On 6/23/22 06:51, Baoquan He wrote:
On 06/21/22 at 08:15pm, Zhipeng Shi wrote:
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.
__purge_vmap_area_lazy() has cond_resched_lock() to reschedule and drop
the lock. From your saying, it's spin_is_contended() which is not
working well to make rescheduling happen during __purge_vmap_area_lazy()
handling. Then the fixing should be done in lock side.
Sebastian had sent out patch last year to fix spin_is_contended().
https://lore.kernel.org/lkml/20210906143004.2259141-3-bigeasy@xxxxxxxxxxxxx/
However, there is no follow-up after some discussion and the patch
wasn't merged.
Cheers,
Longman
