Re: [PATCH -tip 23/32] sched: Add a per-thread core scheduling interface

[Chris Hyser <chris.hyser@oracle.com>]

On Tue, Nov 17, 2020 at 06:19:53PM -0500, Joel Fernandes (Google) wrote:
> Add a per-thread core scheduling interface which allows a thread to share a
> core with another thread, or have a core exclusively for itself.
> 
> ChromeOS uses core-scheduling to securely enable hyperthreading.  This cuts
> down the keypress latency in Google docs from 150ms to 50ms while improving
> the camera streaming frame rate by ~3%.
> 

Inline is a patch for comment to extend this interface to make it more useful.
This patch would still need to provide doc and selftests updates as well.

-chrish

---8<-----------------------

From ec3d6506fee89022d93789e1ba44d49c1b1b04dd Mon Sep 17 00:00:00 2001
From: chris hyser <chris.hyser@oracle.com>
Date: Tue, 10 Nov 2020 15:35:59 -0500
Subject: [PATCH] sched: Provide a more extensive prctl interface for core
 scheduling.

The current prctl interface is a "from" only interface allowing a task to
join an existing core scheduling group by getting the "cookie" from a
specified task/pid.

Additionally, sharing from a task without an existing core scheduling group
(cookie == 0) creates a new cookie shared between the two.

"From" functionality works well for programs modified to use the prctl(),
but many applications will not be modified simply for core scheduling.
Using a wrapper program to share the core scheduling cookie from another
task followed by an "exec" can work, but there is no means to assign a
cookie for an unmodified running task.

Simply inverting the interface to a "to"-only interface, i.e. the calling
task shares it's cookie with the specified task/pid also has limitations,
there being no means to enable tasks to join an existing core scheduling
group for instance.

The solution is to allow both, or more specifically provide a flags
argument to allow various core scheduling commands, currently FROM, TO, and
CLEAR.

The combination of FROM and TO allow a helper program to share the core
scheduling cookie of one task/core scheduling group, with additional tasks.

if (prctl(PR_SCHED_CORE_SHARE, PR_SCHED_CORE_SHARE_FROM, src_pid, 0, 0) < 0)
	handle_error("src_pid sched_core failed");
if (prctl(PR_SCHED_CORE_SHARE, PR_SCHED_CORE_SHARE_TO, dest_pid, 0, 0) < 0)
	handle_error("dest_pid sched_core failed");

Signed-off-by: chris hyser <chris.hyser@oracle.com>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index c9efdf8..eed002e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2084,14 +2084,14 @@ void sched_core_unsafe_enter(void);
 void sched_core_unsafe_exit(void);
 bool sched_core_wait_till_safe(unsigned long ti_check);
 bool sched_core_kernel_protected(void);
-int sched_core_share_pid(pid_t pid);
+int sched_core_share_pid(unsigned long flags, pid_t pid);
 void sched_tsk_free(struct task_struct *tsk);
 #else
 #define sched_core_unsafe_enter(ignore) do { } while (0)
 #define sched_core_unsafe_exit(ignore) do { } while (0)
 #define sched_core_wait_till_safe(ignore) do { } while (0)
 #define sched_core_kernel_protected(ignore) do { } while (0)
-#define sched_core_share_pid(pid) do { } while (0)
+#define sched_core_share_pid(flags, pid) do { } while (0)
 #define sched_tsk_free(tsk) do { } while (0)
 #endif
 
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 217b048..f8e4e96 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -250,5 +250,8 @@ struct prctl_mm_map {
 
 /* Request the scheduler to share a core */
 #define PR_SCHED_CORE_SHARE		59
+# define PR_SCHED_CORE_CLEAR		0  /* clear core_sched cookie of pid */
+# define PR_SCHED_CORE_SHARE_FROM	1  /* get core_sched cookie from pid */
+# define PR_SCHED_CORE_SHARE_TO		2  /* push core_sched cookie to pid */
 
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sched/coretag.c b/kernel/sched/coretag.c
index 800c0f8..14feac1 100644
--- a/kernel/sched/coretag.c
+++ b/kernel/sched/coretag.c
@@ -9,6 +9,7 @@
  */
 
 #include "sched.h"
+#include "linux/prctl.h"
 
 /*
  * Wrapper representing a complete cookie. The address of the cookie is used as
@@ -456,40 +457,45 @@ int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)
 }
 
 /* Called from prctl interface: PR_SCHED_CORE_SHARE */
-int sched_core_share_pid(pid_t pid)
+int sched_core_share_pid(unsigned long flags, pid_t pid)
 {
+	struct task_struct *dest;
+	struct task_struct *src;
 	struct task_struct *task;
 	int err;
 
-	if (pid == 0) { /* Recent current task's cookie. */
-		/* Resetting a cookie requires privileges. */
-		if (current->core_task_cookie)
-			if (!capable(CAP_SYS_ADMIN))
-				return -EPERM;
-		task = NULL;
-	} else {
-		rcu_read_lock();
-		task = pid ? find_task_by_vpid(pid) : current;
-		if (!task) {
-			rcu_read_unlock();
-			return -ESRCH;
-		}
-
-		get_task_struct(task);
-
-		/*
-		 * Check if this process has the right to modify the specified
-		 * process. Use the regular "ptrace_may_access()" checks.
-		 */
-		if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
-			rcu_read_unlock();
-			err = -EPERM;
-			goto out;
-		}
+	rcu_read_lock();
+	task = find_task_by_vpid(pid);
+	if (!task) {
 		rcu_read_unlock();
+		return -ESRCH;
 	}
 
-	err = sched_core_share_tasks(current, task);
+	get_task_struct(task);
+
+	/*
+	 * Check if this process has the right to modify the specified
+	 * process. Use the regular "ptrace_may_access()" checks.
+	 */
+	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
+		rcu_read_unlock();
+		err = -EPERM;
+		goto out;
+	}
+	rcu_read_unlock();
+
+	if (flags == PR_SCHED_CORE_CLEAR) {
+		dest = task;
+		src = NULL;
+	} else if (flags == PR_SCHED_CORE_SHARE_TO) {
+		dest = task;
+		src = current;
+	} else if (flags == PR_SCHED_CORE_SHARE_FROM) {
+		dest = current;
+		src = task;
+	}
+
+	err = sched_core_share_tasks(dest, src);
 out:
 	if (task)
 		put_task_struct(task);
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index cffdfab..50c31f3 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -1030,6 +1030,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
 
 #ifdef CONFIG_SCHED_CORE
 	__PS("core_cookie", p->core_cookie);
+	__PS("core_task_cookie", p->core_task_cookie);
 #endif
 
 	sched_show_numa(p, m);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b3b89bd..eafb399 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1202,7 +1202,7 @@ void sched_core_dequeue(struct rq *rq, struct task_struct *p);
 void sched_core_get(void);
 void sched_core_put(void);
 
-int sched_core_share_pid(pid_t pid);
+int sched_core_share_pid(unsigned long flags, pid_t pid);
 int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2);
 
 #ifdef CONFIG_CGROUP_SCHED
diff --git a/kernel/sys.c b/kernel/sys.c
index 61a3c98..da52a0d 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2530,9 +2530,13 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 
 		error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
 		break;
+#ifdef CONFIG_SCHED_CORE
 	case PR_SCHED_CORE_SHARE:
-		error = sched_core_share_pid(arg2);
+		if (arg4 || arg5)
+			return -EINVAL;
+		error = sched_core_share_pid(arg2, arg3);
 		break;
+#endif
 	default:
 		error = -EINVAL;
 		break;