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percpu_ida: fix a live lock

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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 <shli@fusionio.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Shaohua Li 2013-12-31 11:38:27 +08:00 committed by Jens Axboe
parent 53d8ab29f8
commit d835502f3d
1 changed files with 2 additions and 5 deletions
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7
lib/percpu_ida.c
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@ -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) {