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From:   Chris Metcalf <cmetcalf@mellanox.com>
To:     Davidlohr Bueso <dave@stgolabs.net>,
        Oleg Nesterov <oleg@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Tejun Heo <tj@kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
Cc:     Chris Metcalf <cmetcalf@mellanox.com>
Subject: [PATCH v16 04/13] Add try_get_task_struct_on_cpu() to scheduler for task isolation
Date:   Fri,  3 Nov 2017 13:04:43 -0400
Message-Id: <1509728692-10460-5-git-send-email-cmetcalf@mellanox.com>
In-Reply-To: <1509728692-10460-1-git-send-email-cmetcalf@mellanox.com>
References: <1509728692-10460-1-git-send-email-cmetcalf@mellanox.com>
Sender: linux-kernel-owner@vger.kernel.org
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Task isolation wants to be able to verify that a remote core is
running an isolated task to determine if it should generate a
diagnostic, and also possibly interrupt it.

This API returns a pointer to the task_struct of the task that
was running on the specified core at the moment of the request;
it uses try_get_task_struct() to increment the ref count on the
returned task_struct so that the caller can examine it even if
the actual remote task has already exited by that point.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
---
 include/linux/sched/task.h |  1 +
 kernel/sched/core.c        | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 270ff76d43d9..6785db926857 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -97,6 +97,7 @@ static inline void put_task_struct(struct task_struct *t)
 
 struct task_struct *task_rcu_dereference(struct task_struct **ptask);
 struct task_struct *try_get_task_struct(struct task_struct **ptask);
+struct task_struct *try_get_task_struct_on_cpu(int cpu);
 
 
 #ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d17c5da523a0..2728154057ae 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -670,6 +670,17 @@ bool sched_can_stop_tick(struct rq *rq)
 }
 #endif /* CONFIG_NO_HZ_FULL */
 
+/*
+ * Return a pointer to the task_struct for the task that is running on
+ * the specified cpu at the time of the call (note that the task may have
+ * exited by the time the caller inspects the resulting task_struct).
+ * Caller must put_task_struct() with the pointer when finished with it.
+ */
+struct task_struct *try_get_task_struct_on_cpu(int cpu)
+{
+	return try_get_task_struct(&cpu_rq(cpu)->curr);
+}
+
 void sched_avg_update(struct rq *rq)
 {
 	s64 period = sched_avg_period();
-- 
2.1.2


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