[PATCH 0/5] Arch-generic sched_clock NMI safety and optimizations for -tip

[PATCH 0/5] Arch-generic sched_clock NMI safety and optimizations for -tip

[John Stultz]

Ingo, Peter, Thomas,

  The following patches from Daniel extend the arch-generic sched_clock
implementation so that it can be safely called from NMI (or FIQ on ARM)
context. They also optimize the sched_clock logic to improve cache
performance. I wanted to send them along so they could be queued in
-tip for 4.1.

>From Daniel's last post:
"The data cache profile of sched_clock() in both the original code and
my previous patch was somewhere between 2 and 3 (64-byte) cache lines,
depending on alignment of struct clock_data. After patching, the cache
profile for the normal case should be a single cacheline.

NMI safety was tested on i.MX6 with perf drowning the system in FIQs and
using the perf handler to check that sched_clock() returned monotonic
values. At the same time I forcefully reduced kt_wrap so that
update_sched_clock() is being called at >1000Hz.

Without the patches the above system is grossly unstable, surviving
[9K,115K,25K] perf event cycles during three separate runs. With the
patch I ran for over 9M perf event cycles before getting bored."

I'm relaying these along because in the past I've queued and submitted
arch-generic sched_clock.c changes from Stephen via Thomas, but I wanted
to make sure this got a look from Ingo and Peter.

In the future, I think Stephen is probably the right person to act as
patch-catcher for the kernel/time/sched_clock.c file. But I think those
changes should continue to be merged in via the -tip tree w/ other
scheduler code.

Let me know if you have any objections or concerns.

thanks
-john

Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>

These patches can also be pulled from the pull request below.

The following changes since commit c517d838eb7d07bbe9507871fab3931deccff539:

Linux 4.0-rc1 (2015-02-22 18:21:14 -0800)

are available in the git repository at:

https://git.linaro.org/people/john.stultz/linux.git fortglx/4.1/schedclock

for you to fetch changes up to 4bb74b85d2e74cfec9e21a6de44b232a2983bcc7:

sched_clock: Avoid deadlock during read from NMI (2015-03-10 20:28:58 -0700)

----------------------------------------------------------------

Daniel Thompson (5):
  sched_clock: Match scope of read and write seqcounts
  sched_clock: Optimize cache line usage
  sched_clock: Remove suspend from clock_read_data
  sched_clock: Remove redundant notrace from update function
  sched_clock: Avoid deadlock during read from NMI

 kernel/time/sched_clock.c | 195 ++++++++++++++++++++++++++++++++--------------
 1 file changed, 138 insertions(+), 57 deletions(-)

-- 
1.9.1

[PATCH 1/5] sched_clock: Match scope of read and write seqcounts

[John Stultz]

From: Daniel Thompson <daniel.thompson@linaro.org>

Currently the scope of the raw_write_seqcount_begin/end in
sched_clock_register far exceeds the scope of the read section in
sched_clock. This gives the impression of safety during cursory review
but achieves little.

Note that this is likely to be a latent issue at present because
sched_clock_register() is typically called before we enable interrupts,
however the issue does risk bugs being needlessly introduced as the code
evolves.

This patch fixes the problem by increasing the scope of the read locking
performed by sched_clock() to cover all data modified by
sched_clock_register.

We also improve clarity by moving writes to struct clock_data that do
not impact sched_clock() outside of the critical section.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 kernel/time/sched_clock.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 01d2d15..3d21a87 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -58,23 +58,21 @@ static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
 
 unsigned long long notrace sched_clock(void)
 {
-	u64 epoch_ns;
-	u64 epoch_cyc;
-	u64 cyc;
+	u64 cyc, res;
 	unsigned long seq;
 
-	if (cd.suspended)
-		return cd.epoch_ns;
-
 	do {
 		seq = raw_read_seqcount_begin(&cd.seq);
-		epoch_cyc = cd.epoch_cyc;
-		epoch_ns = cd.epoch_ns;
+
+		res = cd.epoch_ns;
+		if (!cd.suspended) {
+			cyc = read_sched_clock();
+			cyc = (cyc - cd.epoch_cyc) & sched_clock_mask;
+			res += cyc_to_ns(cyc, cd.mult, cd.shift);
+		}
 	} while (read_seqcount_retry(&cd.seq, seq));
 
-	cyc = read_sched_clock();
-	cyc = (cyc - epoch_cyc) & sched_clock_mask;
-	return epoch_ns + cyc_to_ns(cyc, cd.mult, cd.shift);
+	return res;
 }
 
 /*
@@ -124,10 +122,11 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 	clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);
 
 	new_mask = CLOCKSOURCE_MASK(bits);
+	cd.rate = rate;
 
 	/* calculate how many ns until we wrap */
 	wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask);
-	new_wrap_kt = ns_to_ktime(wrap - (wrap >> 3));
+	cd.wrap_kt = ns_to_ktime(wrap - (wrap >> 3));
 
 	/* update epoch for new counter and update epoch_ns from old counter*/
 	new_epoch = read();
@@ -138,8 +137,6 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 	raw_write_seqcount_begin(&cd.seq);
 	read_sched_clock = read;
 	sched_clock_mask = new_mask;
-	cd.rate = rate;
-	cd.wrap_kt = new_wrap_kt;
 	cd.mult = new_mult;
 	cd.shift = new_shift;
 	cd.epoch_cyc = new_epoch;
-- 
1.9.1

[PATCH 2/5] sched_clock: Optimize cache line usage

[John Stultz]

From: Daniel Thompson <daniel.thompson@linaro.org>

Currently sched_clock(), a very hot code path, is not optimized to
minimise its cache profile. In particular:

  1. cd is not ____cacheline_aligned,

  2. struct clock_data does not distinguish between hotpath and
     coldpath data, reducing locality of reference in the hotpath,

  3. Some hotpath data is missing from struct clock_data and is marked
     __read_mostly (which more or less guarantees it will not share a
     cache line with cd).

This patch corrects these problems by extracting all hotpath data
into a separate structure and using ____cacheline_aligned to ensure
the hotpath uses a single (64 byte) cache line.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 kernel/time/sched_clock.c | 113 +++++++++++++++++++++++++++++++---------------
 1 file changed, 77 insertions(+), 36 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 3d21a87..695b2ac 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -18,28 +18,59 @@
 #include <linux/seqlock.h>
 #include <linux/bitops.h>
 
-struct clock_data {
-	ktime_t wrap_kt;
+/**
+ * struct clock_read_data - data required to read from sched_clock
+ *
+ * @epoch_ns:		sched_clock value at last update
+ * @epoch_cyc:		Clock cycle value at last update
+ * @sched_clock_mask:   Bitmask for two's complement subtraction of non 64bit
+ *			clocks
+ * @read_sched_clock:	Current clock source (or dummy source when suspended)
+ * @mult:		Multipler for scaled math conversion
+ * @shift:		Shift value for scaled math conversion
+ * @suspended:		Flag to indicate if the clock is suspended (stopped)
+ *
+ * Care must be taken when updating this structure; it is read by
+ * some very hot code paths. It occupies <=48 bytes and, when combined
+ * with the seqcount used to synchronize access, comfortably fits into
+ * a 64 byte cache line.
+ */
+struct clock_read_data {
 	u64 epoch_ns;
 	u64 epoch_cyc;
-	seqcount_t seq;
-	unsigned long rate;
+	u64 sched_clock_mask;
+	u64 (*read_sched_clock)(void);
 	u32 mult;
 	u32 shift;
 	bool suspended;
 };
 
+/**
+ * struct clock_data - all data needed for sched_clock (including
+ *                     registration of a new clock source)
+ *
+ * @seq:		Sequence counter for protecting updates.
+ * @read_data:		Data required to read from sched_clock.
+ * @wrap_kt:		Duration for which clock can run before wrapping
+ * @rate:		Tick rate of the registered clock
+ * @actual_read_sched_clock: Registered clock read function
+ *
+ * The ordering of this structure has been chosen to optimize cache
+ * performance. In particular seq and read_data (combined) should fit
+ * into a single 64 byte cache line.
+ */
+struct clock_data {
+	seqcount_t seq;
+	struct clock_read_data read_data;
+	ktime_t wrap_kt;
+	unsigned long rate;
+};
+
 static struct hrtimer sched_clock_timer;
 static int irqtime = -1;
 
 core_param(irqtime, irqtime, int, 0400);
 
-static struct clock_data cd = {
-	.mult	= NSEC_PER_SEC / HZ,
-};
-
-static u64 __read_mostly sched_clock_mask;
-
 static u64 notrace jiffy_sched_clock_read(void)
 {
 	/*
@@ -49,7 +80,10 @@ static u64 notrace jiffy_sched_clock_read(void)
 	return (u64)(jiffies - INITIAL_JIFFIES);
 }
 
-static u64 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
+static struct clock_data cd ____cacheline_aligned = {
+	.read_data = { .mult = NSEC_PER_SEC / HZ,
+		       .read_sched_clock = jiffy_sched_clock_read, },
+};
 
 static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
 {
@@ -60,15 +94,16 @@ unsigned long long notrace sched_clock(void)
 {
 	u64 cyc, res;
 	unsigned long seq;
+	struct clock_read_data *rd = &cd.read_data;
 
 	do {
 		seq = raw_read_seqcount_begin(&cd.seq);
 
-		res = cd.epoch_ns;
-		if (!cd.suspended) {
-			cyc = read_sched_clock();
-			cyc = (cyc - cd.epoch_cyc) & sched_clock_mask;
-			res += cyc_to_ns(cyc, cd.mult, cd.shift);
+		res = rd->epoch_ns;
+		if (!rd->suspended) {
+			cyc = rd->read_sched_clock();
+			cyc = (cyc - rd->epoch_cyc) & rd->sched_clock_mask;
+			res += cyc_to_ns(cyc, rd->mult, rd->shift);
 		}
 	} while (read_seqcount_retry(&cd.seq, seq));
 
@@ -83,16 +118,17 @@ static void notrace update_sched_clock(void)
 	unsigned long flags;
 	u64 cyc;
 	u64 ns;
+	struct clock_read_data *rd = &cd.read_data;
 
-	cyc = read_sched_clock();
-	ns = cd.epoch_ns +
-		cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
-			  cd.mult, cd.shift);
+	cyc = rd->read_sched_clock();
+	ns = rd->epoch_ns +
+	     cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
+		       rd->mult, rd->shift);
 
 	raw_local_irq_save(flags);
 	raw_write_seqcount_begin(&cd.seq);
-	cd.epoch_ns = ns;
-	cd.epoch_cyc = cyc;
+	rd->epoch_ns = ns;
+	rd->epoch_cyc = cyc;
 	raw_write_seqcount_end(&cd.seq);
 	raw_local_irq_restore(flags);
 }
@@ -109,9 +145,9 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 {
 	u64 res, wrap, new_mask, new_epoch, cyc, ns;
 	u32 new_mult, new_shift;
-	ktime_t new_wrap_kt;
 	unsigned long r;
 	char r_unit;
+	struct clock_read_data *rd = &cd.read_data;
 
 	if (cd.rate > rate)
 		return;
@@ -130,17 +166,18 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 
 	/* update epoch for new counter and update epoch_ns from old counter*/
 	new_epoch = read();
-	cyc = read_sched_clock();
-	ns = cd.epoch_ns + cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
-			  cd.mult, cd.shift);
+	cyc = rd->read_sched_clock();
+	ns = rd->epoch_ns +
+	     cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
+		       rd->mult, rd->shift);
 
 	raw_write_seqcount_begin(&cd.seq);
-	read_sched_clock = read;
-	sched_clock_mask = new_mask;
-	cd.mult = new_mult;
-	cd.shift = new_shift;
-	cd.epoch_cyc = new_epoch;
-	cd.epoch_ns = ns;
+	rd->read_sched_clock = read;
+	rd->sched_clock_mask = new_mask;
+	rd->mult = new_mult;
+	rd->shift = new_shift;
+	rd->epoch_cyc = new_epoch;
+	rd->epoch_ns = ns;
 	raw_write_seqcount_end(&cd.seq);
 
 	r = rate;
@@ -172,7 +209,7 @@ void __init sched_clock_postinit(void)
 	 * If no sched_clock function has been provided at that point,
 	 * make it the final one one.
 	 */
-	if (read_sched_clock == jiffy_sched_clock_read)
+	if (cd.read_data.read_sched_clock == jiffy_sched_clock_read)
 		sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);
 
 	update_sched_clock();
@@ -188,17 +225,21 @@ void __init sched_clock_postinit(void)
 
 static int sched_clock_suspend(void)
 {
+	struct clock_read_data *rd = &cd.read_data;
+
 	update_sched_clock();
 	hrtimer_cancel(&sched_clock_timer);
-	cd.suspended = true;
+	rd->suspended = true;
 	return 0;
 }
 
 static void sched_clock_resume(void)
 {
-	cd.epoch_cyc = read_sched_clock();
+	struct clock_read_data *rd = &cd.read_data;
+
+	rd->epoch_cyc = rd->read_sched_clock();
 	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
-	cd.suspended = false;
+	rd->suspended = false;
 }
 
 static struct syscore_ops sched_clock_ops = {
-- 
1.9.1

[PATCH 3/5] sched_clock: Remove suspend from clock_read_data

[John Stultz]

From: Daniel Thompson <daniel.thompson@linaro.org>

Currently cd.read_data.suspended is read by the hotpath function
sched_clock(). This variable need not be accessed on the hotpath. In
fact, once it is removed, we can remove the conditional branches from
sched_clock() and install a dummy read_sched_clock function to suspend
the clock.

The new master copy of the function pointer (actual_read_sched_clock) is
introduced and is used for all reads of the clock hardware except those
within sched_clock itself.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 kernel/time/sched_clock.c | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 695b2ac..5d64079 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -28,10 +28,9 @@
  * @read_sched_clock:	Current clock source (or dummy source when suspended)
  * @mult:		Multipler for scaled math conversion
  * @shift:		Shift value for scaled math conversion
- * @suspended:		Flag to indicate if the clock is suspended (stopped)
  *
  * Care must be taken when updating this structure; it is read by
- * some very hot code paths. It occupies <=48 bytes and, when combined
+ * some very hot code paths. It occupies <=40 bytes and, when combined
  * with the seqcount used to synchronize access, comfortably fits into
  * a 64 byte cache line.
  */
@@ -42,7 +41,6 @@ struct clock_read_data {
 	u64 (*read_sched_clock)(void);
 	u32 mult;
 	u32 shift;
-	bool suspended;
 };
 
 /**
@@ -64,6 +62,7 @@ struct clock_data {
 	struct clock_read_data read_data;
 	ktime_t wrap_kt;
 	unsigned long rate;
+	u64 (*actual_read_sched_clock)(void);
 };
 
 static struct hrtimer sched_clock_timer;
@@ -83,6 +82,8 @@ static u64 notrace jiffy_sched_clock_read(void)
 static struct clock_data cd ____cacheline_aligned = {
 	.read_data = { .mult = NSEC_PER_SEC / HZ,
 		       .read_sched_clock = jiffy_sched_clock_read, },
+	.actual_read_sched_clock = jiffy_sched_clock_read,
+
 };
 
 static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
@@ -99,12 +100,9 @@ unsigned long long notrace sched_clock(void)
 	do {
 		seq = raw_read_seqcount_begin(&cd.seq);
 
-		res = rd->epoch_ns;
-		if (!rd->suspended) {
-			cyc = rd->read_sched_clock();
-			cyc = (cyc - rd->epoch_cyc) & rd->sched_clock_mask;
-			res += cyc_to_ns(cyc, rd->mult, rd->shift);
-		}
+		cyc = (rd->read_sched_clock() - rd->epoch_cyc) &
+		      rd->sched_clock_mask;
+		res = rd->epoch_ns + cyc_to_ns(cyc, rd->mult, rd->shift);
 	} while (read_seqcount_retry(&cd.seq, seq));
 
 	return res;
@@ -120,7 +118,7 @@ static void notrace update_sched_clock(void)
 	u64 ns;
 	struct clock_read_data *rd = &cd.read_data;
 
-	cyc = rd->read_sched_clock();
+	cyc = cd.actual_read_sched_clock();
 	ns = rd->epoch_ns +
 	     cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
 		       rd->mult, rd->shift);
@@ -166,10 +164,11 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 
 	/* update epoch for new counter and update epoch_ns from old counter*/
 	new_epoch = read();
-	cyc = rd->read_sched_clock();
+	cyc = cd.actual_read_sched_clock();
 	ns = rd->epoch_ns +
 	     cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
 		       rd->mult, rd->shift);
+	cd.actual_read_sched_clock = read;
 
 	raw_write_seqcount_begin(&cd.seq);
 	rd->read_sched_clock = read;
@@ -209,7 +208,7 @@ void __init sched_clock_postinit(void)
 	 * If no sched_clock function has been provided at that point,
 	 * make it the final one one.
 	 */
-	if (cd.read_data.read_sched_clock == jiffy_sched_clock_read)
+	if (cd.actual_read_sched_clock == jiffy_sched_clock_read)
 		sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);
 
 	update_sched_clock();
@@ -223,13 +222,24 @@ void __init sched_clock_postinit(void)
 	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
 }
 
+/*
+ * Clock read function for use when the clock is suspended.
+ *
+ * This function makes it appear to sched_clock() as if the clock
+ * stopped counting at its last update.
+ */
+static u64 notrace suspended_sched_clock_read(void)
+{
+	return cd.read_data.epoch_cyc;
+}
+
 static int sched_clock_suspend(void)
 {
 	struct clock_read_data *rd = &cd.read_data;
 
 	update_sched_clock();
 	hrtimer_cancel(&sched_clock_timer);
-	rd->suspended = true;
+	rd->read_sched_clock = suspended_sched_clock_read;
 	return 0;
 }
 
@@ -237,9 +247,9 @@ static void sched_clock_resume(void)
 {
 	struct clock_read_data *rd = &cd.read_data;
 
-	rd->epoch_cyc = rd->read_sched_clock();
+	rd->epoch_cyc = cd.actual_read_sched_clock();
 	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
-	rd->suspended = false;
+	rd->read_sched_clock = cd.actual_read_sched_clock;
 }
 
 static struct syscore_ops sched_clock_ops = {
-- 
1.9.1

[PATCH 4/5] sched_clock: Remove redundant notrace from update function

[John Stultz]

From: Daniel Thompson <daniel.thompson@linaro.org>

Currently update_sched_clock() is marked as notrace but this function
is not called by ftrace. This is trivially fixed by removing the mark
up.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 kernel/time/sched_clock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 5d64079..9280327 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -111,7 +111,7 @@ unsigned long long notrace sched_clock(void)
 /*
  * Atomically update the sched_clock epoch.
  */
-static void notrace update_sched_clock(void)
+static void update_sched_clock(void)
 {
 	unsigned long flags;
 	u64 cyc;
-- 
1.9.1

[PATCH 5/5] sched_clock: Avoid deadlock during read from NMI

[John Stultz]

From: Daniel Thompson <daniel.thompson@linaro.org>

Currently it is possible for an NMI (or FIQ on ARM) to come in and
read sched_clock() whilst update_sched_clock() has locked the seqcount
for writing. This results in the NMI handler locking up when it calls
raw_read_seqcount_begin().

This patch fixes the NMI safety issues by providing banked clock data.
This is a similar approach to the one used in Thomas Gleixner's
4396e058c52e("timekeeping: Provide fast and NMI safe access to
CLOCK_MONOTONIC").

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 kernel/time/sched_clock.c | 103 ++++++++++++++++++++++++++++++----------------
 1 file changed, 68 insertions(+), 35 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 9280327..a23d98c 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -47,19 +47,20 @@ struct clock_read_data {
  * struct clock_data - all data needed for sched_clock (including
  *                     registration of a new clock source)
  *
- * @seq:		Sequence counter for protecting updates.
+ * @seq:		Sequence counter for protecting updates. The lowest
+ *			bit is the index for @read_data.
  * @read_data:		Data required to read from sched_clock.
  * @wrap_kt:		Duration for which clock can run before wrapping
  * @rate:		Tick rate of the registered clock
  * @actual_read_sched_clock: Registered clock read function
  *
  * The ordering of this structure has been chosen to optimize cache
- * performance. In particular seq and read_data (combined) should fit
+ * performance. In particular seq and read_data[0] (combined) should fit
  * into a single 64 byte cache line.
  */
 struct clock_data {
 	seqcount_t seq;
-	struct clock_read_data read_data;
+	struct clock_read_data read_data[2];
 	ktime_t wrap_kt;
 	unsigned long rate;
 	u64 (*actual_read_sched_clock)(void);
@@ -80,10 +81,9 @@ static u64 notrace jiffy_sched_clock_read(void)
 }
 
 static struct clock_data cd ____cacheline_aligned = {
-	.read_data = { .mult = NSEC_PER_SEC / HZ,
-		       .read_sched_clock = jiffy_sched_clock_read, },
+	.read_data[0] = { .mult = NSEC_PER_SEC / HZ,
+			  .read_sched_clock = jiffy_sched_clock_read, },
 	.actual_read_sched_clock = jiffy_sched_clock_read,
-
 };
 
 static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
@@ -95,10 +95,11 @@ unsigned long long notrace sched_clock(void)
 {
 	u64 cyc, res;
 	unsigned long seq;
-	struct clock_read_data *rd = &cd.read_data;
+	struct clock_read_data *rd;
 
 	do {
-		seq = raw_read_seqcount_begin(&cd.seq);
+		seq = raw_read_seqcount(&cd.seq);
+		rd = cd.read_data + (seq & 1);
 
 		cyc = (rd->read_sched_clock() - rd->epoch_cyc) &
 		      rd->sched_clock_mask;
@@ -109,26 +110,50 @@ unsigned long long notrace sched_clock(void)
 }
 
 /*
+ * Updating the data required to read the clock.
+ *
+ * sched_clock will never observe mis-matched data even if called from
+ * an NMI. We do this by maintaining an odd/even copy of the data and
+ * steering sched_clock to one or the other using a sequence counter.
+ * In order to preserve the data cache profile of sched_clock as much
+ * as possible the system reverts back to the even copy when the update
+ * completes; the odd copy is used *only* during an update.
+ */
+static void update_clock_read_data(struct clock_read_data *rd)
+{
+	/* update the backup (odd) copy with the new data */
+	cd.read_data[1] = *rd;
+
+	/* steer readers towards the odd copy */
+	raw_write_seqcount_latch(&cd.seq);
+
+	/* now its safe for us to update the normal (even) copy */
+	cd.read_data[0] = *rd;
+
+	/* switch readers back to the even copy */
+	raw_write_seqcount_latch(&cd.seq);
+}
+
+/*
  * Atomically update the sched_clock epoch.
  */
 static void update_sched_clock(void)
 {
-	unsigned long flags;
 	u64 cyc;
 	u64 ns;
-	struct clock_read_data *rd = &cd.read_data;
+	struct clock_read_data rd;
+
+	rd = cd.read_data[0];
 
 	cyc = cd.actual_read_sched_clock();
-	ns = rd->epoch_ns +
-	     cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
-		       rd->mult, rd->shift);
-
-	raw_local_irq_save(flags);
-	raw_write_seqcount_begin(&cd.seq);
-	rd->epoch_ns = ns;
-	rd->epoch_cyc = cyc;
-	raw_write_seqcount_end(&cd.seq);
-	raw_local_irq_restore(flags);
+	ns = rd.epoch_ns +
+	     cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask,
+		       rd.mult, rd.shift);
+
+	rd.epoch_ns = ns;
+	rd.epoch_cyc = cyc;
+
+	update_clock_read_data(&rd);
 }
 
 static enum hrtimer_restart sched_clock_poll(struct hrtimer *hrt)
@@ -145,7 +170,7 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 	u32 new_mult, new_shift;
 	unsigned long r;
 	char r_unit;
-	struct clock_read_data *rd = &cd.read_data;
+	struct clock_read_data rd;
 
 	if (cd.rate > rate)
 		return;
@@ -162,22 +187,23 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 	wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask);
 	cd.wrap_kt = ns_to_ktime(wrap - (wrap >> 3));
 
+	rd = cd.read_data[0];
+
 	/* update epoch for new counter and update epoch_ns from old counter*/
 	new_epoch = read();
 	cyc = cd.actual_read_sched_clock();
-	ns = rd->epoch_ns +
-	     cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
-		       rd->mult, rd->shift);
+	ns = rd.epoch_ns +
+	     cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask,
+		       rd.mult, rd.shift);
 	cd.actual_read_sched_clock = read;
 
-	raw_write_seqcount_begin(&cd.seq);
-	rd->read_sched_clock = read;
-	rd->sched_clock_mask = new_mask;
-	rd->mult = new_mult;
-	rd->shift = new_shift;
-	rd->epoch_cyc = new_epoch;
-	rd->epoch_ns = ns;
-	raw_write_seqcount_end(&cd.seq);
+	rd.read_sched_clock = read;
+	rd.sched_clock_mask = new_mask;
+	rd.mult = new_mult;
+	rd.shift = new_shift;
+	rd.epoch_cyc = new_epoch;
+	rd.epoch_ns = ns;
+	update_clock_read_data(&rd);
 
 	r = rate;
 	if (r >= 4000000) {
@@ -227,15 +253,22 @@ void __init sched_clock_postinit(void)
  *
  * This function makes it appear to sched_clock() as if the clock
  * stopped counting at its last update.
+ *
+ * This function must only be called from the critical
+ * section in sched_clock(). It relies on the read_seqcount_retry()
+ * at the end of the critical section to be sure we observe the
+ * correct copy of epoch_cyc.
  */
 static u64 notrace suspended_sched_clock_read(void)
 {
-	return cd.read_data.epoch_cyc;
+	unsigned long seq = raw_read_seqcount(&cd.seq);
+
+	return cd.read_data[seq & 1].epoch_cyc;
 }
 
 static int sched_clock_suspend(void)
 {
-	struct clock_read_data *rd = &cd.read_data;
+	struct clock_read_data *rd = &cd.read_data[0];
 
 	update_sched_clock();
 	hrtimer_cancel(&sched_clock_timer);
@@ -245,7 +278,7 @@ static int sched_clock_suspend(void)
 
 static void sched_clock_resume(void)
 {
-	struct clock_read_data *rd = &cd.read_data;
+	struct clock_read_data *rd = &cd.read_data[0];
 
 	rd->epoch_cyc = cd.actual_read_sched_clock();
 	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
-- 
1.9.1

Re: [PATCH 0/5] Arch-generic sched_clock NMI safety and optimizations for -tip

[Peter Zijlstra]

On Tue, Mar 17, 2015 at 10:38:21AM -0700, John Stultz wrote:
> Daniel Thompson (5):
>   sched_clock: Match scope of read and write seqcounts
>   sched_clock: Optimize cache line usage
>   sched_clock: Remove suspend from clock_read_data
>   sched_clock: Remove redundant notrace from update function
>   sched_clock: Avoid deadlock during read from NMI
> 
>  kernel/time/sched_clock.c | 195 ++++++++++++++++++++++++++++++++--------------
>  1 file changed, 138 insertions(+), 57 deletions(-)

Looks sane; makes one wonder if there's more code to share between this
and the core timekeeping; there's some strong resemblance ;-)

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Re: [PATCH 0/5] Arch-generic sched_clock NMI safety and optimizations for -tip

[Ingo Molnar]

* John Stultz <john.stultz@linaro.org> wrote:

> Ingo, Peter, Thomas,
> 
>   The following patches from Daniel extend the arch-generic sched_clock
> implementation so that it can be safely called from NMI (or FIQ on ARM)
> context. They also optimize the sched_clock logic to improve cache
> performance. I wanted to send them along so they could be queued in
> -tip for 4.1.

Hm, the first patch does not apply to tip:timers/core, as it conflicts 
with:

  fb82fe2fe858 clocksource: Add 'max_cycles' to 'struct clocksource'
  362fde041037 clocksource: Simplify the logic around clocksource wrapping safety margins

Mind resending your latest queue as patches, on top of tip:timers/core 
or tip:master?

Thanks,

	Ingo