[PATCH v3 1/5] nohz_full: add support for "cpu_isolated" mode

Chris Metcalf
2015-06-03T15:30:29Z

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 <cmetcalf@ezchip.com>
---
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..e20c3f4a6a82 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()
+{
+ 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 8222ae40ecb0..fb4ba400d7e1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1732,6 +1732,9 @@ struct task_struct {
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
unsigned long task_state_change;
#endif
+#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 f8492da57ad3..ec1953474a65 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);
@@ -134,11 +135,18 @@ static inline bool tick_nohz_full_cpu(int cpu)
return cpumask_test_cpu(cpu, 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; }
@@ -147,6 +155,8 @@ 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 72d59a1a6eb6..66739d7c1350 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>
@@ -85,6 +86,8 @@ 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 a4e372b798a5..3fd9e47f8fc8 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2243,6 +2243,14 @@ 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 914259128145..f6236b66788f 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()
+{
+ 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)
+{
+ struct clock_event_device *dev =
+ __this_cpu_read(tick_cpu_device.evtdev);
+ struct task_struct *task = current;
+ unsigned long start = jiffies;
+ bool warned = false;
+
+ /* Drain the pagevecs to avoid unnecessary IPI flushes later. */
+ lru_add_drain();
+
+ while (READ_ONCE(dev->next_event.tv64) != KTIME_MAX) {
+ if (!warned && (jiffies - start) >= (5 * HZ)) {
+ pr_warn("%s/%d: cpu %d: cpu_isolated task blocked for %ld seconds\n",
+ task->comm, task->pid, smp_processor_id(),
+ (jiffies - start) / HZ);
+ warned = true;
+ }
+ if (should_resched())
+ schedule();
+ if (test_thread_flag(TIF_SIGPENDING))
+ break;
+ tick_nohz_cpu_isolated_wait();
+ }
+ if (warned) {
+ pr_warn("%s/%d: cpu %d: cpu_isolated task unblocked after %ld seconds\n",
+ task->comm, task->pid, smp_processor_id(),
+ (jiffies - start) / HZ);
+ dump_stack();
+ }
+}
+
#endif

/*
--
2.1.2