Messages in this thread | | | Date | Thu, 9 Apr 2015 13:58:16 -0700 | Subject | Re: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() | From | Jason Low <> |
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On Thu, Apr 9, 2015 at 12:58 PM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > On Thu, Apr 09, 2015 at 12:43:38PM -0700, Jason Low wrote: >> On Thu, 2015-04-09 at 11:16 -0700, Linus Torvalds wrote: >> > On Thu, Apr 9, 2015 at 11:08 AM, Linus Torvalds >> > <torvalds@linux-foundation.org> wrote: >> > > >> > > The pointer is a known-safe kernel pointer - it's just that it was >> > > "known safe" a few instructions ago, and might be rcu-free'd at any >> > > time. >> > >> > Actually, we could even do something like this: >> > >> > static inline int sem_owner_on_cpu(struct semaphore *sem, struct >> > task_struct *owner) >> > { >> > int on_cpu; >> > >> > #ifdef CONFIG_DEBUG_PAGEALLOC >> > rcu_read_lock(); >> > #endif >> > on_cpu = sem->owner == owner && owner->on_cpu; >> > #ifdef CONFIG_DEBUG_PAGEALLOC >> > rcu_read_unlock(); >> > #endif >> > return on_cpu; >> > } >> > >> > because we really don't need to hold the RCU lock over the whole loop, >> > we just need to validate that the semaphore owner still matches, and >> > if so, check that it's on_cpu. >> > >> > And if CONFIG_DEBUG_PAGEALLOC is set, we don't care about performance >> > *at*all*. We will have worse performance problems than doing some RCU >> > read-locking inside the loop. >> > >> > And if CONFIG_DEBUG_PAGEALLOC isn't set, we don't really care about >> > locking, since at worst we just access stale memory for one iteration. >> > >> > Hmm. It's not pretty, but neither is the current "let's just take a >> > rcu lock that we don't really need over a loop that doesn't have very >> > strict bounding". >> > >> > Comments? >> >> So that looks more similar to how the original code was where the >> rcu_read_lock() and rcu_read_unlock() was done inside the owner_running >> helper function (though without the CONFIG_DEBUG_PAGEALLOC), before >> commit 307bf9803f25 ("sched: Simplify mutex_spin_on_owner()") modified >> it to be done outside the loop. > > Another approach would be to post a timer before entering the spinloop, > and have the timer handler set the resched bit. Then the loop would > be bounded, safe, and would run at full speed.
Though posting a timer, ect... would also add more overhead right?
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