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The use of rcuref_t for reference counting introduces a performance bottleneck when accessed concurrently by multiple threads during futex operations. Replace rcuref_t with special crafted per-CPU reference counters. The lifetime logic remains the same. The newly allocate private hash starts in FR_PERCPU state. In this state, each futex operation that requires the private hash uses a per-CPU counter (an unsigned int) for incrementing or decrementing the reference count. When the private hash is about to be replaced, the per-CPU counters are migrated to a atomic_t counter mm_struct::futex_atomic. The migration process: - Waiting for one RCU grace period to ensure all users observe the current private hash. This can be skipped if a grace period elapsed since the private hash was assigned. - futex_private_hash::state is set to FR_ATOMIC, forcing all users to use mm_struct::futex_atomic for reference counting. - After a RCU grace period, all users are guaranteed to be using the atomic counter. The per-CPU counters can now be summed up and added to the atomic_t counter. If the resulting count is zero, the hash can be safely replaced. Otherwise, active users still hold a valid reference. - Once the atomic reference count drops to zero, the next futex operation will switch to the new private hash. call_rcu_hurry() is used to speed up transition which otherwise might be delay with RCU_LAZY. There is nothing wrong with using call_rcu(). The side effects would be that on auto scaling the new hash is used later and the SET_SLOTS prctl() will block longer. [bigeasy: commit description + mm get/ put_async] Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20250710110011.384614-3-bigeasy@linutronix.de |
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