167 lines
6.4 KiB
Plaintext
167 lines
6.4 KiB
Plaintext
lglock - local/global locks for mostly local access patterns
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------------------------------------------------------------
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Origin: Nick Piggin's VFS scalability series introduced during
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2.6.35++ [1] [2]
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Location: kernel/locking/lglock.c
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include/linux/lglock.h
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Users: currently only the VFS and stop_machine related code
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Design Goal:
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------------
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Improve scalability of globally used large data sets that are
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distributed over all CPUs as per_cpu elements.
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To manage global data structures that are partitioned over all CPUs
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as per_cpu elements but can be mostly handled by CPU local actions
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lglock will be used where the majority of accesses are cpu local
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reading and occasional cpu local writing with very infrequent
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global write access.
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* deal with things locally whenever possible
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- very fast access to the local per_cpu data
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- reasonably fast access to specific per_cpu data on a different
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CPU
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* while making global action possible when needed
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- by expensive access to all CPUs locks - effectively
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resulting in a globally visible critical section.
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Design:
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-------
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Basically it is an array of per_cpu spinlocks with the
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lg_local_lock/unlock accessing the local CPUs lock object and the
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lg_local_lock_cpu/unlock_cpu accessing a remote CPUs lock object
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the lg_local_lock has to disable preemption as migration protection so
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that the reference to the local CPUs lock does not go out of scope.
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Due to the lg_local_lock/unlock only touching cpu-local resources it
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is fast. Taking the local lock on a different CPU will be more
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expensive but still relatively cheap.
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One can relax the migration constraints by acquiring the current
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CPUs lock with lg_local_lock_cpu, remember the cpu, and release that
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lock at the end of the critical section even if migrated. This should
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give most of the performance benefits without inhibiting migration
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though needs careful considerations for nesting of lglocks and
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consideration of deadlocks with lg_global_lock.
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The lg_global_lock/unlock locks all underlying spinlocks of all
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possible CPUs (including those off-line). The preemption disable/enable
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are needed in the non-RT kernels to prevent deadlocks like:
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on cpu 1
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task A task B
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lg_global_lock
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got cpu 0 lock
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<<<< preempt <<<<
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lg_local_lock_cpu for cpu 0
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spin on cpu 0 lock
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On -RT this deadlock scenario is resolved by the arch_spin_locks in the
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lglocks being replaced by rt_mutexes which resolve the above deadlock
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by boosting the lock-holder.
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Implementation:
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---------------
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The initial lglock implementation from Nick Piggin used some complex
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macros to generate the lglock/brlock in lglock.h - they were later
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turned into a set of functions by Andi Kleen [7]. The change to functions
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was motivated by the presence of multiple lock users and also by them
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being easier to maintain than the generating macros. This change to
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functions is also the basis to eliminated the restriction of not
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being initializeable in kernel modules (the remaining problem is that
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locks are not explicitly initialized - see lockdep-design.txt)
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Declaration and initialization:
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-------------------------------
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#include <linux/lglock.h>
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DEFINE_LGLOCK(name)
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or:
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DEFINE_STATIC_LGLOCK(name);
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lg_lock_init(&name, "lockdep_name_string");
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on UP this is mapped to DEFINE_SPINLOCK(name) in both cases, note
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also that as of 3.18-rc6 all declaration in use are of the _STATIC_
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variant (and it seems that the non-static was never in use).
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lg_lock_init is initializing the lockdep map only.
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Usage:
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------
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From the locking semantics it is a spinlock. It could be called a
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locality aware spinlock. lg_local_* behaves like a per_cpu
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spinlock and lg_global_* like a global spinlock.
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No surprises in the API.
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lg_local_lock(*lglock);
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access to protected per_cpu object on this CPU
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lg_local_unlock(*lglock);
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lg_local_lock_cpu(*lglock, cpu);
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access to protected per_cpu object on other CPU cpu
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lg_local_unlock_cpu(*lglock, cpu);
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lg_global_lock(*lglock);
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access all protected per_cpu objects on all CPUs
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lg_global_unlock(*lglock);
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There are no _trylock variants of the lglocks.
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Note that the lg_global_lock/unlock has to iterate over all possible
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CPUs rather than the actually present CPUs or a CPU could go off-line
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with a held lock [4] and that makes it very expensive. A discussion on
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these issues can be found at [5]
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Constraints:
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------------
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* currently the declaration of lglocks in kernel modules is not
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possible, though this should be doable with little change.
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* lglocks are not recursive.
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* suitable for code that can do most operations on the CPU local
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data and will very rarely need the global lock
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* lg_global_lock/unlock is *very* expensive and does not scale
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* on UP systems all lg_* primitives are simply spinlocks
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* in PREEMPT_RT the spinlock becomes an rt-mutex and can sleep but
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does not change the tasks state while sleeping [6].
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* in PREEMPT_RT the preempt_disable/enable in lg_local_lock/unlock
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is downgraded to a migrate_disable/enable, the other
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preempt_disable/enable are downgraded to barriers [6].
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The deadlock noted for non-RT above is resolved due to rt_mutexes
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boosting the lock-holder in this case which arch_spin_locks do
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not do.
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lglocks were designed for very specific problems in the VFS and probably
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only are the right answer in these corner cases. Any new user that looks
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at lglocks probably wants to look at the seqlock and RCU alternatives as
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her first choice. There are also efforts to resolve the RCU issues that
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currently prevent using RCU in place of view remaining lglocks.
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Note on brlock history:
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-----------------------
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The 'Big Reader' read-write spinlocks were originally introduced by
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Ingo Molnar in 2000 (2.4/2.5 kernel series) and removed in 2003. They
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later were introduced by the VFS scalability patch set in 2.6 series
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again as the "big reader lock" brlock [2] variant of lglock which has
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been replaced by seqlock primitives or by RCU based primitives in the
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3.13 kernel series as was suggested in [3] in 2003. The brlock was
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entirely removed in the 3.13 kernel series.
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Link: 1 http://lkml.org/lkml/2010/8/2/81
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Link: 2 http://lwn.net/Articles/401738/
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Link: 3 http://lkml.org/lkml/2003/3/9/205
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Link: 4 https://lkml.org/lkml/2011/8/24/185
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Link: 5 http://lkml.org/lkml/2011/12/18/189
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Link: 6 https://www.kernel.org/pub/linux/kernel/projects/rt/
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patch series - lglocks-rt.patch.patch
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Link: 7 http://lkml.org/lkml/2012/3/5/26
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