Messages in this thread | | | Date | Tue, 12 Jul 2005 14:04:23 -0700 | From | "Paul E. McKenney" <> | Subject | Re: PREEMPT/PREEMPT_RT question |
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On Tue, Jul 12, 2005 at 09:26:35PM +0200, Ingo Molnar wrote: > * Paul E. McKenney <paulmck@us.ibm.com> wrote: > > So, time to get serious about a bit of code cleanup: > > > > o The heavyweight atomic operations in rcu_read_lock() and > > rcu_read_unlock() are not needed in UP kernels, since > > interrupts are disabled. > > atomic_*() ops should already be lightweight on UP.
Agreed, will worry about architectures where they might not be later.
> > o In order to get things to work in both CONFIG_PREEMPT and > > CONFIG_PREEMPT_RT, I ended up using the following: > > > > #ifdef CONFIG_PREEMPT_RT > > > > #define rcu_spinlock_t _raw_spinlock_t > > #define rcu_spin_lock(l, f) _raw_spin_lock(l) > > #define rcu_spin_trylock(l, f) _raw_spin_trylock(l) > > #define rcu_spin_unlock(l, f) _raw_spin_unlock(l) > > #define RCU_SPIN_LOCK_UNLOCKED RAW_SPIN_LOCK_UNLOCKED > > > > #else /* #ifdef CONFIG_PREEMPT_RT */ > > > > #define rcu_spinlock_t spinlock_t > > #define rcu_spin_lock(l, f) spin_lock_irqsave(l, f) > > #define rcu_spin_trylock(l, f) spin_trylock_irqsave(l, f) > > #define rcu_spin_unlock(l, f) spin_unlock_irqrestore(l, f) > > #define RCU_SPIN_LOCK_UNLOCKED SPIN_LOCK_UNLOCKED > > > > #endif /* #else #ifdef CONFIG_PREEMPT_RT */ > > > > Then using rcu_spin_lock() &c everywhere. The problem is > > that (as near as I can tell) the only way to prevent interrupts > > from running on the current CPU in CONFIG_PREEMPT kernels is > > to use the _irq spinlock primitives, but _raw_spin_lock() does > > the job in CONFIG_PREEMPT_RT (since interrupts are run in process > > context, right). I could use _irq in both, but that would > > unnecessarily degrade interrupt latency in CONFIG_PREEMPT_RT. > > > > Suggestions??? > > in PREEMPT_RT, if you define a spinlock type as raw_spinlock_t (via > DEFINE_RAW_SPINLOCK(lock)) then the spin_lock*() APIs automatically > switch over to that type. I.e. no need to do the rcu_ stuff AFAICT. > (unless i missed some detail about what you are trying to do.) Raw > spinlocks under PREEMPT_RT pair with the raw IRQ flag, i.e. > spin_lock_irq() will disable hard interrupts. Generally i'd suggest to > go for the raw spinlock and raw-irq-flag-disabling variant, because RCU > locking is core enough functionality to warrant atomic treatment. If > there's any latency path in it, we can work on breaking it up later.
Fair enough!
The thing I am (perhaps foolishly) trying to do is get to a single source base that provides PREEMPT_RCU under both CONFIG_PREEMPT and CONFIG_PREEMPT_RT. Since you are OK with RCU disabling interrupts, I am happy.
> it's perfectly fine to disable raw interrupts with the RCU code, as long > as you do O(1) amount of work. (where the constant factor isnt 2^(2^32) > ;-)
Will try to keep it down to a dull roar. ;-)
> > Some remaining shortcomings of the current code: > > > > o Untested on CONFIG_PREEMPT_RT (working on this, suggestions > > for 4-CPU-stable CONFIG_PREEMPT_RT versions most welcome). > > latest (-51-28) has no known regressions, and i regularly boot on 4-way > (and irregularly on 8-way) boxes. So if you see something strange on the > latest -RT kernel, please report it.
And stock -51-7 just passed nine rounds of kernbench + LTP on some 4-CPU x86 machines, with one still running. Good stuff!!!
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