linux-kernel - Re: [RFC PATCH v3 10/16] sched: Core-wide rq->lock

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Date:   Fri, 31 May 2019 13:08:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vineeth Remanan Pillai <vpillai@...italocean.com>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, subhra.mazumdar@...cle.com,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>, Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v3 10/16] sched: Core-wide rq->lock

On Wed, May 29, 2019 at 08:36:46PM +0000, Vineeth Remanan Pillai wrote:

> + * The static-key + stop-machine variable are needed such that:
> + *
> + *	spin_lock(rq_lockp(rq));
> + *	...
> + *	spin_unlock(rq_lockp(rq));
> + *
> + * ends up locking and unlocking the _same_ lock, and all CPUs
> + * always agree on what rq has what lock.

> @@ -5790,8 +5854,15 @@ int sched_cpu_activate(unsigned int cpu)
>  	/*
>  	 * When going up, increment the number of cores with SMT present.
>  	 */
> -	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> +	if (cpumask_weight(cpu_smt_mask(cpu)) == 2) {
>  		static_branch_inc_cpuslocked(&sched_smt_present);
> +#ifdef CONFIG_SCHED_CORE
> +		if (static_branch_unlikely(&__sched_core_enabled)) {
> +			rq->core_enabled = true;
> +		}
> +#endif
> +	}
> +
>  #endif
>  	set_cpu_active(cpu, true);
>  
> @@ -5839,8 +5910,16 @@ int sched_cpu_deactivate(unsigned int cpu)
>  	/*
>  	 * When going down, decrement the number of cores with SMT present.
>  	 */
> -	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> +	if (cpumask_weight(cpu_smt_mask(cpu)) == 2) {
> +#ifdef CONFIG_SCHED_CORE
> +		struct rq *rq = cpu_rq(cpu);
> +		if (static_branch_unlikely(&__sched_core_enabled)) {
> +			rq->core_enabled = false;
> +		}
> +#endif
>  		static_branch_dec_cpuslocked(&sched_smt_present);
> +
> +	}
>  #endif

I'm confused, how doesn't this break the invariant above?

That is, all CPUs must at all times agree on the value of rq_lockp(),
and I'm not seeing how that is true with the above changes.