Re: PREEMPT/PREEMPT_RT question

[Ingo Molnar]

* Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:

> Hello!
> 
> OK, counter-flip RCU actually survives a pair of overnight runs on 
> CONFIG_PREEMPT running on 4-CPU machines, and also survives five 
> kernbenches and an LTP on another 4-CPU machine. [...]

cool!

> 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.

> 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.

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) 
;-)

> 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.

	Ingo
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