[PATCH 1/2] s390/gmap: voluntarily schedule during key setting

[Christian Borntraeger <borntraeger@linux.ibm.com>]

With large and many guest with storage keys it is possible to create
large latencies or stalls during initial key setting:

rcu: INFO: rcu_sched self-detected stall on CPU
rcu:   18-....: (2099 ticks this GP) idle=54e/1/0x4000000000000002 softirq=35598716/35598716 fqs=998
       (t=2100 jiffies g=155867385 q=20879)
Task dump for CPU 18:
CPU 1/KVM       R  running task        0 1030947 256019 0x06000004
Call Trace:
sched_show_task
rcu_dump_cpu_stacks
rcu_sched_clock_irq
update_process_times
tick_sched_handle
tick_sched_timer
__hrtimer_run_queues
hrtimer_interrupt
do_IRQ
ext_int_handler
ptep_zap_key

The mmap lock is held during the page walking but since this is a
semaphore scheduling is still possible. Same for the kvm srcu.
To minimize overhead do this on every segment table entry or large page.

Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
---
 arch/s390/mm/gmap.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index 1ac73917a8d3..b8ae4a4aa2ba 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -2608,6 +2608,18 @@ static int __s390_enable_skey_pte(pte_t *pte, unsigned long addr,
 	return 0;
 }
 
+/*
+ * Give a chance to schedule after setting a key to 256 pages.
+ * We only hold the mm lock, which is a rwsem and the kvm srcu.
+ * Both can sleep.
+ */
+static int __s390_enable_skey_pmd(pmd_t *pmd, unsigned long addr,
+				  unsigned long next, struct mm_walk *walk)
+{
+	cond_resched();
+	return 0;
+}
+
 static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
 				      unsigned long hmask, unsigned long next,
 				      struct mm_walk *walk)
@@ -2630,12 +2642,14 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
 	end = start + HPAGE_SIZE - 1;
 	__storage_key_init_range(start, end);
 	set_bit(PG_arch_1, &page->flags);
+	cond_resched();
 	return 0;
 }
 
 static const struct mm_walk_ops enable_skey_walk_ops = {
 	.hugetlb_entry		= __s390_enable_skey_hugetlb,
 	.pte_entry		= __s390_enable_skey_pte,
+	.pmd_entry		= __s390_enable_skey_pmd,
 };
 
 int s390_enable_skey(void)
-- 
2.35.1