linux-kernel - Re: [PATCH 4/7] x86/intel_rdt: Implement scheduling support for Intel RDT

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Date:	Sat, 2 May 2015 20:51:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>
Cc:	vikas.shivappa@...el.com, linux-kernel@...r.kernel.org,
	x86@...nel.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...nel.org, tj@...nel.org, matt.fleming@...el.com,
	will.auld@...el.com, peter.zijlstra@...el.com,
	h.peter.anvin@...el.com, kanaka.d.juvva@...el.com
Subject: Re: [PATCH 4/7] x86/intel_rdt: Implement scheduling support for
 Intel RDT

On Fri, May 01, 2015 at 06:36:38PM -0700, Vikas Shivappa wrote:
> Adds support for IA32_PQR_ASSOC MSR writes during task scheduling.
> 
> The high 32 bits in the per processor MSR IA32_PQR_ASSOC represents the
> CLOSid. During context switch kernel implements this by writing the
> CLOSid of the cgroup to which the task belongs to the CPU's
> IA32_PQR_ASSOC MSR.
> 
> For Cache Allocation, this would let the task fill in the cache 'subset'
> represented by the cgroup's Cache bit mask(CBM).
> 

Are you guys for real? Have you even looked at the trainwreck this
makes?

> +static inline bool rdt_enabled(void)
> +{
> +	return static_key_false(&rdt_enable_key);
> +}

> +/*
> + * rdt_sched_in() - Writes the task's CLOSid to IA32_PQR_MSR
> + * if the current Closid is different than the new one.
> + */
> +static inline void rdt_sched_in(struct task_struct *task)
> +{
> +	struct intel_rdt *ir;
> +	unsigned int clos;
> +
> +	if (!rdt_enabled())
> +		return;
> +
> +	/*
> +	 * This needs to be fixed
> +	 * to cache the whole PQR instead of just CLOSid.
> +	 * PQR has closid in high 32 bits and CQM-RMID in low 10 bits.
> +	 * Should not write a 0 to the low 10 bits of PQR
> +	 * and corrupt RMID.
> +	 */
> +	clos = this_cpu_read(x86_cpu_clos);
> +
> +	rcu_read_lock();
> +	ir = task_rdt(task);
> +	if (ir->clos == clos) {
> +		rcu_read_unlock();
> +		return;
> +	}
> +
> +	wrmsr(MSR_IA32_PQR_ASSOC, 0, ir->clos);
> +	this_cpu_write(x86_cpu_clos, ir->clos);
> +	rcu_read_unlock();
> +}

You inject _ALL_ that into the scheduler hot path. Insane much?

> +
> +#else
> +
> +static inline void rdt_sched_in(struct task_struct *task) {}
> +
>  #endif
>  #endif
> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index 751bf4b..82ef4b3 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -8,6 +8,9 @@ struct tss_struct;
>  void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>  		      struct tss_struct *tss);
>  
> +#include <asm/intel_rdt.h>
> +#define post_arch_switch(current)	rdt_sched_in(current)
> +
>  #ifdef CONFIG_X86_32
>  
>  #ifdef CONFIG_CC_STACKPROTECTOR

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f9123a8..cacb490 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2241,6 +2241,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>  	prev_state = prev->state;
>  	vtime_task_switch(prev);
>  	finish_arch_switch(prev);
> +	post_arch_switch(current);
>  	perf_event_task_sched_in(prev, current);
>  	finish_lock_switch(rq, prev);
>  	finish_arch_post_lock_switch();

Not a word in the Changelog on this hook; that's double fail.

Please _THINK_ before writing code.
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