From d835502f3dacad1638d516ab156d66f0ba377cf5 Mon Sep 17 00:00:00 2001 From: Shaohua Li Date: Tue, 31 Dec 2013 11:38:27 +0800 Subject: [PATCH] percpu_ida: fix a live lock steal_tags only happens when free tags is more than half of the total tags. This is too strict and can cause live lock. I found that if one cpu has free tags, but other cpu can't steal (thread is bound to specific cpus), threads which want 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), no performance changes were observed. Signed-off-by: Shaohua Li Signed-off-by: Jens Axboe --- lib/percpu_ida.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c index 9d054bf91d0f..85f43b1718d4 100644 --- a/lib/percpu_ida.c +++ b/lib/percpu_ida.c @@ -54,9 +54,7 @@ static inline void move_tags(unsigned *dst, unsigned *dst_nr, /* * 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 percpu_ida *pool, 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) {