This reverts commit b0defa7ae03ecf91b8bfd10ede430cff12fcbd06. b0defa7ae03ec changed the load balancing logic to ignore env.max_loop if all tasks examined to that point were pinned. The goal of the patch was to make it more likely to be able to detach a task buried in a long list of pinned tasks. However, this has the unfortunate side effect of creating an O(n) iteration in detach_tasks(), as we now must fully iterate every task on a cpu if all or most are pinned. Since this load balance code is done with rq lock held, and often in softirq context, it is very easy to trigger hard lockups. We observed such hard lockups with a user who affined O(10k) threads to a single cpu. When I discussed this with Vincent he initially suggested that we keep the limit on the number of tasks to detach, but increase the number of tasks we can search. However, after some back and forth on the mailing list, he recommended we instead revert the original patch, as it seems likely no one was actually getting hit by the original issue.
Unified · Split
| @@ -9149,12 +9149,8 @@ static int detach_tasks(struct lb_env *env) · 9148 unmodified lines | |||
| 9149 | break; | 9149 | break; |
| 9150 | 9150 | ||
| 9151 | env->loop++; | 9151 | env->loop++; |
| 9152 | /* | 9152 | /* We've more or less seen every task there is, call it quits */ |
| 9153 | * We've more or less seen every task there is, call it quits | 9153 | if (env->loop > env->loop_max) |
| 9154 | * unless we haven't found any movable task yet. | ||
| 9155 | */ | ||
| 9156 | if (env->loop > env->loop_max && | ||
| 9157 | !(env->flags & LBF_ALL_PINNED)) | ||
| 9158 | break; | 9154 | break; |
| 9159 | 9155 | ||
| 9160 | /* take a breather every nr_migrate tasks */ | 9156 | /* take a breather every nr_migrate tasks */ |
| @@ -11393,9 +11389,7 @@ more_balance: · 2232 unmodified lines | |||
| 11393 | 11389 | ||
| 11394 | if (env.flags & LBF_NEED_BREAK) { | 11390 | if (env.flags & LBF_NEED_BREAK) { |
| 11395 | env.flags &= ~LBF_NEED_BREAK; | 11391 | env.flags &= ~LBF_NEED_BREAK; |
| 11396 | /* Stop if we tried all running tasks */ | 11392 | goto more_balance; |
| 11397 | if (env.loop < busiest->nr_running) | ||
| 11398 | goto more_balance; | ||
| 11399 | } | 11393 | } |
| 11400 | 11394 | ||
| 11401 | /* | 11395 | /* |