Subject: [PATCH v4 03/17] sched_clock: Use an hrtimer instead of timer

From: [email-masked] (John Stultz)
Date: Mon, 22 Jul 2013 11:21:19 -0700

On 07/18/2013 04:21 PM, Stephen Boyd wrote:
> In the next patch we're going to increase the number of bits that
> the generic sched_clock can handle to be greater than 32. With
> more than 32 bits the wraparound time can be larger than what can
> fit into the units that msecs_to_jiffies takes (unsigned int).
> Luckily, the wraparound is initially calculated in nanoseconds
> which we can easily use with hrtimers, so switch to using an
> hrtimer.
>
> Cc: Russell King <[email-masked]>
> Signed-off-by: Stephen Boyd <[email-masked]>

Hrmm. So in my testing (under qemu), this patch causes bootup to hang.

qemu-system-arm -kernel zImage-arm -M vexpress-a9 -cpu cortex-a9 
-nographic -m 1024  -append 'root=/dev/mmcblk0p2 rw mem=1024M 
raid=noautodetect console=ttyAMA0,38400n8 rootwait vmalloc=256MB 
devtmpfs.mount=0' -sd test-arm.img -redir tcp:4300::22

Config file attached.

I haven't gotten a chance to look very closely, but it seems the 
folowing patch resolves the issue. I'm not sure if we're seeing callers 
to setup_sched_clock happen after sched_clock_postinit or what, but it 
probably needs another look over.

thanks
-john

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index a269890b..c018ffc 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -138,12 +138,6 @@ void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
  	pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lluns\n",
  		bits, r, r_unit, res, wrap);
  
-	/*
-	 * Start the timer to keep sched_clock() properly updated and
-	 * sets the initial epoch.
-	 */
-	hrtimer_init(&sched_clock_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
-	sched_clock_timer.function = sched_clock_poll;
  	update_sched_clock();
  
  	/*
@@ -175,6 +169,13 @@ void __init sched_clock_postinit(void)
  		setup_sched_clock(jiffy_sched_clock_read, 32, HZ);
  
  	update_sched_clock();
+
+	/*
+	 * Start the timer to keep sched_clock() properly updated and
+	 * sets the initial epoch.
+	 */
+	hrtimer_init(&sched_clock_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	sched_clock_timer.function = sched_clock_poll;
  	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
  }

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