linux-kernel - Re: [patch V2 08/28] sched/smt: Make sched_smt

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Date:   Thu, 29 Nov 2018 09:42:56 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch V2 08/28] sched/smt: Make sched_smt_present track topology

On Sun, Nov 25, 2018 at 07:33:36PM +0100, Thomas Gleixner wrote:
> Currently the 'sched_smt_present' static key is enabled when at CPU bringup
> SMT topology is observed, but it is never disabled. However there is demand
> to also disable the key when the topology changes such that there is no SMT
> present anymore.
> 
> Implement this by making the key count the number of cores that have SMT
> enabled.
> 
> In particular, the SMT topology bits are set before interrrupts are enabled
> and similarly, are cleared after interrupts are disabled for the last time
> and the CPU dies.

I see that the number you used is '2', but I thought that there are some
CPUs out there (Knights Landing?) that could have four threads?

Would it be better to have a generic function that would provide the
amount of threads the platform does expose - and use that instead
of a constant value? 

> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> ---
>  kernel/sched/core.c |   19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5738,15 +5738,10 @@ int sched_cpu_activate(unsigned int cpu)
>  
>  #ifdef CONFIG_SCHED_SMT
>  	/*
> -	 * The sched_smt_present static key needs to be evaluated on every
> -	 * hotplug event because at boot time SMT might be disabled when
> -	 * the number of booted CPUs is limited.
> -	 *
> -	 * If then later a sibling gets hotplugged, then the key would stay
> -	 * off and SMT scheduling would never be functional.
> +	 * When going up, increment the number of cores with SMT present.
>  	 */
> -	if (cpumask_weight(cpu_smt_mask(cpu)) > 1)
> -		static_branch_enable_cpuslocked(&sched_smt_present);
> +	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> +		static_branch_inc_cpuslocked(&sched_smt_present);
>  #endif
>  	set_cpu_active(cpu, true);
>  
> @@ -5790,6 +5785,14 @@ int sched_cpu_deactivate(unsigned int cp
>  	 */
>  	synchronize_rcu_mult(call_rcu, call_rcu_sched);
>  
> +#ifdef CONFIG_SCHED_SMT
> +	/*
> +	 * When going down, decrement the number of cores with SMT present.
> +	 */
> +	if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
> +		static_branch_dec_cpuslocked(&sched_smt_present);
> +#endif
> +
>  	if (!sched_smp_initialized)
>  		return 0;
>  
> 
>