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From: Will Deacon <will@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Qais Yousef <qais.yousef@arm.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	kernel-team@android.com
Subject: Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems
Date: Wed, 18 Aug 2021 11:42:28 +0100	[thread overview]
Message-ID: <20210818104227.GA13828@willie-the-truck> (raw)
In-Reply-To: <YRvRfZ/NnuNyIu3s@hirez.programming.kicks-ass.net>

Hi Peter,

On Tue, Aug 17, 2021 at 05:10:53PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 30, 2021 at 12:24:35PM +0100, Will Deacon wrote:
> > @@ -2783,20 +2778,173 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
> >  
> >  	__do_set_cpus_allowed(p, new_mask, flags);
> >  
> > -	return affine_move_task(rq, p, &rf, dest_cpu, flags);
> > +	if (flags & SCA_USER)
> > +		release_user_cpus_ptr(p);
> > +
> > +	return affine_move_task(rq, p, rf, dest_cpu, flags);
> >  
> >  out:
> > -	task_rq_unlock(rq, p, &rf);
> > +	task_rq_unlock(rq, p, rf);
> >  
> >  	return ret;
> >  }
> 
> > +void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
> > +{
> > +	unsigned long flags;
> > +	struct cpumask *mask = p->user_cpus_ptr;
> > +
> > +	/*
> > +	 * Try to restore the old affinity mask. If this fails, then
> > +	 * we free the mask explicitly to avoid it being inherited across
> > +	 * a subsequent fork().
> > +	 */
> > +	if (!mask || !__sched_setaffinity(p, mask))
> > +		return;
> > +
> > +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> > +	release_user_cpus_ptr(p);
> > +	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> > +}
> 
> Both these are a problem on RT.

Ah, sorry. I didn't realise you couldn't _free_ with a raw lock held in RT.
Is there somewhere I can read up on that?

> The easiest recourse is simply never freeing the CPU mask (except on
> exit). The alternative is something like the below I suppose..
> 
> I'm leaning towards the former option, wdyt?

Defering the freeing until exit feels like a little fiddly, as we still
want to clear ->user_cpus_ptr on affinity changes when SCA_USER is set
so we'd have to keep track of the mask somewhere and reuse it instead
of allocating a new one if we need it later on. Do-able, but feels a bit
nasty, particular across fork().

As for your other suggestion:

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2733,6 +2733,7 @@ static int __set_cpus_allowed_ptr_locked
>  	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
>  	const struct cpumask *cpu_valid_mask = cpu_active_mask;
>  	bool kthread = p->flags & PF_KTHREAD;
> +	struct cpumask *user_mask = NULL;
>  	unsigned int dest_cpu;
>  	int ret = 0;
>  
> @@ -2792,9 +2793,13 @@ static int __set_cpus_allowed_ptr_locked
>  	__do_set_cpus_allowed(p, new_mask, flags);
>  
>  	if (flags & SCA_USER)
> -		release_user_cpus_ptr(p);
> +		swap(user_mask, p->user_cpus_ptr);
> +
> +	ret = affine_move_task(rq, p, rf, dest_cpu, flags);
> +
> +	kfree(user_mask);
>  
> -	return affine_move_task(rq, p, rf, dest_cpu, flags);
> +	return ret;
>  
>  out:
>  	task_rq_unlock(rq, p, rf);
> @@ -2954,8 +2959,10 @@ void relax_compatible_cpus_allowed_ptr(s
>  		return;
>  
>  	raw_spin_lock_irqsave(&p->pi_lock, flags);
> -	release_user_cpus_ptr(p);
> +	p->user_cpus_ptr = NULL;
>  	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +
> +	kfree(mask);

I think the idea looks good, but perhaps we could wrap things up a bit:

/* Comment about why this is useful with RT */
static cpumask_t *clear_user_cpus_ptr(struct task_struct *p)
{
	struct cpumask *user_mask = NULL;

	swap(user_mask, p->user_cpus_ptr);
	return user_mask;
}

void release_user_cpus_ptr(struct task_struct *p)
{
	kfree(clear_user_cpus_ptr(p));
}

Then just use clear_user_cpus_ptr() in sched/core.c where we know what
we're doing (well, at least one of us does!).

Will

WARNING: multiple messages have this Message-ID
From: Will Deacon <will@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Qais Yousef <qais.yousef@arm.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	kernel-team@android.com
Subject: Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems
Date: Wed, 18 Aug 2021 11:42:28 +0100	[thread overview]
Message-ID: <20210818104227.GA13828@willie-the-truck> (raw)
In-Reply-To: <YRvRfZ/NnuNyIu3s@hirez.programming.kicks-ass.net>

Hi Peter,

On Tue, Aug 17, 2021 at 05:10:53PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 30, 2021 at 12:24:35PM +0100, Will Deacon wrote:
> > @@ -2783,20 +2778,173 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
> >  
> >  	__do_set_cpus_allowed(p, new_mask, flags);
> >  
> > -	return affine_move_task(rq, p, &rf, dest_cpu, flags);
> > +	if (flags & SCA_USER)
> > +		release_user_cpus_ptr(p);
> > +
> > +	return affine_move_task(rq, p, rf, dest_cpu, flags);
> >  
> >  out:
> > -	task_rq_unlock(rq, p, &rf);
> > +	task_rq_unlock(rq, p, rf);
> >  
> >  	return ret;
> >  }
> 
> > +void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
> > +{
> > +	unsigned long flags;
> > +	struct cpumask *mask = p->user_cpus_ptr;
> > +
> > +	/*
> > +	 * Try to restore the old affinity mask. If this fails, then
> > +	 * we free the mask explicitly to avoid it being inherited across
> > +	 * a subsequent fork().
> > +	 */
> > +	if (!mask || !__sched_setaffinity(p, mask))
> > +		return;
> > +
> > +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> > +	release_user_cpus_ptr(p);
> > +	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> > +}
> 
> Both these are a problem on RT.

Ah, sorry. I didn't realise you couldn't _free_ with a raw lock held in RT.
Is there somewhere I can read up on that?

> The easiest recourse is simply never freeing the CPU mask (except on
> exit). The alternative is something like the below I suppose..
> 
> I'm leaning towards the former option, wdyt?

Defering the freeing until exit feels like a little fiddly, as we still
want to clear ->user_cpus_ptr on affinity changes when SCA_USER is set
so we'd have to keep track of the mask somewhere and reuse it instead
of allocating a new one if we need it later on. Do-able, but feels a bit
nasty, particular across fork().

As for your other suggestion:

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2733,6 +2733,7 @@ static int __set_cpus_allowed_ptr_locked
>  	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
>  	const struct cpumask *cpu_valid_mask = cpu_active_mask;
>  	bool kthread = p->flags & PF_KTHREAD;
> +	struct cpumask *user_mask = NULL;
>  	unsigned int dest_cpu;
>  	int ret = 0;
>  
> @@ -2792,9 +2793,13 @@ static int __set_cpus_allowed_ptr_locked
>  	__do_set_cpus_allowed(p, new_mask, flags);
>  
>  	if (flags & SCA_USER)
> -		release_user_cpus_ptr(p);
> +		swap(user_mask, p->user_cpus_ptr);
> +
> +	ret = affine_move_task(rq, p, rf, dest_cpu, flags);
> +
> +	kfree(user_mask);
>  
> -	return affine_move_task(rq, p, rf, dest_cpu, flags);
> +	return ret;
>  
>  out:
>  	task_rq_unlock(rq, p, rf);
> @@ -2954,8 +2959,10 @@ void relax_compatible_cpus_allowed_ptr(s
>  		return;
>  
>  	raw_spin_lock_irqsave(&p->pi_lock, flags);
> -	release_user_cpus_ptr(p);
> +	p->user_cpus_ptr = NULL;
>  	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +
> +	kfree(mask);

I think the idea looks good, but perhaps we could wrap things up a bit:

/* Comment about why this is useful with RT */
static cpumask_t *clear_user_cpus_ptr(struct task_struct *p)
{
	struct cpumask *user_mask = NULL;

	swap(user_mask, p->user_cpus_ptr);
	return user_mask;
}

void release_user_cpus_ptr(struct task_struct *p)
{
	kfree(clear_user_cpus_ptr(p));
}

Then just use clear_user_cpus_ptr() in sched/core.c where we know what
we're doing (well, at least one of us does!).

Will

_______________________________________________
linux-arm-kernel mailing list
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http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2021-08-18 10:42 UTC|newest]

Thread overview: 67+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30 11:24 [PATCH v11 00/16] Add support for 32-bit tasks on asymmetric AArch32 systems Will Deacon
2021-07-30 11:24 ` Will Deacon
2021-07-30 11:24 ` [PATCH v11 01/16] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 02/16] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1 Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 03/16] cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus() Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 04/16] cpuset: Cleanup cpuset_cpus_allowed_fallback() use in select_fallback_rq() Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 05/16] sched: Reject CPU affinity changes based on task_cpu_possible_mask() Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 06/16] sched: Introduce task_struct::user_cpus_ptr to track requested affinity Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 07/16] sched: Split the guts of sched_setaffinity() into a helper function Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-17 15:40   ` Peter Zijlstra
2021-08-17 15:40     ` Peter Zijlstra
2021-08-18 10:50     ` Will Deacon
2021-08-18 10:50       ` Will Deacon
2021-08-18 10:56       ` Peter Zijlstra
2021-08-18 10:56         ` Peter Zijlstra
2021-08-18 11:11         ` Will Deacon
2021-08-18 11:11           ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-17 15:10   ` Peter Zijlstra
2021-08-17 15:10     ` Peter Zijlstra
2021-08-18 10:42     ` Will Deacon [this message]
2021-08-18 10:42       ` Will Deacon
2021-08-18 10:56       ` Peter Zijlstra
2021-08-18 10:56         ` Peter Zijlstra
2021-08-18 11:53         ` Peter Zijlstra
2021-08-18 11:53           ` Peter Zijlstra
2021-08-18 12:19           ` Will Deacon
2021-08-18 12:19             ` Will Deacon
2021-08-18 11:06       ` Peter Zijlstra
2021-08-18 11:06         ` Peter Zijlstra
2021-08-17 15:41   ` Peter Zijlstra
2021-08-17 15:41     ` Peter Zijlstra
2021-08-18 10:43     ` Will Deacon
2021-08-18 10:43       ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 09/16] sched: Introduce dl_task_check_affinity() to check proposed affinity Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-08-23  9:26   ` [tip: sched/core] " tip-bot2 for Will Deacon
2021-07-30 11:24 ` [PATCH v11 10/16] arm64: Implement task_cpu_possible_mask() Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-07-30 11:24 ` [PATCH v11 11/16] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0 Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-07-30 11:24 ` [PATCH v11 12/16] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-07-30 11:24 ` [PATCH v11 13/16] arm64: Advertise CPUs capable of running 32-bit applications in sysfs Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-07-30 11:24 ` [PATCH v11 14/16] arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0 Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-07-30 11:24 ` [PATCH v11 15/16] arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores Will Deacon
2021-07-30 11:24   ` Will Deacon
2021-07-30 11:24 ` [PATCH v11 16/16] Documentation: arm64: describe asymmetric 32-bit support Will Deacon
2021-07-30 11:24   ` Will Deacon

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