Messages in this thread Patch in this message | | | Date | Thu, 07 May 2009 15:40:14 -0700 | From | Darren Hart <> | Subject | [tip PATCH] futex: add requeue-pi documentation |
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From: Darren Hart <dvhltc@us.ibm.com>
Add Documentation/futex-requeue-pi.txt describing the motivation for the newly added FUTEX_*REQUEUE_PI op codes and their implementation.
Signed-off-by: Darren Hart <dvhltc@us.ibm.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Sripathi Kodi <sripathik@in.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: John Stultz <johnstul@us.ibm.com> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Dinakar Guniguntala <dino@in.ibm.com> Cc: Ulrich Drepper <drepper@redhat.com> Cc: Eric Dumazet <dada1@cosmosbay.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Jakub Jelinek <jakub@redhat.com> ---
Documentation/futex-requeue-pi.txt | 124 ++++++++++++++++++++++++++++++++++++ 1 files changed, 124 insertions(+), 0 deletions(-) create mode 100644 Documentation/futex-requeue-pi.txt
diff --git a/Documentation/futex-requeue-pi.txt b/Documentation/futex-requeue-pi.txt new file mode 100644 index 0000000..7933394 --- /dev/null +++ b/Documentation/futex-requeue-pi.txt @@ -0,0 +1,124 @@ +Futex Requeue PI +---------------- + +Requeueing of tasks from a non-PI futex to a PI futex requires special handling +in order to ensure the underlying rt_mutex is never left without an owner if it +has waiters; doing so would break the PI boosting logic [see +rt-mutex-desgin.txt] For the purposes of brevity, this action will be referred +to as "requeue_pi" throughout this document. Priority inheritance is +abbreviated throughout as "PI". + +Motivation +---------- + +Without requeue_pi, the glibc implementation of pthread_cond_broadcast() must +resort to waking all the tasks waiting on a pthread_condvar and letting them +try to sort out which task gets to run first in classic thundering-herd +formation. An ideal implementation would wake the highest-priority waiter, and +leave the rest to the natural wakeup inherent in unlocking the mutex associated +with the condvar. + +Consider the simplified glibc calls: + +/* caller must lock mutex */ +pthread_cond_wait(cond, mutex) +{ + lock(cond->__data.__lock); + unlock(mutex); + do { + unlock(cond->__data.__lock); + futex_wait(cond->__data.__futex); + lock(cond->__data.__lock); + } while(...) + unlock(cond->__data.__lock); + lock(mutex); +} + +pthread_cond_broadcast(cond) +{ + lock(cond->__data.__lock); + unlock(cond->__data.__lock); + futex_requeue(cond->data.__futex, cond->mutex); +} + +Once pthread_cond_broadcast() requeues the tasks, the cond->mutex has waiters. +Note that pthread_cond_wait() attempts to lock the mutex only after it has +returned to user space. This will leave the underlying rt_mutex with waiters, +and no owner, breaking the previously mentioned PI-boosting algorithms. + +In order to support PI-aware pthread_condvar's, the kernel needs to be able to +requeue tasks to PI futexes. This support implies that upon a successful +futex_wait system call, the caller would return to user space already holding +the PI futex. The glibc implementation would be modified as follows: + + +/* caller must lock mutex */ +pthread_cond_wait_pi(cond, mutex) +{ + lock(cond->__data.__lock); + unlock(mutex); + do { + unlock(cond->__data.__lock); + futex_wait_requeue_pi(cond->__data.__futex); + lock(cond->__data.__lock); + } while(...) + unlock(cond->__data.__lock); + /* the kernel acquired the the mutex for us */ +} + +pthread_cond_broadcast_pi(cond) +{ + lock(cond->__data.__lock); + unlock(cond->__data.__lock); + futex_requeue_pi(cond->data.__futex, cond->mutex); +} + +The actual glibc implementation will likely test for PI and make the +necessary changes inside the existing calls rather than creating new calls +for the PI cases. Similar changes are needed for pthread_cond_timedwait() +and pthread_cond_signal(). + +Implementation +-------------- + +In order to ensure the rt_mutex has an owner if it has waiters, it is necessary +for both the requeue code, as well as the waiting code, to be able to acquire +the rt_mutex before returning to user space. The requeue code cannot simply +wake the waiter and leave it to acquire the rt_mutex as it would open a race +window between the requeue call returning to user space and the waiter waking +and starting to run. This is especially true in the uncontended case. + +The solution involves two new rt_mutex helper routines, +rt_mutex_start_proxy_lock() and rt_mutex_finish_proxy_lock(), which allow the +requeue code to acquire an uncontended rt_mutex on behalf of the waiter and to +enqueue the waiter on a contended rt_mutex. Two new system calls provide the +kernel<->user interface to requeue_pi: FUTEX_WAIT_REQUEUE_PI and +FUTEX_REQUEUE_CMP_PI. + +FUTEX_WAIT_REQUEUE_PI is called by the waiter (pthread_cond_wait() and +pthread_cond_timedwait()) to block on the initial futex and wait to be requeued +to a PI-aware futex. The implementation is the result of a high-speed +collision between futex_wait() and futex_lock_pi(), with some extra logic to +check for the additional wake-up scenarios. + +FUTEX_REQUEUE_CMP_PI is called by the waker (pthread_cond_broadcast() and +pthread_cond_signal()) to requeue and possibly wake the waiting tasks. +Internally, this system call is still handled by futex_requeue (by passing +requeue_pi=1). Before requeueing, futex_requeue() attempts to acquire the +requeue target PI futex on behalf of the top waiter. If it can, this waiter is +woken. futex_requeue() then proceeds to requeue the remaining +nr_wake+nr_requeue tasks to the PI futex, calling rt_mutex_start_proxy_lock() +prior to each requeue to prepare the task as a waiter on the underlying +rt_mutex. It is possible that the lock can be acquired at this stage as well, +if so, the next waiter is woken to finish the acquisition of the lock. +FUTEX_REQUEUE_PI accepts nr_wake and nr_requeue as arguments, but their sum is +all that really matters. futex_requeue() will wake or requeue up to nr_wake + +nr_requeue tasks. It will wake only as many tasks as it can acquire the lock +for, which in the majority of cases should be 0 as good programming practice +dictates that the caller of either pthread_cond_broadcast() or +pthread_cond_signal() acquire the mutex prior to making the call. +FUTEX_REQUEUE_PI requires that nr_wake=1. nr_requeue should be INT_MAX for +broadcast and 0 for signal. + + + -- Darren Hart IBM Linux Technology Center Real-Time Linux Team
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