[Question] vmalloc latency in RT-Linux

[Zhipeng Shi <zhipeng.shi0@gmail.com>]

I noticed in rt-linux, vmalloc has a large latency. This is because the 
free_vmap_area_lock is held for a long time in the function  
__purge_vmap_area_lazy. 

In non-RT-Linux, because the function spin_is_contended is well 
implemented, so there will be no such problem.

But in RT-Linux, spin_is_contended simply returns 0. I don't understand 
why this function was implemented like this before, but in order to
solve this problem, I thought of two ways.

The first is to modify the spin_is_contended definition in spinlock_rt.h 
as shown below, but I'm not sure if the change has side-effects:

-#define spin_is_contended(lock)    (((void)(lock), 0))
+static inline int spin_is_contended(spinlock_t *lock)
+{
+ unsigned long *p = (unsigned long *) &lock->lock.owner;
+
+ return (READ_ONCE(*p) & RT_MUTEX_HAS_WAITERS);
+}

The second is by reducing the number of lazy_max_pages, but it will lead 
to lower performance of vmalloc.

Guys, Do you have any good ideas?

Best regards,
Zhipeng