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From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	"Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King <linux@arm.linux.org.uk>,
	Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
	Ben Maurer <bmaurer@fb.com>, Steven Rostedt <rostedt@goodmis.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Joel Fernandes <joelaf@google.com>
Subject: Re: [RFC PATCH for 4.21 03/16] sched: Implement push_task_to_cpu (v2)
Date: Wed, 17 Oct 2018 12:21:45 +0530
Message-ID: <20181017065145.GA7111@linux.vnet.ibm.com> (raw)
In-Reply-To: <20181010191936.7495-4-mathieu.desnoyers@efficios.com>

Hi Mathieu,

> +int push_task_to_cpu(struct task_struct *p, unsigned int dest_cpu)
> +{

In your use case, is the task going to be current?
If yes, we should simply be using migrate_task_to.

> +	struct rq_flags rf;
> +	struct rq *rq;
> +	int ret = 0;
> +
> +	rq = task_rq_lock(p, &rf);
> +	update_rq_clock(rq);
> +
> +	if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed)) {
> +		ret = -EINVAL;
> +		goto out;
> +	}

Ideally we should have checked cpus_allowed/cpu_active_mask before taking
the lock. This would help reduce the contention on the rqlock when the
passed parameter is not correct.

> +
> +	if (!cpumask_test_cpu(dest_cpu, cpu_active_mask)) {
> +		ret = -EBUSY;
> +		goto out;
> +	}
> +
> +	if (task_cpu(p) == dest_cpu)
> +		goto out;

Same as above.

> +
> +	if (task_running(rq, p) || p->state == TASK_WAKING) {

Why are we using migration thread to move a task in TASK_WAKING state?

> +		struct migration_arg arg = { p, dest_cpu };
> +		/* Need help from migration thread: drop lock and wait. */
> +		task_rq_unlock(rq, p, &rf);
> +		stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
> +		tlb_migrate_finish(p->mm);
> +		return 0;

Why cant we use migrate_task_to instead?

> +	} else if (task_on_rq_queued(p)) {
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 455fa330de04..27ad25780204 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1340,6 +1340,15 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
>  #endif
>  }
>  
> +#ifdef CONFIG_SMP
> +int push_task_to_cpu(struct task_struct *p, unsigned int dest_cpu);
> +#else
> +static inline int push_task_to_cpu(struct task_struct *p, unsigned int dest_cpu)
> +{
> +	return 0;
> +}
> +#endif
> +

Your usecase is outside kernel/sched. So I am not sure if this is the right
place for the declaration.


  reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-10 19:19 [RFC PATCH for 4.21 00/16] rseq updates, new cpu_opv system call Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc Mathieu Desnoyers
2018-10-11 10:37   ` Szabolcs Nagy
2018-10-11 15:13     ` Mathieu Desnoyers
2018-10-11 16:20       ` Szabolcs Nagy
2018-10-11 16:37         ` Mathieu Desnoyers
2018-10-11 17:04           ` Szabolcs Nagy
2018-10-11 19:42             ` Mathieu Desnoyers
2018-10-12  9:59               ` Szabolcs Nagy
2018-10-23 14:59                 ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 02/16] rseq/selftests: Adapt number of threads to the number of detected cpus Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 03/16] sched: Implement push_task_to_cpu (v2) Mathieu Desnoyers
2018-10-17  6:51   ` Srikar Dronamraju [this message]
2018-10-17 15:09     ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 04/16] mm: Introduce vm_map_user_ram, vm_unmap_user_ram Mathieu Desnoyers
2018-10-16 18:30   ` Steven Rostedt
2018-10-16 19:21     ` Mathieu Desnoyers
2018-10-16 19:40       ` Steven Rostedt
2018-10-17  0:27     ` Sergey Senozhatsky
2018-10-17 15:00       ` Mathieu Desnoyers
2018-10-17 15:04         ` Mathieu Desnoyers
2018-10-17 15:34           ` Sergey Senozhatsky
2018-10-10 19:19 ` [RFC PATCH for 4.21 05/16] mm: Provide is_vma_noncached Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call (v8) Mathieu Desnoyers
2018-10-16  8:10   ` Sergey Senozhatsky
2018-10-16 19:17     ` Mathieu Desnoyers
2018-10-17  1:46       ` Sergey Senozhatsky
2018-10-17  7:19   ` Srikar Dronamraju
2018-10-17 15:11     ` Mathieu Desnoyers
2018-10-17 16:09       ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 07/16] cpu_opv: limit amount of virtual address space used by cpu_opv Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 08/16] x86: Wire up cpu_opv system call Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 09/16] powerpc: " Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 10/16] arm: " Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 11/16] cpu-opv/selftests: Provide cpu-op library Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 12/16] cpu-opv/selftests: Provide basic test Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 13/16] cpu-opv/selftests: Provide percpu_op API Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 14/16] cpu-opv/selftests: Provide basic percpu ops test Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 15/16] cpu-opv/selftests: Provide parametrized tests Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 16/16] cpu-opv/selftests: Provide Makefile, scripts, gitignore Mathieu Desnoyers

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