Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context

[Heiko Carstens]

On Tue, Jun 06, 2017 at 04:45:57PM +0200, Christian Borntraeger wrote:
> Adding s390 folks and list
> >> Only s390 is TSO, arm64 is very much a weak arch.
> > 
> > Right, and thus arm64 can implement a fast this_cpu_inc using LL/SC.
> > s390 cannot because its atomic_inc has implicit memory barriers.
> > 
> > s390's this_cpu_inc is *faster* than the generic one, but still pretty slow.
> 
> FWIW, we improved the performance of local_irq_save/restore some time ago
> with commit 204ee2c5643199a2 ("s390/irqflags: optimize irq restore") and
> disable/enable seem to be reasonably fast (3-5ns on my system doing both
> disable/enable in a loop) on todays systems. So  I would assume that the
> generic implementation would not be that bad. 
> 
> A the same time, the implicit memory barrier of the atomic_inc should be
> even cheaper. In contrast to x86, a full smp_mb seems to be almost for
> free (looks like <= 1 cycle for a bcr 14,0 and no contention). So I
> _think_ that this should be really fast enough.
> 
> As a side note, I am asking myself, though, why we do need the
> preempt_disable/enable for the cases where we use the opcodes 
> like lao (atomic load and or to a memory location) and friends.

Because you want the atomic instruction to be executed on the local cpu for
which you have to per cpu pointer. If you get preempted to a different cpu
between the ptr__ assignment and lan instruction it might be executed not
on the local cpu. It's not really a correctness issue.

#define arch_this_cpu_to_op(pcp, val, op)				\
{									\
	typedef typeof(pcp) pcp_op_T__;					\
	pcp_op_T__ val__ = (val);					\
	pcp_op_T__ old__, *ptr__;					\
	preempt_disable();						\
	ptr__ = raw_cpu_ptr(&(pcp));					\
	asm volatile(							\
		op "	%[old__],%[val__],%[ptr__]\n"			\
		: [old__] "=d" (old__), [ptr__] "+Q" (*ptr__)		\
		: [val__] "d" (val__)					\
		: "cc");						\
	preempt_enable();						\
}

#define this_cpu_and_4(pcp, val)	arch_this_cpu_to_op(pcp, val, "lan")

However in reality it doesn't matter at all, since all distributions we
care about have preemption disabled.

So this_cpu_inc() should just generate three instructions: two to calculate
the percpu pointer and an additional asi for the atomic increment, with
operand specific serialization. This is supposed to be a lot faster than
disabling/enabling interrupts around a non-atomic operation.

But maybe I didn't get the point of this thread :)