Re: [PATCH 1/6] sched: migration changes for core scheduling

[Peter Zijlstra]

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.