[GIT PULL v2] printk: Make it usable on nohz cpus

[GIT PULL v2] printk: Make it usable on nohz cpus

[Frederic Weisbecker]

Ingo,

Please pull the printk support in dynticks mode patches that can
be found at:

  git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git tags/printk-dynticks-for-mingo-v2

for you to fetch changes up to 74876a98a87a115254b3a66a14b27320b7f0acaa:

  printk: Wake up klogd using irq_work (2012-11-18 01:01:49 +0100)

It is based on v3.7-rc4.

Changes since previous pull request include support for irq work flush
on CPU offlining and acks from Steve. The rest hasn't changed except some
comment fix.

Thanks.

----------------------------------------------------------------
Support for printk in dynticks mode:

* Fix two races in irq work claiming

* Generalize irq_work support to all archs

* Don't stop tick with irq works pending. This
fix is generally useful and concerns archs that
can't raise self IPIs.

* Flush irq works before CPU offlining.

* Introduce "lazy" irq works that can wait for the
next tick to be executed, unless it's stopped.

* Implement klogd wake up using irq work. This
removes the ad-hoc printk_tick()/printk_needs_cpu()
hooks and make it working even in dynticks mode.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

----------------------------------------------------------------
Frederic Weisbecker (7):
      irq_work: Fix racy IRQ_WORK_BUSY flag setting
      irq_work: Fix racy check on work pending flag
      irq_work: Remove CONFIG_HAVE_IRQ_WORK
      nohz: Add API to check tick state
      irq_work: Don't stop the tick with pending works
      irq_work: Make self-IPIs optable
      printk: Wake up klogd using irq_work

Steven Rostedt (2):
      irq_work: Flush work on CPU_DYING
      irq_work: Warn if there's still work on cpu_down

 arch/alpha/Kconfig                  |    1 -
 arch/arm/Kconfig                    |    1 -
 arch/arm64/Kconfig                  |    1 -
 arch/blackfin/Kconfig               |    1 -
 arch/frv/Kconfig                    |    1 -
 arch/hexagon/Kconfig                |    1 -
 arch/mips/Kconfig                   |    1 -
 arch/parisc/Kconfig                 |    1 -
 arch/powerpc/Kconfig                |    1 -
 arch/s390/Kconfig                   |    1 -
 arch/sh/Kconfig                     |    1 -
 arch/sparc/Kconfig                  |    1 -
 arch/x86/Kconfig                    |    1 -
 drivers/staging/iio/trigger/Kconfig |    1 -
 include/linux/irq_work.h            |   20 ++++++
 include/linux/printk.h              |    3 -
 include/linux/tick.h                |   17 ++++-
 init/Kconfig                        |    5 +-
 kernel/irq_work.c                   |  131 ++++++++++++++++++++++++++---------
 kernel/printk.c                     |   36 +++++-----
 kernel/time/tick-sched.c            |    7 +-
 kernel/timer.c                      |    1 -
 22 files changed, 161 insertions(+), 73 deletions(-)

[PATCH 1/9] irq_work: Fix racy IRQ_WORK_BUSY flag setting

[Frederic Weisbecker]

The IRQ_WORK_BUSY flag is set right before we execute the
work. Once this flag value is set, the work enters a
claimable state again.

So if we have specific data to compute in our work, we ensure it's
either handled by another CPU or locally by enqueuing the work again.
This state machine is guanranteed by atomic operations on the flags.

So when we set IRQ_WORK_BUSY without using an xchg-like operation,
we break this guarantee as in the following summarized scenario:

        CPU 1                                   CPU 2
        -----                                   -----
                                                (flags = 0)
                                                old_flags = flags;
        (flags = 0)
        cmpxchg(flags, old_flags,
                old_flags | IRQ_WORK_FLAGS)
        (flags = 3)
        [...]
        flags = IRQ_WORK_BUSY
        (flags = 2)
        func()
                                                (sees flags = 3)
                                                cmpxchg(flags, old_flags,
                                                        old_flags | IRQ_WORK_FLAGS)
                                                (give up)

        cmpxchg(flags, 2, 0);
        (flags = 0)

CPU 1 claims a work and executes it, so it sets IRQ_WORK_BUSY and
the work is again in a claimable state. Now CPU 2 has new data to process
and try to claim that work but it may see a stale value of the flags
and think the work is still pending somewhere that will handle our data.
This is because CPU 1 doesn't set IRQ_WORK_BUSY atomically.

As a result, the data expected to be handle by CPU 2 won't get handled.

To fix this, use xchg() to set IRQ_WORK_BUSY, this way we ensure the CPU 2
will see the correct value with cmpxchg() using the expected ordering.

Changelog-heavily-inspired-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Anish Kumar <anish198519851985@gmail.com>
---
 kernel/irq_work.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 1588e3b..57be1a6 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -119,8 +119,11 @@ void irq_work_run(void)
 		/*
 		 * Clear the PENDING bit, after this point the @work
 		 * can be re-used.
+		 * Make it immediately visible so that other CPUs trying
+		 * to claim that work don't rely on us to handle their data
+		 * while we are in the middle of the func.
 		 */
-		work->flags = IRQ_WORK_BUSY;
+		xchg(&work->flags, IRQ_WORK_BUSY);
 		work->func(work);
 		/*
 		 * Clear the BUSY bit and return to the free state if
-- 
1.7.5.4

[PATCH 2/9] irq_work: Fix racy check on work pending flag

[Frederic Weisbecker]

Work claiming wants to be SMP-safe.

And by the time we try to claim a work, if it is already executing
concurrently on another CPU, we want to succeed the claiming and queue
the work again because the other CPU may have missed the data we wanted
to handle in our work if it's about to complete there.

This scenario is summarized below:

        CPU 1                                   CPU 2
        -----                                   -----
        (flags = 0)
        cmpxchg(flags, 0, IRQ_WORK_FLAGS)
        (flags = 3)
        [...]
        xchg(flags, IRQ_WORK_BUSY)
        (flags = 2)
        func()
                                                if (flags & IRQ_WORK_PENDING)
                                                        (not true)
                                                cmpxchg(flags, flags, IRQ_WORK_FLAGS)
                                                (flags = 3)
                                                [...]
        cmpxchg(flags, IRQ_WORK_BUSY, 0);
        (fail, pending on CPU 2)

This state machine is synchronized using [cmp]xchg() on the flags.
As such, the early IRQ_WORK_PENDING check in CPU 2 above is racy.
By the time we check it, we may be dealing with a stale value because
we aren't using an atomic accessor. As a result, CPU 2 may "see"
that the work is still pending on another CPU while it may be
actually completing the work function exection already, leaving
our data unprocessed.

To fix this, we start by speculating about the value we wish to be
in the work->flags but we only make any conclusion after the value
returned by the cmpxchg() call that either claims the work or let
the current owner handle the pending work for us.

Changelog-heavily-inspired-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Anish Kumar <anish198519851985@gmail.com>
---
 kernel/irq_work.c |   16 +++++++++++-----
 1 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 57be1a6..64eddd5 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -34,15 +34,21 @@ static DEFINE_PER_CPU(struct llist_head, irq_work_list);
  */
 static bool irq_work_claim(struct irq_work *work)
 {
-	unsigned long flags, nflags;
+	unsigned long flags, oflags, nflags;
 
+	/*
+	 * Start with our best wish as a premise but only trust any
+	 * flag value after cmpxchg() result.
+	 */
+	flags = work->flags & ~IRQ_WORK_PENDING;
 	for (;;) {
-		flags = work->flags;
-		if (flags & IRQ_WORK_PENDING)
-			return false;
 		nflags = flags | IRQ_WORK_FLAGS;
-		if (cmpxchg(&work->flags, flags, nflags) == flags)
+		oflags = cmpxchg(&work->flags, flags, nflags);
+		if (oflags == flags)
 			break;
+		if (oflags & IRQ_WORK_PENDING)
+			return false;
+		flags = oflags;
 		cpu_relax();
 	}
 
-- 
1.7.5.4

[PATCH 3/9] irq_work: Remove CONFIG_HAVE_IRQ_WORK

[Frederic Weisbecker]

irq work can run on any arch even without IPI
support because of the hook on update_process_times().

So lets remove HAVE_IRQ_WORK because it doesn't reflect
any backend requirement.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 arch/alpha/Kconfig                  |    1 -
 arch/arm/Kconfig                    |    1 -
 arch/arm64/Kconfig                  |    1 -
 arch/blackfin/Kconfig               |    1 -
 arch/frv/Kconfig                    |    1 -
 arch/hexagon/Kconfig                |    1 -
 arch/mips/Kconfig                   |    1 -
 arch/parisc/Kconfig                 |    1 -
 arch/powerpc/Kconfig                |    1 -
 arch/s390/Kconfig                   |    1 -
 arch/sh/Kconfig                     |    1 -
 arch/sparc/Kconfig                  |    1 -
 arch/x86/Kconfig                    |    1 -
 drivers/staging/iio/trigger/Kconfig |    1 -
 init/Kconfig                        |    4 ----
 15 files changed, 0 insertions(+), 18 deletions(-)

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 5dd7f5d..e56c2d1 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -5,7 +5,6 @@ config ALPHA
 	select HAVE_IDE
 	select HAVE_OPROFILE
 	select HAVE_SYSCALL_WRAPPERS
-	select HAVE_IRQ_WORK
 	select HAVE_PCSPKR_PLATFORM
 	select HAVE_PERF_EVENTS
 	select HAVE_DMA_ATTRS
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ade7e92..22d378b 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -36,7 +36,6 @@ config ARM
 	select HAVE_GENERIC_HARDIRQS
 	select HAVE_HW_BREAKPOINT if (PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7))
 	select HAVE_IDE if PCI || ISA || PCMCIA
-	select HAVE_IRQ_WORK
 	select HAVE_KERNEL_GZIP
 	select HAVE_KERNEL_LZMA
 	select HAVE_KERNEL_LZO
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index ef54a59..dd50d72 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -17,7 +17,6 @@ config ARM64
 	select HAVE_GENERIC_DMA_COHERENT
 	select HAVE_GENERIC_HARDIRQS
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS
-	select HAVE_IRQ_WORK
 	select HAVE_MEMBLOCK
 	select HAVE_PERF_EVENTS
 	select HAVE_SPARSE_IRQ
diff --git a/arch/blackfin/Kconfig b/arch/blackfin/Kconfig
index b6f3ad5..86f891f 100644
--- a/arch/blackfin/Kconfig
+++ b/arch/blackfin/Kconfig
@@ -24,7 +24,6 @@ config BLACKFIN
 	select HAVE_FUNCTION_TRACER
 	select HAVE_FUNCTION_TRACE_MCOUNT_TEST
 	select HAVE_IDE
-	select HAVE_IRQ_WORK
 	select HAVE_KERNEL_GZIP if RAMKERNEL
 	select HAVE_KERNEL_BZIP2 if RAMKERNEL
 	select HAVE_KERNEL_LZMA if RAMKERNEL
diff --git a/arch/frv/Kconfig b/arch/frv/Kconfig
index df2eb4b..c44fd6e 100644
--- a/arch/frv/Kconfig
+++ b/arch/frv/Kconfig
@@ -3,7 +3,6 @@ config FRV
 	default y
 	select HAVE_IDE
 	select HAVE_ARCH_TRACEHOOK
-	select HAVE_IRQ_WORK
 	select HAVE_PERF_EVENTS
 	select HAVE_UID16
 	select HAVE_GENERIC_HARDIRQS
diff --git a/arch/hexagon/Kconfig b/arch/hexagon/Kconfig
index 0744f7d..40a3185 100644
--- a/arch/hexagon/Kconfig
+++ b/arch/hexagon/Kconfig
@@ -14,7 +14,6 @@ config HEXAGON
 	# select HAVE_CLK
 	# select IRQ_PER_CPU
 	# select GENERIC_PENDING_IRQ if SMP
-	select HAVE_IRQ_WORK
 	select GENERIC_ATOMIC64
 	select HAVE_PERF_EVENTS
 	select HAVE_GENERIC_HARDIRQS
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index dba9390..3d86d69 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -4,7 +4,6 @@ config MIPS
 	select HAVE_GENERIC_DMA_COHERENT
 	select HAVE_IDE
 	select HAVE_OPROFILE
-	select HAVE_IRQ_WORK
 	select HAVE_PERF_EVENTS
 	select PERF_USE_VMALLOC
 	select HAVE_ARCH_KGDB
diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index 11def45..8f0df47 100644
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -9,7 +9,6 @@ config PARISC
 	select RTC_DRV_GENERIC
 	select INIT_ALL_POSSIBLE
 	select BUG
-	select HAVE_IRQ_WORK
 	select HAVE_PERF_EVENTS
 	select GENERIC_ATOMIC64 if !64BIT
 	select HAVE_GENERIC_HARDIRQS
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index a902a5c..a90f0c9 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -118,7 +118,6 @@ config PPC
 	select HAVE_SYSCALL_WRAPPERS if PPC64
 	select GENERIC_ATOMIC64 if PPC32
 	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
-	select HAVE_IRQ_WORK
 	select HAVE_PERF_EVENTS
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS && PPC_BOOK3S_64
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 5dba755..0816ff0 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -78,7 +78,6 @@ config S390
 	select HAVE_KVM if 64BIT
 	select HAVE_ARCH_TRACEHOOK
 	select INIT_ALL_POSSIBLE
-	select HAVE_IRQ_WORK
 	select HAVE_PERF_EVENTS
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
 	select HAVE_DEBUG_KMEMLEAK
diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index babc2b8..996e008 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -11,7 +11,6 @@ config SUPERH
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_ATTRS
-	select HAVE_IRQ_WORK
 	select HAVE_PERF_EVENTS
 	select HAVE_DEBUG_BUGVERBOSE
 	select ARCH_HAVE_CUSTOM_GPIO_H
diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index b6b442b..05a478f 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -22,7 +22,6 @@ config SPARC
 	select ARCH_WANT_OPTIONAL_GPIOLIB
 	select RTC_CLASS
 	select RTC_DRV_M48T59
-	select HAVE_IRQ_WORK
 	select HAVE_DMA_ATTRS
 	select HAVE_DMA_API_DEBUG
 	select HAVE_ARCH_JUMP_LABEL
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 46c3bff..c13e07a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -26,7 +26,6 @@ config X86
 	select HAVE_OPROFILE
 	select HAVE_PCSPKR_PLATFORM
 	select HAVE_PERF_EVENTS
-	select HAVE_IRQ_WORK
 	select HAVE_IOREMAP_PROT
 	select HAVE_KPROBES
 	select HAVE_MEMBLOCK
diff --git a/drivers/staging/iio/trigger/Kconfig b/drivers/staging/iio/trigger/Kconfig
index 7d32075..d44d3ad 100644
--- a/drivers/staging/iio/trigger/Kconfig
+++ b/drivers/staging/iio/trigger/Kconfig
@@ -21,7 +21,6 @@ config IIO_GPIO_TRIGGER
 config IIO_SYSFS_TRIGGER
 	tristate "SYSFS trigger"
 	depends on SYSFS
-	depends on HAVE_IRQ_WORK
 	select IRQ_WORK
 	help
 	  Provides support for using SYSFS entry as IIO triggers.
diff --git a/init/Kconfig b/init/Kconfig
index 6fdd6e3..cdc152c 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -20,12 +20,8 @@ config CONSTRUCTORS
 	bool
 	depends on !UML
 
-config HAVE_IRQ_WORK
-	bool
-
 config IRQ_WORK
 	bool
-	depends on HAVE_IRQ_WORK
 
 config BUILDTIME_EXTABLE_SORT
 	bool
-- 
1.7.5.4

[PATCH 4/9] nohz: Add API to check tick state

[Frederic Weisbecker]

We need some quick way to check if the CPU has stopped
its tick. This will be useful to implement the printk tick
using the irq work subsystem.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 include/linux/tick.h     |   17 ++++++++++++++++-
 kernel/time/tick-sched.c |    2 +-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index f37fceb..2307dd3 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -8,6 +8,8 @@
 
 #include <linux/clockchips.h>
 #include <linux/irqflags.h>
+#include <linux/percpu.h>
+#include <linux/hrtimer.h>
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
 
@@ -122,13 +124,26 @@ static inline int tick_oneshot_mode_active(void) { return 0; }
 #endif /* !CONFIG_GENERIC_CLOCKEVENTS */
 
 # ifdef CONFIG_NO_HZ
+DECLARE_PER_CPU(struct tick_sched, tick_cpu_sched);
+
+static inline int tick_nohz_tick_stopped(void)
+{
+	return __this_cpu_read(tick_cpu_sched.tick_stopped);
+}
+
 extern void tick_nohz_idle_enter(void);
 extern void tick_nohz_idle_exit(void);
 extern void tick_nohz_irq_exit(void);
 extern ktime_t tick_nohz_get_sleep_length(void);
 extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
 extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
-# else
+
+# else /* !CONFIG_NO_HZ */
+static inline int tick_nohz_tick_stopped(void)
+{
+	return 0;
+}
+
 static inline void tick_nohz_idle_enter(void) { }
 static inline void tick_nohz_idle_exit(void) { }
 
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a402608..9e945aa 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -28,7 +28,7 @@
 /*
  * Per cpu nohz control structure
  */
-static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);
+DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);
 
 /*
  * The time, when the last jiffy update happened. Protected by xtime_lock.
-- 
1.7.5.4

[PATCH 5/9] irq_work: Don't stop the tick with pending works

[Frederic Weisbecker]

Don't stop the tick if we have pending irq works on the
queue, otherwise if the arch can't raise self-IPIs, we may not
find an opportunity to execute the pending works for a while.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 include/linux/irq_work.h |    6 ++++++
 kernel/irq_work.c        |   11 +++++++++++
 kernel/time/tick-sched.c |    3 ++-
 3 files changed, 19 insertions(+), 1 deletions(-)

diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
index 6a9e8f5..a69704f 100644
--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -20,4 +20,10 @@ bool irq_work_queue(struct irq_work *work);
 void irq_work_run(void);
 void irq_work_sync(struct irq_work *work);
 
+#ifdef CONFIG_IRQ_WORK
+bool irq_work_needs_cpu(void);
+#else
+static bool irq_work_needs_cpu(void) { return false; }
+#endif
+
 #endif /* _LINUX_IRQ_WORK_H */
diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 64eddd5..b3c113a 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -99,6 +99,17 @@ bool irq_work_queue(struct irq_work *work)
 }
 EXPORT_SYMBOL_GPL(irq_work_queue);
 
+bool irq_work_needs_cpu(void)
+{
+	struct llist_head *this_list;
+
+	this_list = &__get_cpu_var(irq_work_list);
+	if (llist_empty(this_list))
+		return false;
+
+	return true;
+}
+
 /*
  * Run the irq_work entries on this cpu. Requires to be ran from hardirq
  * context with local IRQs disabled.
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 9e945aa..f249e8c 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -20,6 +20,7 @@
 #include <linux/profile.h>
 #include <linux/sched.h>
 #include <linux/module.h>
+#include <linux/irq_work.h>
 
 #include <asm/irq_regs.h>
 
@@ -289,7 +290,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 	} while (read_seqretry(&xtime_lock, seq));
 
 	if (rcu_needs_cpu(cpu, &rcu_delta_jiffies) || printk_needs_cpu(cpu) ||
-	    arch_needs_cpu(cpu)) {
+	    arch_needs_cpu(cpu) || irq_work_needs_cpu()) {
 		next_jiffies = last_jiffies + 1;
 		delta_jiffies = 1;
 	} else {
-- 
1.7.5.4

[PATCH 6/9] irq_work: Flush work on CPU_DYING

[Frederic Weisbecker]

From: Steven Rostedt <rostedt@goodmis.org>

In order not to offline a CPU with pending irq works, flush the
queue from CPU_DYING. The notifier is called by stop_machine on
the CPU that is going down. The code will not be called from irq context
(so things like get_irq_regs() wont work) but I'm not sure what the
requirements are for irq_work in that regard (Peter?). But irqs are
disabled and the CPU is about to go offline. Might as well flush the work.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 kernel/irq_work.c |   51 +++++++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index b3c113a..4ed1749 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -12,6 +12,8 @@
 #include <linux/percpu.h>
 #include <linux/hardirq.h>
 #include <linux/irqflags.h>
+#include <linux/cpu.h>
+#include <linux/notifier.h>
 #include <asm/processor.h>
 
 /*
@@ -110,11 +112,7 @@ bool irq_work_needs_cpu(void)
 	return true;
 }
 
-/*
- * Run the irq_work entries on this cpu. Requires to be ran from hardirq
- * context with local IRQs disabled.
- */
-void irq_work_run(void)
+static void __irq_work_run(void)
 {
 	struct irq_work *work;
 	struct llist_head *this_list;
@@ -124,7 +122,6 @@ void irq_work_run(void)
 	if (llist_empty(this_list))
 		return;
 
-	BUG_ON(!in_irq());
 	BUG_ON(!irqs_disabled());
 
 	llnode = llist_del_all(this_list);
@@ -149,6 +146,16 @@ void irq_work_run(void)
 		(void)cmpxchg(&work->flags, IRQ_WORK_BUSY, 0);
 	}
 }
+
+/*
+ * Run the irq_work entries on this cpu. Requires to be ran from hardirq
+ * context with local IRQs disabled.
+ */
+void irq_work_run(void)
+{
+	BUG_ON(!in_irq());
+	__irq_work_run();
+}
 EXPORT_SYMBOL_GPL(irq_work_run);
 
 /*
@@ -163,3 +170,35 @@ void irq_work_sync(struct irq_work *work)
 		cpu_relax();
 }
 EXPORT_SYMBOL_GPL(irq_work_sync);
+
+#ifdef CONFIG_HOTPLUG_CPU
+static int irq_work_cpu_notify(struct notifier_block *self,
+			       unsigned long action, void *hcpu)
+{
+	long cpu = (long)hcpu;
+
+	switch (action) {
+	case CPU_DYING:
+		/* Called from stop_machine */
+		if (WARN_ON_ONCE(cpu != smp_processor_id()))
+			break;
+		__irq_work_run();
+		break;
+	default:
+		break;
+	}
+	return NOTIFY_OK;
+}
+
+static struct notifier_block cpu_notify;
+
+static __init int irq_work_init_cpu_notifier(void)
+{
+	cpu_notify.notifier_call = irq_work_cpu_notify;
+	cpu_notify.priority = 0;
+	register_cpu_notifier(&cpu_notify);
+	return 0;
+}
+device_initcall(irq_work_init_cpu_notifier);
+
+#endif /* CONFIG_HOTPLUG_CPU */
-- 
1.7.5.4

[PATCH 7/9] irq_work: Warn if there's still work on cpu_down

[Frederic Weisbecker]

From: Steven Rostedt <rostedt@goodmis.org>

If we are in nohz and there's still irq_work to be done when the idle
task is about to go offline, give a nasty warning. Everything should
have been flushed from the CPU_DYING notifier already. Further attempts
to enqueue an irq_work are buggy because irqs are disabled by
__cpu_disable(). The best we can do is to report the issue to the user.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 kernel/irq_work.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 4ed1749..480f747 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -109,6 +109,9 @@ bool irq_work_needs_cpu(void)
 	if (llist_empty(this_list))
 		return false;
 
+	/* All work should have been flushed before going offline */
+	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
+
 	return true;
 }
 
-- 
1.7.5.4

[PATCH 8/9] irq_work: Make self-IPIs optable

[Frederic Weisbecker]

On irq work initialization, let the user choose to define it
as "lazy" or not. "Lazy" means that we don't want to send
an IPI (provided the arch can anyway) when we enqueue this
work but we rather prefer to wait for the next timer tick
to execute our work if possible.

This is going to be a benefit for non-urgent enqueuers
(like printk in the future) that may prefer not to raise
an IPI storm in case of frequent enqueuing on short periods
of time.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 include/linux/irq_work.h |   14 +++++++++++++
 kernel/irq_work.c        |   47 ++++++++++++++++++++++++++-------------------
 2 files changed, 41 insertions(+), 20 deletions(-)

diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
index a69704f..b28eb60 100644
--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -3,6 +3,20 @@
 
 #include <linux/llist.h>
 
+/*
+ * An entry can be in one of four states:
+ *
+ * free	     NULL, 0 -> {claimed}       : free to be used
+ * claimed   NULL, 3 -> {pending}       : claimed to be enqueued
+ * pending   next, 3 -> {busy}          : queued, pending callback
+ * busy      NULL, 2 -> {free, claimed} : callback in progress, can be claimed
+ */
+
+#define IRQ_WORK_PENDING	1UL
+#define IRQ_WORK_BUSY		2UL
+#define IRQ_WORK_FLAGS		3UL
+#define IRQ_WORK_LAZY		4UL /* Doesn't want IPI, wait for tick */
+
 struct irq_work {
 	unsigned long flags;
 	struct llist_node llnode;
diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 480f747..7f3a59b 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -12,24 +12,15 @@
 #include <linux/percpu.h>
 #include <linux/hardirq.h>
 #include <linux/irqflags.h>
+#include <linux/sched.h>
+#include <linux/tick.h>
 #include <linux/cpu.h>
 #include <linux/notifier.h>
 #include <asm/processor.h>
 
-/*
- * An entry can be in one of four states:
- *
- * free	     NULL, 0 -> {claimed}       : free to be used
- * claimed   NULL, 3 -> {pending}       : claimed to be enqueued
- * pending   next, 3 -> {busy}          : queued, pending callback
- * busy      NULL, 2 -> {free, claimed} : callback in progress, can be claimed
- */
-
-#define IRQ_WORK_PENDING	1UL
-#define IRQ_WORK_BUSY		2UL
-#define IRQ_WORK_FLAGS		3UL
 
 static DEFINE_PER_CPU(struct llist_head, irq_work_list);
+static DEFINE_PER_CPU(int, irq_work_raised);
 
 /*
  * Claim the entry so that no one else will poke at it.
@@ -69,14 +60,19 @@ void __weak arch_irq_work_raise(void)
  */
 static void __irq_work_queue(struct irq_work *work)
 {
-	bool empty;
-
 	preempt_disable();
 
-	empty = llist_add(&work->llnode, &__get_cpu_var(irq_work_list));
-	/* The list was empty, raise self-interrupt to start processing. */
-	if (empty)
-		arch_irq_work_raise();
+	llist_add(&work->llnode, &__get_cpu_var(irq_work_list));
+
+	/*
+	 * If the work is not "lazy" or the tick is stopped, raise the irq
+	 * work interrupt (if supported by the arch), otherwise, just wait
+	 * for the next tick.
+	 */
+	if (!(work->flags & IRQ_WORK_LAZY) || tick_nohz_tick_stopped()) {
+		if (!this_cpu_cmpxchg(irq_work_raised, 0, 1))
+			arch_irq_work_raise();
+	}
 
 	preempt_enable();
 }
@@ -117,10 +113,19 @@ bool irq_work_needs_cpu(void)
 
 static void __irq_work_run(void)
 {
+	unsigned long flags;
 	struct irq_work *work;
 	struct llist_head *this_list;
 	struct llist_node *llnode;
 
+
+	/*
+	 * Reset the "raised" state right before we check the list because
+	 * an NMI may enqueue after we find the list empty from the runner.
+	 */
+	__this_cpu_write(irq_work_raised, 0);
+	barrier();
+
 	this_list = &__get_cpu_var(irq_work_list);
 	if (llist_empty(this_list))
 		return;
@@ -140,13 +145,15 @@ static void __irq_work_run(void)
 		 * to claim that work don't rely on us to handle their data
 		 * while we are in the middle of the func.
 		 */
-		xchg(&work->flags, IRQ_WORK_BUSY);
+		flags = work->flags & ~IRQ_WORK_PENDING;
+		xchg(&work->flags, flags);
+
 		work->func(work);
 		/*
 		 * Clear the BUSY bit and return to the free state if
 		 * no-one else claimed it meanwhile.
 		 */
-		(void)cmpxchg(&work->flags, IRQ_WORK_BUSY, 0);
+		(void)cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
 	}
 }
 
-- 
1.7.5.4

[PATCH 9/9] printk: Wake up klogd using irq_work

[Frederic Weisbecker]

klogd is woken up asynchronously from the tick in order
to do it safely.

However if printk is called when the tick is stopped, the reader
won't be woken up until the next interrupt, which might not fire
for a while. As a result, the user may miss some message.

To fix this, lets implement the printk tick using a lazy irq work.
This subsystem takes care of the timer tick state and can
fix up accordingly.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 include/linux/printk.h   |    3 ---
 init/Kconfig             |    1 +
 kernel/printk.c          |   36 ++++++++++++++++++++----------------
 kernel/time/tick-sched.c |    2 +-
 kernel/timer.c           |    1 -
 5 files changed, 22 insertions(+), 21 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9afc01e..86c4b62 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -98,9 +98,6 @@ int no_printk(const char *fmt, ...)
 extern asmlinkage __printf(1, 2)
 void early_printk(const char *fmt, ...);
 
-extern int printk_needs_cpu(int cpu);
-extern void printk_tick(void);
-
 #ifdef CONFIG_PRINTK
 asmlinkage __printf(5, 0)
 int vprintk_emit(int facility, int level,
diff --git a/init/Kconfig b/init/Kconfig
index cdc152c..c575566 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1196,6 +1196,7 @@ config HOTPLUG
 config PRINTK
 	default y
 	bool "Enable support for printk" if EXPERT
+	select IRQ_WORK
 	help
 	  This option enables normal printk support. Removing it
 	  eliminates most of the message strings from the kernel image
diff --git a/kernel/printk.c b/kernel/printk.c
index 2d607f4..c9104fe 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -42,6 +42,7 @@
 #include <linux/notifier.h>
 #include <linux/rculist.h>
 #include <linux/poll.h>
+#include <linux/irq_work.h>
 
 #include <asm/uaccess.h>
 
@@ -1955,30 +1956,32 @@ int is_console_locked(void)
 static DEFINE_PER_CPU(int, printk_pending);
 static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf);
 
-void printk_tick(void)
+static void wake_up_klogd_work_func(struct irq_work *irq_work)
 {
-	if (__this_cpu_read(printk_pending)) {
-		int pending = __this_cpu_xchg(printk_pending, 0);
-		if (pending & PRINTK_PENDING_SCHED) {
-			char *buf = __get_cpu_var(printk_sched_buf);
-			printk(KERN_WARNING "[sched_delayed] %s", buf);
-		}
-		if (pending & PRINTK_PENDING_WAKEUP)
-			wake_up_interruptible(&log_wait);
+	int pending = __this_cpu_xchg(printk_pending, 0);
+
+	if (pending & PRINTK_PENDING_SCHED) {
+		char *buf = __get_cpu_var(printk_sched_buf);
+		printk(KERN_WARNING "[sched_delayed] %s", buf);
 	}
-}
 
-int printk_needs_cpu(int cpu)
-{
-	if (cpu_is_offline(cpu))
-		printk_tick();
-	return __this_cpu_read(printk_pending);
+	if (pending & PRINTK_PENDING_WAKEUP)
+		wake_up_interruptible(&log_wait);
 }
 
+static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
+	.func = wake_up_klogd_work_func,
+	.flags = IRQ_WORK_LAZY,
+};
+
 void wake_up_klogd(void)
 {
-	if (waitqueue_active(&log_wait))
+	preempt_disable();
+	if (waitqueue_active(&log_wait)) {
 		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
+		irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
+	}
+	preempt_enable();
 }
 
 static void console_cont_flush(char *text, size_t size)
@@ -2458,6 +2461,7 @@ int printk_sched(const char *fmt, ...)
 	va_end(args);
 
 	__this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
+	irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
 	local_irq_restore(flags);
 
 	return r;
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index f249e8c..822d757 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -289,7 +289,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 		time_delta = timekeeping_max_deferment();
 	} while (read_seqretry(&xtime_lock, seq));
 
-	if (rcu_needs_cpu(cpu, &rcu_delta_jiffies) || printk_needs_cpu(cpu) ||
+	if (rcu_needs_cpu(cpu, &rcu_delta_jiffies) ||
 	    arch_needs_cpu(cpu) || irq_work_needs_cpu()) {
 		next_jiffies = last_jiffies + 1;
 		delta_jiffies = 1;
diff --git a/kernel/timer.c b/kernel/timer.c
index 367d008..ff3b516 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1351,7 +1351,6 @@ void update_process_times(int user_tick)
 	account_process_tick(p, user_tick);
 	run_local_timers();
 	rcu_check_callbacks(cpu, user_tick);
-	printk_tick();
 #ifdef CONFIG_IRQ_WORK
 	if (in_irq())
 		irq_work_run();
-- 
1.7.5.4

Re: [GIT PULL v2] printk: Make it usable on nohz cpus

[Ingo Molnar]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> Ingo,
> 
> Please pull the printk support in dynticks mode patches that can
> be found at:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git tags/printk-dynticks-for-mingo-v2
> 
> for you to fetch changes up to 74876a98a87a115254b3a66a14b27320b7f0acaa:
> 
>   printk: Wake up klogd using irq_work (2012-11-18 01:01:49 +0100)
> 
> It is based on v3.7-rc4.
> 
> Changes since previous pull request include support for irq 
> work flush on CPU offlining and acks from Steve. The rest 
> hasn't changed except some comment fix.

Since this changes kernel/printk.c it needs Linus's ack.

I looked through the older submissions but found no good summary 
of these changes: it would be nice if you could write up a good 
high level description of these changes - why has printk based 
kernel message logging become problematic on nohz, what are the 
symptoms to users, and what are the solution alternatives you 
found and please justify the irq_work extension variant you 
picked.

I more or less accept the fact that fixes are needed here, but 
the linecount appears a bit high. It's possibly unavoidable, but 
would be nice to have a discussion of it, as printk is something 
we really, really want to keep as simple as possible.

Thanks,

	Ingo

Re: [GIT PULL v2] printk: Make it usable on nohz cpus

[Frederic Weisbecker]

2012/12/8 Ingo Molnar <mingo@kernel.org>:
>
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
>
>> Ingo,
>>
>> Please pull the printk support in dynticks mode patches that can
>> be found at:
>>
>>   git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git tags/printk-dynticks-for-mingo-v2
>>
>> for you to fetch changes up to 74876a98a87a115254b3a66a14b27320b7f0acaa:
>>
>>   printk: Wake up klogd using irq_work (2012-11-18 01:01:49 +0100)
>>
>> It is based on v3.7-rc4.
>>
>> Changes since previous pull request include support for irq
>> work flush on CPU offlining and acks from Steve. The rest
>> hasn't changed except some comment fix.
>
> Since this changes kernel/printk.c it needs Linus's ack.
>
> I looked through the older submissions but found no good summary
> of these changes: it would be nice if you could write up a good
> high level description of these changes - why has printk based
> kernel message logging become problematic on nohz, what are the
> symptoms to users, and what are the solution alternatives you
> found and please justify the irq_work extension variant you
> picked.
>
> I more or less accept the fact that fixes are needed here, but
> the linecount appears a bit high. It's possibly unavoidable, but
> would be nice to have a discussion of it, as printk is something
> we really, really want to keep as simple as possible.

Sorry, I focused too much on details and it indeed lacks a detailed
general explanation.

printk() is problematic for the full dynticks implementation that we
are working on because it depends on the tick to stay periodic in
order to wake up the potential readers sleeping on the syslog()
syscall.

printk() can be called about anywhere (sort of), so when a new message
is written in the buffer while there are pending readers, printk wakes
them up asynchronously instead of doing it in place in order to avoid
some gory locking scenarios with the scheduler locks.

This asynchronous wake up is handled by a hook on the timer tick:
printk_tick(). This is called every timer interrupt. So if we stop the
tick outside idle, and some printk() is called while the CPU runs in
full dynticks mode while we have pending readers, those may not be
woken for a while. Hence the user may miss some important message.

So if we want to enter in full dynticks mode safely, I fear we don't
have much choices. We need to find some event driven rather than
periodic driven way to perform the asynchronous wake up. The irq work
subsystem is a very good fit for that because:

* it can typically raise self-IPIs (some archs haven't implemented
that yet but this will be a requirement for full dynticks mode)
* it's light, simple and standalone. This is what we want for an API
used by printk()
* this is lockless, which is a requirement for printk at this stage.

Also irq work subsystem has its own hook on the timer tick in case the
arch can't perform self-IPIs. So if the tick is not stopped while
printk() is called, we can even avoid the IPI. This may be desirable
if we have situations with lots of printk() in a short period of time:
this can avoid an IPI storm.

Also this is an immediate benefit for mainline because we can now
remove the ad-hoc printk_tick() timer hook. One less function call and
per-cpu check from the timer interrupt is a win.

What do you guys think?