Re: [RFC] timekeeping: Use cached readouts for monotonic and raw clocks in suspend

[joelaf <joelaf@google.com>]

Hi Thomas,

On 11/20/2016 05:24 AM, Thomas Gleixner wrote:
> On Sat, 19 Nov 2016, Joel Fernandes wrote:
>
>> I am planning to add boot clock as a trace clock that can account suspend time
>> during tracing, however ktime_get_with_offset throws a warning as the
>> clocksource is attempted to be accessed in suspend.
>
> ktime_get_with_offset() cannot be used as trace clock at all because it can
> life lock in NMI context. That's why we have ktime_get_mono_fast().

But ktime_get_mono_fast doesn't account the suspend time, only boot 
clock (accessed with ktime_get_with_offset) does and while tracing it is 
useful for the trace clock to account suspended time for my usecase.

Instead, would it be ok to introduce a fast boot clock derived from fast 
monotonic clock to address the NMI live lock issues you mentioned? Below 
is an untested patch just to show the idea. Let me know your suggestions 
and Thanks,

Joel
----------8<--------------
 From 78c4f89e6f39cdd32e91883f2d2a80c7d97e34cf Mon Sep 17 00:00:00 2001
From: Joel Fernandes <joelaf@google.com>
Date: Sun, 20 Nov 2016 18:58:28 -0800
Subject: [RFC] timekeeping: Add a fast boot clock derived from fast
  monotonic clock

Signed-off-by: Joel Fernandes <joelaf@google.com>
---
  kernel/time/timekeeping.c | 36 ++++++++++++++++++++++++++++++------
  1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 37dec7e..41afa1e 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -55,6 +55,12 @@ static struct timekeeper shadow_timekeeper;
   */
  struct tk_fast {
  	seqcount_t		seq;
+
+	/*
+	 * first dimension is based on lower seq bit,
+	 * second dimension is for offset type (real, boot, tai)
+	 */
+	ktime_t			offsets[2][3];
  	struct tk_read_base	base[2];
  };

@@ -350,14 +356,20 @@ static void update_fast_timekeeper(struct 
tk_read_base *tkr, struct tk_fast *tkf
  	/* Force readers off to base[1] */
  	raw_write_seqcount_latch(&tkf->seq);

-	/* Update base[0] */
+	/* Update base[0] and offsets*/
  	memcpy(base, tkr, sizeof(*base));
+	tkf->offsets[0][TK_OFFS_REAL] = tk_core.timekeeper.offs_real;
+	tkf->offsets[0][TK_OFFS_BOOT] = tk_core.timekeeper.offs_boot;
+	tkf->offsets[0][TK_OFFS_TAI] = tk_core.timekeeper.offs_tai;

  	/* Force readers back to base[0] */
  	raw_write_seqcount_latch(&tkf->seq);

-	/* Update base[1] */
+	/* Update base[1] and offsets*/
  	memcpy(base + 1, base, sizeof(*base));
+	tkf->offsets[1][TK_OFFS_REAL] = tk_core.timekeeper.offs_real;
+	tkf->offsets[1][TK_OFFS_BOOT] = tk_core.timekeeper.offs_boot;
+	tkf->offsets[1][TK_OFFS_TAI] = tk_core.timekeeper.offs_tai;
  }

  /**
@@ -392,16 +404,23 @@ static void update_fast_timekeeper(struct 
tk_read_base *tkr, struct tk_fast *tkf
   * of the following timestamps. Callers need to be aware of that and
   * deal with it.
   */
-static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
+static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf, int 
offset)
  {
  	struct tk_read_base *tkr;
  	unsigned int seq;
  	u64 now;
+	ktime_t *off;

  	do {
  		seq = raw_read_seqcount_latch(&tkf->seq);
  		tkr = tkf->base + (seq & 0x01);
-		now = ktime_to_ns(tkr->base);
+
+		if (offset >= 0) {
+			off = tkf->offsets[seq & 0x01];
+			now = ktime_to_ns(ktime_add(tkr->base, off[offset]));
+		} else {
+			now = ktime_to_ns(tkr->base);
+		}

  		now += timekeeping_delta_to_ns(tkr,
  				clocksource_delta(
@@ -415,16 +434,21 @@ static __always_inline u64 
__ktime_get_fast_ns(struct tk_fast *tkf)

  u64 ktime_get_mono_fast_ns(void)
  {
-	return __ktime_get_fast_ns(&tk_fast_mono);
+	return __ktime_get_fast_ns(&tk_fast_mono, -1);
  }
  EXPORT_SYMBOL_GPL(ktime_get_mono_fast_ns);

  u64 ktime_get_raw_fast_ns(void)
  {
-	return __ktime_get_fast_ns(&tk_fast_raw);
+	return __ktime_get_fast_ns(&tk_fast_raw, -1);
  }
  EXPORT_SYMBOL_GPL(ktime_get_raw_fast_ns);

+u64 ktime_get_boot_fast_ns(void)
+{
+	return __ktime_get_fast_ns(&tk_fast_mono, TK_OFFS_BOOT);
+}
+
  /* Suspend-time cycles value for halted fast timekeeper. */
  static cycle_t cycles_at_suspend;

-- 
2.8.0.rc3.226.g39d4020