linux-kernel - Re: Warning on boot on SAMA5D2 with Linux 4.11-rc1

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Date:   Tue, 7 Mar 2017 16:17:03 +0100
From:   David Engraf <david.engraf@...go.com>
To:     Romain Izard <romain.izard.pro@...il.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Warning on boot on SAMA5D2 with Linux 4.11-rc1

Am 07.03.2017 um 16:05 schrieb Romain Izard:
> 2017-03-06 12:28 GMT+01:00 Romain Izard <romain.izard.pro@...il.com>:
>>
>> While looking for another issue, I tried Linux 4.11-rc1 on a SAMA5D2 Xplained
>> board. The boot log contains the following warning:
>>
>> [    0.100000] ------------[ cut here ]------------
>> [    0.100000] WARNING: CPU: 0 PID: 1 at
>> ../kernel/time/sched_clock.c:180 sched_clock_register+0x44/0x1e4
>> [    0.100000] CPU: 0 PID: 1 Comm: swapper Not tainted 4.11.0-rc1+ #3
>> [    0.100000] Hardware name: Atmel SAMA5
>> [    0.100000] [<c010c494>] (unwind_backtrace) from [<c010a558>]
>> (show_stack+0x10/0x14)
>> [    0.100000] [<c010a558>] (show_stack) from [<c0115654>] (__warn+0xe0/0xf8)
>> [    0.100000] [<c0115654>] (__warn) from [<c011571c>]
>> (warn_slowpath_null+0x20/0x28)
>> [    0.100000] [<c011571c>] (warn_slowpath_null) from [<c090b0d0>]
>> (sched_clock_register+0x44/0x1e4)
>> [    0.100000] [<c090b0d0>] (sched_clock_register) from [<c091fb98>]
>> (tcb_clksrc_init+0x1ac/0x360)
>> [    0.100000] [<c091fb98>] (tcb_clksrc_init) from [<c0900d8c>]
>> (do_one_initcall+0xb4/0x15c)
>> [    0.100000] [<c0900d8c>] (do_one_initcall) from [<c0900f68>]
>> (kernel_init_freeable+0x134/0x1c4)
>> [    0.100000] [<c0900f68>] (kernel_init_freeable) from [<c06bfc64>]
>> (kernel_init+0x8/0x10c)
>> [    0.100000] [<c06bfc64>] (kernel_init) from [<c0107318>]
>> (ret_from_fork+0x14/0x3c)
>> [    0.100000] ---[ end trace 7ce9be9d7cf6f800 ]---
>> [    0.100012] sched_clock: 32 bits at 10MHz, resolution 96ns, wraps
>> every 206986376143ns
>>
>> This is related to the following commit:
>> 7b9f1d16e6d1 clocksource/drivers/tcb_clksrc: Use 32 bit tcb as sched_clock
>>
>> When we call sched_clock_register from tcb_clksrc_init from
>> arch_initcall, we are too late as sched expects all the candidates for
>> its clock to be registered before interrupts are enabled. This warning
>> does not prevent the tcb clock from being used.

I have no idea why sched_clock_register complains when interrupts are 
already enabled. Form the code it doesn't look like this is a real issue 
and it works for me.

> After some more use with 4.11-rc1, I also noticed that the timestamp for
> printk rolls over to 0 after only 413s. Reverting the aforementioned commit
> fixes it.

I had this issue as well so I proposed the following patch a few weeks ago.

-------- Forwarded Message --------
Betreff: [PATCH resend] timers, sched_clock: Update timeout for clock wrap
Datum: Thu,  2 Mar 2017 10:02:16 +0100
Von: David Engraf <david.engraf@...go.com>
An: tglx@...utronix.de, john.stultz@...aro.org
Kopie (CC): linux-kernel@...r.kernel.org, David Engraf 
<david.engraf@...go.com>

The scheduler clock framework may not use the correct timeout for the clock
wrap. This happens when a new clock driver calls sched_clock_register()
after the kernel called sched_clock_postinit(). In this case the clock wrap
timeout is too long thus sched_clock_poll() is called too late and the clock
already wrapped.

On my ARM system the scheduler was no longer scheduling any other task than
the idle task because the sched_clock() wrapped.

Signed-off-by: David Engraf <david.engraf@...go.com>
---
  kernel/time/sched_clock.c | 5 +++++
  1 file changed, 5 insertions(+)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index a26036d..382b159 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -205,6 +205,11 @@ sched_clock_register(u64 (*read)(void), int bits, 
unsigned long rate)
   	update_clock_read_data(&rd);
  +	if (sched_clock_timer.function != NULL) {
+		/* update timeout for clock wrap */
+		hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
+	}
+
  	r = rate;
  	if (r >= 4000000) {
  		r /= 1000000;
-- 
2.9.3