. Signed-off-by:
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Ingo Molnar authored
Add ptr_to_compat() to s390 - needed by the new robust-futex code. Signed-off-by:
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Ingo Molnar authored
Add ptr_to_compat() to ia64 - needed by the robust-futex code. Signed-off-by:
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Dave Hansen authored
Just about every architecture defines some macros to do operations on pfns. They're all virtually identical. This patch consolidates all of them. One minor glitch is that at least i386 uses them in a very skeletal header file. To keep away from #include dependency hell, I stuck the new definitions in a new, isolated header. Of all of the implementations, sh64 is the only one that varied by a bit. It used some masks to ensure that any sign-extension got ripped away before the arithmetic is done. This has been posted to that sh64 maintainers and the development list. Compiles on x86, x86_64, ia64 and ppc64. Signed-off-by:
Dave Hansen <haveblue@us.ibm.com> Signed-off-by:
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KAMEZAWA Hiroyuki authored
Helper functions for for_each_online_pgdat/for_each_zone look too big to be inlined. Speed of these helper macro itself is not very important. (inner loops are tend to do more work than this) This patch make helper function to be out-of-lined. inline out-of-line .text 005c0680 005bf6a0 005c0680 - 005bf6a0 = FE0 = 4Kbytes. Signed-off-by:
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Signed-off-by:
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KAMEZAWA Hiroyuki authored
By using for_each_online_pgdat(), pgdat_list is not necessary now. This patch removes it. Signed-off-by:
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KAMEZAWA Hiroyuki authored
Because pgdat_list was linked to pgdat_list in *reverse* order, (By default) some of arch has to sort it by themselves. for_each_pgdat has gone..for_each_online_pgdat() uses node_online_map, which doesn't need to be sorted. This patch removes codes for sorting pgdat. Signed-off-by:
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KAMEZAWA Hiroyuki authored
Replace for_each_pgdat() with for_each_online_pgdat(). Signed-off-by:
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KAMEZAWA Hiroyuki authored
Add a list_head to bootmem_data_t and make bootmems use it. bootmem list is sorted by node_boot_start. Only nodes against which init_bootmem() is called are linked to the list. (i386 allocates bootmem only from one node(0) not from all online nodes.) A summary: 1. for_each_online_pgdat() traverses all *online* nodes. 2. alloc_bootmem() allocates memory only from initialized-for-bootmem nodes. Signed-off-by:
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KAMEZAWA Hiroyuki authored
This patch defines for_each_online_pgdat() as a replacement of for_each_pgdat() Now, online nodes are managed by node_online_map. But for_each_pgdat() uses pgdat_link to iterate over all nodes(pgdat). This means management structure for online pgdat is duplicated. I think using node_online_map for for_each_pgdat() is simple and sane rather ather than pgdat_link. New macro is named as for_each_online_pgdat(). Following patch will fix callers of for_each_pgdat(). The bootmem allocater uses for_each_pgdat() before pgdat initialization. I don't think it's sane. Following patch will fix it. Signed-off-by:
Yasunori Goto <y-goto@jp.fujitsu.com> Signed-off-by:
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KAMEZAWA Hiroyuki authored
This patch removes zone_mem_map. pfn_to_page uses pgdat, page_to_pfn uses zone. page_to_pfn can use pgdat instead of zone, which is only one user of zone_mem_map. By modifing it, we can remove zone_mem_map. Signed-off-by:
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KAMEZAWA Hiroyuki authored
ia64 has special config CONFIG_VIRTUAL_MEM_MAP. CONFIG_DISCONTIGMEM=y && CONFIG_VIRTUAL_MEM_MAP!=y is bug ? Signed-off-by:
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KAMEZAWA Hiroyuki authored
xtensa can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
v850 can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
UML can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
sparc can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
sh64 can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
sh can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
s390 can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
PPC can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
PARISC can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
MIPS can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
m32r can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
H8300 can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
FRV can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
cris can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
arm26 can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
ARM can use generic funcs. PFN_TO_NID, LOCAL_MAP_NR are defined by sub-archs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
Alpha can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
PowerPC can use generic ones. Signed-off-by:
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KAMEZAWA Hiroyuki authored
x86_64 can use generic funcs. For DISCONTIGMEM, CONFIG_OUT_OF_LINE_PFN_TO_PAGE is selected. Signed-off-by:
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KAMEZAWA Hiroyuki authored
i386 can use generic funcs. Signed-off-by:
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KAMEZAWA Hiroyuki authored
There are 3 memory models, FLATMEM, DISCONTIGMEM, SPARSEMEM. Each arch has its own page_to_pfn(), pfn_to_page() for each models. But most of them can use the same arithmetic. This patch adds asm-generic/memory_model.h, which includes generic page_to_pfn(), pfn_to_page() definitions for each memory model. When CONFIG_OUT_OF_LINE_PFN_TO_PAGE=y, out-of-line functions are used instead of macro. This is enabled by some archs and reduces text size. Signed-off-by:
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Shaohua Li authored
Don't use cpuid.2 to determine cache info if cpuid.4 is supported. The exception is P4 trace cache. We always use cpuid.2 to get trace cache under P4. Signed-off-by:
Shaohua Li <shaohua.li@intel.com> Cc: Andi Kleen <ak@muc.de> Signed-off-by:
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Siddha, Suresh B authored
Current sched groups power calculation for allnodes_domains is wrong. We should really be using cumulative power of the physical packages in that group (similar to the calculation in node_domains) Signed-off-by:
Suresh Siddha <suresh.b.siddha@intel.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Signed-off-by:
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Siddha, Suresh B authored
Add a new sched domain for representing multi-core with shared caches between cores. Consider a dual package system, each package containing two cores and with last level cache shared between cores with in a package. If there are two runnable processes, with this appended patch those two processes will be scheduled on different packages. On such systems, with this patch we have observed 8% perf improvement with specJBB(2 warehouse) benchmark and 35% improvement with CFP2000 rate(with 2 users). This new domain will come into play only on multi-core systems with shared caches. On other systems, this sched domain will be removed by domain degeneration code. This new domain can be also used for implementing power savings policy (see OLS 2005 CMP kernel scheduler paper for more details.. I will post another patch for power savings policy soon) Most of the arch/* file changes are for cpu_coregroup_map() implementation. Signed-off-by:
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Andreas Mohr authored
small schedule() microoptimization. Signed-off-by:
Andreas Mohr <andi@lisas.de> Signed-off-by:
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Martin Andersson authored
Is a truncation error in kernel/sched.c triggered when the nice value is negative. The affected code is used in the TASK_INTERACTIVE macro. The code is: #define SCALE(v1,v1_max,v2_max) \ (v1) * (v2_max) / (v1_max) which is used in this way: SCALE(TASK_NICE(p), 40, MAX_BONUS) Comments in the code says: * This part scales the interactivity limit depending on niceness. * * We scale it linearly, offset by the INTERACTIVE_DELTA delta. * Here are a few examples of different nice levels: * * TASK_INTERACTIVE(-20): [1,1,1,1,1,1,1,1,1,0,0] * TASK_INTERACTIVE(-10): [1,1,1,1,1,1,1,0,0,0,0] * TASK_INTERACTIVE( 0): [1,1,1,1,0,0,0,0,0,0,0] * TASK_INTERACTIVE( 10): [1,1,0,0,0,0,0,0,0,0,0] * TASK_INTERACTIVE( 19): [0,0,0,0,0,0,0,0,0,0,0] * * (the X axis represents the possible -5 ... 0 ... +5 dynamic * priority range a task can explore, a value of '1' means the * task is rated interactive.) However, the current code does not scale it linearly and the result differs from the given examples. If the mathematical function "floor" is used when the nice value is negative instead of the truncation one gets when using integer division, the result conforms to the documentation. Output of TASK_INTERACTIVE when using the kernel code: nice dynamic priorities -20 1 1 1 1 1 1 1 1 1 0 0 -19 1 1 1 1 1 1 1 1 0 0 0 -18 1 1 1 1 1 1 1 1 0 0 0 -17 1 1 1 1 1 1 1 1 0 0 0 -16 1 1 1 1 1 1 1 1 0 0 0 -15 1 1 1 1 1 1 1 0 0 0 0 -14 1 1 1 1 1 1 1 0 0 0 0 -13 1 1 1 1 1 1 1 0 0 0 0 -12 1 1 1 1 1 1 1 0 0 0 0 -11 1 1 1 1 1 1 0 0 0 0 0 -10 1 1 1 1 1 1 0 0 0 0 0 -9 1 1 1 1 1 1 0 0 0 0 0 -8 1 1 1 1 1 1 0 0 0 0 0 -7 1 1 1 1 1 0 0 0 0 0 0 -6 1 1 1 1 1 0 0 0 0 0 0 -5 1 1 1 1 1 0 0 0 0 0 0 -4 1 1 1 1 1 0 0 0 0 0 0 -3 1 1 1 1 0 0 0 0 0 0 0 -2 1 1 1 1 0 0 0 0 0 0 0 -1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 2 1 1 1 1 0 0 0 0 0 0 0 3 1 1 1 1 0 0 0 0 0 0 0 4 1 1 1 0 0 0 0 0 0 0 0 5 1 1 1 0 0 0 0 0 0 0 0 6 1 1 1 0 0 0 0 0 0 0 0 7 1 1 1 0 0 0 0 0 0 0 0 8 1 1 0 0 0 0 0 0 0 0 0 9 1 1 0 0 0 0 0 0 0 0 0 10 1 1 0 0 0 0 0 0 0 0 0 11 1 1 0 0 0 0 0 0 0 0 0 12 1 0 0 0 0 0 0 0 0 0 0 13 1 0 0 0 0 0 0 0 0 0 0 14 1 0 0 0 0 0 0 0 0 0 0 15 1 0 0 0 0 0 0 0 0 0 0 16 0 0 0 0 0 0 0 0 0 0 0 17 0 0 0 0 0 0 0 0 0 0 0 18 0 0 0 0 0 0 0 0 0 0 0 19 0 0 0 0 0 0 0 0 0 0 0 Output of TASK_INTERACTIVE when using "floor" nice dynamic priorities -20 1 1 1 1 1 1 1 1 1 0 0 -19 1 1 1 1 1 1 1 1 1 0 0 -18 1 1 1 1 1 1 1 1 1 0 0 -17 1 1 1 1 1 1 1 1 1 0 0 -16 1 1 1 1 1 1 1 1 0 0 0 -15 1 1 1 1 1 1 1 1 0 0 0 -14 1 1 1 1 1 1 1 1 0 0 0 -13 1 1 1 1 1 1 1 1 0 0 0 -12 1 1 1 1 1 1 1 0 0 0 0 -11 1 1 1 1 1 1 1 0 0 0 0 -10 1 1 1 1 1 1 1 0 0 0 0 -9 1 1 1 1 1 1 1 0 0 0 0 -8 1 1 1 1 1 1 0 0 0 0 0 -7 1 1 1 1 1 1 0 0 0 0 0 -6 1 1 1 1 1 1 0 0 0 0 0 -5 1 1 1 1 1 1 0 0 0 0 0 -4 1 1 1 1 1 0 0 0 0 0 0 -3 1 1 1 1 1 0 0 0 0 0 0 -2 1 1 1 1 1 0 0 0 0 0 0 -1 1 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 2 1 1 1 1 0 0 0 0 0 0 0 3 1 1 1 1 0 0 0 0 0 0 0 4 1 1 1 0 0 0 0 0 0 0 0 5 1 1 1 0 0 0 0 0 0 0 0 6 1 1 1 0 0 0 0 0 0 0 0 7 1 1 1 0 0 0 0 0 0 0 0 8 1 1 0 0 0 0 0 0 0 0 0 9 1 1 0 0 0 0 0 0 0 0 0 10 1 1 0 0 0 0 0 0 0 0 0 11 1 1 0 0 0 0 0 0 0 0 0 12 1 0 0 0 0 0 0 0 0 0 0 13 1 0 0 0 0 0 0 0 0 0 0 14 1 0 0 0 0 0 0 0 0 0 0 15 1 0 0 0 0 0 0 0 0 0 0 16 0 0 0 0 0 0 0 0 0 0 0 17 0 0 0 0 0 0 0 0 0 0 0 18 0 0 0 0 0 0 0 0 0 0 0 19 0 0 0 0 0 0 0 0 0 0 0 Signed-off-by:
Martin Andersson <martin.andersson@control.lth.se> Acked-by:
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