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Date:   Sat, 20 Mar 2021 16:34:57 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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Subject: Re: [PATCH 1/6] sched: migration changes for core scheduling
Message-ID: <20210320153457.GX4746@worktop.programming.kicks-ass.net>
References: <20210319203253.3352417-1-joel@joelfernandes.org>
 <20210319203253.3352417-2-joel@joelfernandes.org>
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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.