[patch 1/2]percpu_ida: fix a live lock

From: Shaohua Li
Date: Mon Dec 30 2013 - 22:39:11 EST

steal_tags only happens when free tags is more than half of the total tags.
This is too restrict and can cause live lock. I found one cpu has free tags,
but other cpu can't steal (thread is bound to specific cpus), threads which
wants to allocate tags are always sleeping. I found this when I run next patch,
but this could happen without it I think.

I did performance test too with null_blk. Two cases (each cpu has enough percpu
tags, or total tags are limited) haven't performance changes.

Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
lib/percpu_ida.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

Index: linux/lib/percpu_ida.c
--- linux.orig/lib/percpu_ida.c 2013-11-25 09:36:52.196626486 +0800
+++ linux/lib/percpu_ida.c 2013-12-31 11:20:03.205400612 +0800
@@ -54,9 +54,7 @@ static inline void move_tags(unsigned *d
* Try to steal tags from a remote cpu's percpu freelist.
- * We first check how many percpu freelists have tags - we don't steal tags
- * unless enough percpu freelists have tags on them that it's possible more than
- * half the total tags could be stuck on remote percpu freelists.
+ * We first check how many percpu freelists have tags
* Then we iterate through the cpus until we find some tags - we don't attempt
* to find the "best" cpu to steal from, to keep cacheline bouncing to a
@@ -69,8 +67,7 @@ static inline void steal_tags(struct per
struct percpu_ida_cpu *remote;

for (cpus_have_tags = cpumask_weight(&pool->cpus_have_tags);
- cpus_have_tags * pool->percpu_max_size > pool->nr_tags / 2;
- cpus_have_tags--) {
+ cpus_have_tags; cpus_have_tags--) {
cpu = cpumask_next(cpu, &pool->cpus_have_tags);

if (cpu >= nr_cpu_ids) {
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