lkml.org 
[lkml]   [2015]   [Jan]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v2] sched_clock: Avoid deadlock during read from NMI
Date
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 that problem by providing banked clock data in a
similar manner to Thomas Gleixner's 4396e058c52e("timekeeping: Provide
fast and NMI safe access to CLOCK_MONOTONIC").

Changing the mode of operation of the seqcount away from the traditional
LSB-means-interrupted-write to a banked approach also revealed a good deal
of "fake" write locking within sched_clock_register(). This is likely
to be a latent issue because sched_clock_register() is typically called
before we enable interrupts. Nevertheless the issue has been eliminated
by increasing the scope of the read locking performed by sched_clock().

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
---

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

Without the patch the above system is grossly unstable, surviving
[9K,115K,25K] perf event cycles during three separate runs. With the
patch I ran for over 11M perf event cycles before getting bored.

v2:

* Extended the scope of the read lock in sched_clock() so we can bank
all data consumed there (John Stultz)


kernel/time/sched_clock.c | 157 +++++++++++++++++++++++++++++-----------------
1 file changed, 100 insertions(+), 57 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 01d2d15aa662..a2ea66944bc1 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -18,28 +18,28 @@
#include <linux/seqlock.h>
#include <linux/bitops.h>

-struct clock_data {
- ktime_t wrap_kt;
+struct clock_data_banked {
u64 epoch_ns;
u64 epoch_cyc;
- seqcount_t seq;
- unsigned long rate;
+ u64 (*read_sched_clock)(void);
+ u64 sched_clock_mask;
u32 mult;
u32 shift;
bool suspended;
};

+struct clock_data {
+ ktime_t wrap_kt;
+ seqcount_t seq;
+ unsigned long rate;
+ struct clock_data_banked bank[2];
+};
+
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 +49,14 @@ 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 = {
+ .bank = {
+ [0] = {
+ .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)
{
@@ -58,50 +65,82 @@ 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;
unsigned long seq;
-
- if (cd.suspended)
- return cd.epoch_ns;
+ struct clock_data_banked *b;
+ u64 res;

do {
- seq = raw_read_seqcount_begin(&cd.seq);
- epoch_cyc = cd.epoch_cyc;
- epoch_ns = cd.epoch_ns;
+ seq = raw_read_seqcount(&cd.seq);
+ b = cd.bank + (seq & 1);
+ if (b->suspended) {
+ res = b->epoch_ns;
+ } else {
+ cyc = b->read_sched_clock();
+ cyc = (cyc - b->epoch_cyc) & b->sched_clock_mask;
+ res = b->epoch_ns + cyc_to_ns(cyc, b->mult, b->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;
+}
+
+/*
+ * Start updating the banked clock data.
+ *
+ * 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.
+ *
+ * The caller is responsible for avoiding simultaneous updates.
+ */
+static struct clock_data_banked *update_bank_begin(void)
+{
+ /* update the backup (odd) bank and steer readers towards it */
+ memcpy(cd.bank + 1, cd.bank, sizeof(struct clock_data_banked));
+ raw_write_seqcount_latch(&cd.seq);
+
+ return cd.bank;
+}
+
+/*
+ * Finalize update of banked clock data.
+ *
+ * This is just a trivial switch back to the primary (even) copy.
+ */
+static void update_bank_end(void)
+{
+ raw_write_seqcount_latch(&cd.seq);
}

/*
* Atomically update the sched_clock epoch.
*/
-static void notrace update_sched_clock(void)
+static void notrace update_sched_clock(bool suspended)
{
- unsigned long flags;
+ struct clock_data_banked *b;
u64 cyc;
u64 ns;

- cyc = read_sched_clock();
- ns = cd.epoch_ns +
- cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
- cd.mult, cd.shift);
-
- raw_local_irq_save(flags);
- raw_write_seqcount_begin(&cd.seq);
- cd.epoch_ns = ns;
- cd.epoch_cyc = cyc;
- raw_write_seqcount_end(&cd.seq);
- raw_local_irq_restore(flags);
+ b = update_bank_begin();
+
+ cyc = b->read_sched_clock();
+ ns = b->epoch_ns + cyc_to_ns((cyc - b->epoch_cyc) & b->sched_clock_mask,
+ b->mult, b->shift);
+
+ b->epoch_ns = ns;
+ b->epoch_cyc = cyc;
+ b->suspended = suspended;
+
+ update_bank_end();
}

static enum hrtimer_restart sched_clock_poll(struct hrtimer *hrt)
{
- update_sched_clock();
+ update_sched_clock(false);
hrtimer_forward_now(hrt, cd.wrap_kt);
return HRTIMER_RESTART;
}
@@ -111,9 +150,9 @@ 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_data_banked *b;

if (cd.rate > rate)
return;
@@ -122,29 +161,30 @@ void __init sched_clock_register(u64 (*read)(void), int bits,

/* calculate the mult/shift to convert counter ticks to ns. */
clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);
+ cd.rate = rate;

new_mask = CLOCKSOURCE_MASK(bits);

/* 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));
+
+ b = update_bank_begin();

/* 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 = b->read_sched_clock();
+ ns = b->epoch_ns + cyc_to_ns((cyc - b->epoch_cyc) & b->sched_clock_mask,
+ b->mult, b->shift);

- 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;
- cd.epoch_ns = ns;
- raw_write_seqcount_end(&cd.seq);
+ b->read_sched_clock = read;
+ b->sched_clock_mask = new_mask;
+ b->mult = new_mult;
+ b->shift = new_shift;
+ b->epoch_cyc = new_epoch;
+ b->epoch_ns = ns;
+
+ update_bank_end();

r = rate;
if (r >= 4000000) {
@@ -175,10 +215,10 @@ 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.bank[0].read_sched_clock == jiffy_sched_clock_read)
sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);

- update_sched_clock();
+ update_sched_clock(false);

/*
* Start the timer to keep sched_clock() properly updated and
@@ -191,17 +231,20 @@ void __init sched_clock_postinit(void)

static int sched_clock_suspend(void)
{
- update_sched_clock();
+ update_sched_clock(true);
hrtimer_cancel(&sched_clock_timer);
- cd.suspended = true;
return 0;
}

static void sched_clock_resume(void)
{
- cd.epoch_cyc = read_sched_clock();
+ struct clock_data_banked *b;
+
+ b = update_bank_begin();
+ b->epoch_cyc = b->read_sched_clock();
hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
- cd.suspended = false;
+ b->suspended = false;
+ update_bank_end();
}

static struct syscore_ops sched_clock_ops = {
--
1.9.3


\
 
 \ /
  Last update: 2015-01-22 14:21    [W:0.243 / U:2.848 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site