On Sun, Mar 21, 2021 at 09:34:00PM +0800, Li, Aubrey wrote: > Hi Peter, > > On 2021/3/20 23:34, Peter Zijlstra wrote: > > On Fri, Mar 19, 2021 at 04:32:48PM -0400, Joel Fernandes (Google) wrote: > >> @@ -7530,8 +7543,9 @@ int can_migrate_task(struct task_struct *p, struct > >> lb_env *env) > >> * We do not migrate tasks that are: > >> * 1) throttled_lb_pair, or > >> * 2) cannot be migrated to this CPU due to cpus_ptr, or > >> - * 3) running (obviously), or > >> - * 4) are cache-hot on their current CPU. > >> + * 3) task's cookie does not match with this CPU's core cookie > >> + * 4) running (obviously), or > >> + * 5) are cache-hot on their current CPU. > >> */ > >> if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu)) > >> return 0; > >> @@ -7566,6 +7580,13 @@ int can_migrate_task(struct task_struct *p, struct > >> lb_env *env) > >> return 0; > >> } > >> > >> + /* > >> + * Don't migrate task if the task's cookie does not match > >> + * with the destination CPU's core cookie. > >> + */ > >> + if (!sched_core_cookie_match(cpu_rq(env->dst_cpu), p)) > >> + return 0; > >> + > >> /* Record that we found atleast one task that could run on dst_cpu */ > >> env->flags &= ~LBF_ALL_PINNED; > >> > > > > This one is too strong.. persistent imbalance should be able to override > > it. > > > > IIRC, this change can avoid the following scenario: > > One sysbench cpu thread(cookieA) and sysbench mysql thread(cookieB) running > on the two siblings of core_1, the other sysbench cpu thread(cookieA) and > sysbench mysql thread(cookieB) running on the two siblings of core2, which > causes 50% force idle. > > This is not an imbalance case.
But suppose there is an imbalance; then this cookie crud can forever stall balance. Imagine this cpu running a while(1); with a uniqie cookie on, then it will _never_ accept other tasks == BAD.
