Re: [PATCH v4 1/5] nohz_full: add support for "cpu_isolated" mode

The existing nohz_full mode makes tradeoffs to minimize userspace
interruptions while still attempting to avoid overheads in the
kernel entry/exit path, to provide 100% kernel semantics, etc.

However, some applications require a stronger commitment from the
kernel to avoid interruptions, in particular userspace device
driver style applications, such as high-speed networking code.

This change introduces a framework to allow applications to elect
to have the stronger semantics as needed, specifying
prctl(PR_SET_CPU_ISOLATED, PR_CPU_ISOLATED_ENABLE) to do so.
Subsequent commits will add additional flags and additional
semantics.

The "cpu_isolated" state is indicated by setting a new task struct
field, cpu_isolated_flags, to the value passed by prctl().  When the
_ENABLE bit is set for a task, and it is returning to userspace
on a nohz_full core, it calls the new tick_nohz_cpu_isolated_enter()
routine to take additional actions to help the task avoid being
interrupted in the future.

Initially, there are only two actions taken.  First, the task
calls lru_add_drain() to prevent being interrupted by a subsequent
lru_add_drain_all() call on another core.  Then, the code checks for
pending timer interrupts and quiesces until they are no longer pending.
As a result, sys calls (and page faults, etc.) can be inordinately slow.
However, this quiescing guarantees that no unexpected interrupts will
occur, even if the application intentionally calls into the kernel.

Signed-off-by: Chris Metcalf <redacted>
---
 arch/tile/kernel/process.c |  9 ++++++++
 include/linux/sched.h      |  3 +++
 include/linux/tick.h       | 10 ++++++++
 include/uapi/linux/prctl.h |  5 ++++
 kernel/context_tracking.c  |  3 +++
 kernel/sys.c               |  8 +++++++
 kernel/time/tick-sched.c   | 57 ++++++++++++++++++++++++++++++++++++++++++++++
 7 files changed, 95 insertions(+)

diff --git a/arch/tile/kernel/process.c b/arch/tile/kernel/process.c
index e036c0aa9792..3625e839ad62 100644
--- a/arch/tile/kernel/process.c
+++ b/arch/tile/kernel/process.c

@@ -70,6 +70,15 @@ void arch_cpu_idle(void)
 	_cpu_idle();
 }
 
+#ifdef CONFIG_NO_HZ_FULL

+void tick_nohz_cpu_isolated_wait(void)
+{
+	set_current_state(TASK_INTERRUPTIBLE);
+	_cpu_idle();
+	set_current_state(TASK_RUNNING);
+}
+#endif
+
 /*
  * Release a thread_info structure
  */

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ae21f1591615..f350b0c20bbc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -1778,6 +1778,9 @@ struct task_struct {
 	unsigned long	task_state_change;
 #endif
 	int pagefault_disabled;
+#ifdef CONFIG_NO_HZ_FULL
+	unsigned int	cpu_isolated_flags;
+#endif
 };
 
 /* Future-safe accessor for struct task_struct's cpus_allowed. */

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 3741ba1a652c..cb5569181359 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h

@@ -10,6 +10,7 @@
 #include <linux/context_tracking_state.h>
 #include <linux/cpumask.h>
 #include <linux/sched.h>
+#include <linux/prctl.h>
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
 extern void __init tick_init(void);

@@ -144,11 +145,18 @@ static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
 		cpumask_or(mask, mask, tick_nohz_full_mask);
 }
 
+static inline bool tick_nohz_is_cpu_isolated(void)
+{
+	return tick_nohz_full_cpu(smp_processor_id()) &&
+		(current->cpu_isolated_flags & PR_CPU_ISOLATED_ENABLE);
+}
+
 extern void __tick_nohz_full_check(void);
 extern void tick_nohz_full_kick(void);
 extern void tick_nohz_full_kick_cpu(int cpu);
 extern void tick_nohz_full_kick_all(void);
 extern void __tick_nohz_task_switch(struct task_struct *tsk);
+extern void tick_nohz_cpu_isolated_enter(void);
 #else
 static inline bool tick_nohz_full_enabled(void) { return false; }
 static inline bool tick_nohz_full_cpu(int cpu) { return false; }

@@ -158,6 +166,8 @@ static inline void tick_nohz_full_kick_cpu(int cpu) { }
 static inline void tick_nohz_full_kick(void) { }
 static inline void tick_nohz_full_kick_all(void) { }
 static inline void __tick_nohz_task_switch(struct task_struct *tsk) { }
+static inline bool tick_nohz_is_cpu_isolated(void) { return false; }
+static inline void tick_nohz_cpu_isolated_enter(void) { }
 #endif
 
 static inline bool is_housekeeping_cpu(int cpu)

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 31891d9535e2..edb40b6b84db 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h

@@ -190,4 +190,9 @@ struct prctl_mm_map {
 # define PR_FP_MODE_FR		(1 << 0)	/* 64b FP registers */
 # define PR_FP_MODE_FRE		(1 << 1)	/* 32b compatibility */
 
+/* Enable/disable or query cpu_isolated mode for NO_HZ_FULL kernels. */
+#define PR_SET_CPU_ISOLATED	47
+#define PR_GET_CPU_ISOLATED	48
+# define PR_CPU_ISOLATED_ENABLE	(1 << 0)
+
 #endif /* _LINUX_PRCTL_H */

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 0a495ab35bc7..f9de3ee12723 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c

@@ -20,6 +20,7 @@
 #include <linux/hardirq.h>
 #include <linux/export.h>
 #include <linux/kprobes.h>
+#include <linux/tick.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/context_tracking.h>

@@ -99,6 +100,8 @@ void context_tracking_enter(enum ctx_state state)
 			 * on the tick.
 			 */
 			if (state == CONTEXT_USER) {
+				if (tick_nohz_is_cpu_isolated())
+					tick_nohz_cpu_isolated_enter();
 				trace_user_enter(0);
 				vtime_user_enter(current);
 			}

diff --git a/kernel/sys.c b/kernel/sys.c
index 259fda25eb6b..36eb9a839f1f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c

@@ -2267,6 +2267,14 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case PR_GET_FP_MODE:
 		error = GET_FP_MODE(me);
 		break;
+#ifdef CONFIG_NO_HZ_FULL
+	case PR_SET_CPU_ISOLATED:
+		me->cpu_isolated_flags = arg2;
+		break;
+	case PR_GET_CPU_ISOLATED:
+		error = me->cpu_isolated_flags;
+		break;
+#endif
 	default:
 		error = -EINVAL;
 		break;

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index c792429e98c6..4cf093c012d1 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c

@@ -24,6 +24,7 @@
 #include <linux/posix-timers.h>
 #include <linux/perf_event.h>
 #include <linux/context_tracking.h>
+#include <linux/swap.h>
 
 #include <asm/irq_regs.h>
 

@@ -389,6 +390,62 @@ void __init tick_nohz_init(void)
 	pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n",
 		cpumask_pr_args(tick_nohz_full_mask));
 }
+
+/*
+ * Rather than continuously polling for the next_event in the
+ * tick_cpu_device, architectures can provide a method to save power
+ * by sleeping until an interrupt arrives.
+ */
+void __weak tick_nohz_cpu_isolated_wait(void)
+{
+	cpu_relax();
+}
+
+/*
+ * We normally return immediately to userspace.
+ *
+ * In "cpu_isolated" mode we wait until no more interrupts are
+ * pending.  Otherwise we nap with interrupts enabled and wait for the
+ * next interrupt to fire, then loop back and retry.
+ *
+ * Note that if you schedule two "cpu_isolated" processes on the same
+ * core, neither will ever leave the kernel, and one will have to be
+ * killed manually.  Otherwise in situations where another process is
+ * in the runqueue on this cpu, this task will just wait for that
+ * other task to go idle before returning to user space.
+ */
+void tick_nohz_cpu_isolated_enter(void)