Subject: [PATCH v2] powerpc/pseries: explicitly reschedule during drmem_lmb list traversal

From: Nathan Lynch <[email-masked]>
Date: Tue, 11 Aug 2020 20:20:05 -0500

The drmem lmb list can have hundreds of thousands of entries, and
unfortunately lookups take the form of linear searches. As long as
this is the case, traversals have the potential to monopolize the CPU
and provoke lockup reports, workqueue stalls, and the like unless
they explicitly yield.

Rather than placing cond_resched() calls within various
for_each_drmem_lmb() loop blocks in the code, put it in the iteration
expression of the loop macro itself so users can't omit it.

Call cond_resched() on every 20th element. Each iteration of the loop
in DLPAR code paths can involve around ten RTAS calls which can each
take up to 250us, so this ensures the check is performed at worst
every few milliseconds.

Fixes: 6c6ea53725b3 ("powerpc/mm: Separate ibm, dynamic-memory data from DT format")
Signed-off-by: Nathan Lynch <[email-masked]>
---
 arch/powerpc/include/asm/drmem.h | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

Changes since v1:
* Add bounds assertions in drmem_lmb_next().
* Call cond_resched() in the iterator on only every 20th element
  instead of on every iteration, to reduce overhead in tight loops.

diff --git a/arch/powerpc/include/asm/drmem.h b/arch/powerpc/include/asm/drmem.h
index 17ccc6474ab6..583277e30dd2 100644
--- a/arch/powerpc/include/asm/drmem.h
+++ b/arch/powerpc/include/asm/drmem.h
@@ -8,6 +8,9 @@
 #ifndef _ASM_POWERPC_LMB_H
 #define _ASM_POWERPC_LMB_H
 
+#include <linux/bug.h>
+#include <linux/sched.h>
+
 struct drmem_lmb {
 	u64     base_addr;
 	u32     drc_index;
@@ -26,8 +29,21 @@ struct drmem_lmb_info {
 
 extern struct drmem_lmb_info *drmem_info;
 
+static inline struct drmem_lmb *drmem_lmb_next(struct drmem_lmb *lmb)
+{
+	const unsigned int resched_interval = 20;
+
+	BUG_ON(lmb < drmem_info->lmbs);
+	BUG_ON(lmb >= drmem_info->lmbs + drmem_info->n_lmbs);
+
+	if ((lmb - drmem_info->lmbs) % resched_interval == 0)
+		cond_resched();
+
+	return ++lmb;
+}
+
 #define for_each_drmem_lmb_in_range(lmb, start, end)		\
-	for ((lmb) = (start); (lmb) < (end); (lmb)++)
+	for ((lmb) = (start); (lmb) < (end); lmb = drmem_lmb_next(lmb))
 
 #define for_each_drmem_lmb(lmb)					\
 	for_each_drmem_lmb_in_range((lmb),			\
-- 
2.25.4