RE: IRQ Tracing Problem in Linux 2.6.28 Kernel

["Sol Kavy" <skavy@xxxxxxxxxx>]

I have discovered why other architectures do not have the same problem.   The back trace indicates a real defect (i.e. deadlock) in the generic code.  

Most architectures override sched_clock() with their own version.  Kernel/sched_clock.c:38 is a weak alias that will be overridden if an arch directory supplies its own.

Most of the arch directories provide an implementation that directly access the jiffies_64 variable "without" acquiring the xtime_lock.  

Some of the implementations provide a "poor" implementation in that the value of the jiffies_64 during a 32 rollover is not taken into account.  If sched_clock() is to be called while holding xtime_lock, the core implementation should be modified not to call get_jiffies_64() (which requires the xlock) but to use something like the following:

unsigned long long sched_clock(void)
{
	unsigned long long my_jiffies;
	unsigned long jiffies_top;
	unsigned long jiffies_bottom;

	do {
		jiffies_top = jiffies_64 >> 32;
		jiffies_bottom = jiffies_64 & 0xffffffff;
	} while(unlikely(jiffies_top != (unsigned long)(jiffies_64 >> 32))); 

	my_jiffies = ((unsigned long long)jiffies_top << 32) | (jiffies_bottom);
	return (my_jiffies - INITIAL_JIFFIES) * (NSEC_PER_SEC / HZ);
}

-----Original Message-----
From: linux-newbie-owner@xxxxxxxxxxxxxxx [mailto:linux-newbie-owner@xxxxxxxxxxxxxxx] On Behalf Of Sol Kavy
Sent: Monday, April 13, 2009 3:01 PM
To: linux-newbie@xxxxxxxxxxxxxxx
Subject: IRQ Tracing Problem in Linux 2.6.28 Kernel

The following back trace represents a deadlock in Ubicom's SMP port of 2.6.28 kernel.   I am sure that we are doing something unexpected.  I would appreciate the community's help in understanding what is going wrong.

Thanks in advance for any pointers,

Sol Kavy

Problem:
Ubicom's initial port does not use GENERIC_CLOCKEVENTS.  Instead it uses a periodic timer based on HZ.  The periodic timer calls do_timer() on each tick.

>From the arch directory perspective, we are required to hold the xtime_lock before calling do_timer().  The lock is indeed help by cpu 3 as evidenced in the output below.

The call to get_jiffies_64() at the top of the backtrace is attempting to read the jiffies in a reliable fashion.  The caller is required to wait for the xtime_lock not to be held.  Clearly, since we are in  a path that is holding the xtime_lock, this will never make forward progress.

What is unclear to me is why other ports are not seeing the same problem?  

Perhaps it is because most ports now set GENERIC_CLOCKEVENTS which uses an entirely different mechanism for doing things.  I am in the middle of switching the port to use GENERIC_CLOCKEVENTS but would like to understand this failure in more detail.

Any feedback is greatly appreciated,

Sol

Config Flags:
# CONFIG_GENERIC_CLOCKEVENTS is not set
CONFIG_GENERIC_HARDIRQS=y
CONFIG_IRQ_PER_CPU=y
CONFIG_TRACE_IRQFLAGS_SUPPORT=y
# CONFIG_DEBUG_SHIRQ is not set
CONFIG_TRACE_IRQFLAGS=y
CONFIG_IRQSOFF_TRACER=y

State of the lock:
(gdb) p xtime_lock
$5 = {sequence = 47089, lock = {raw_lock = {lock = 1}, magic = 3735899821, 
    owner_cpu = 3, owner = 0x42b01160}}

This is a backtrace from CPU 3:

 (gdb) bt
#0  get_jiffies_64 () at include/linux/seqlock.h:94
#1  0x4044f558 in sched_clock () at kernel/sched_clock.c:40
#2  0x4045f108 in ring_buffer_time_stamp (cpu=3) at kernel/trace/ring_buffer.c:58
#3  0x40464c50 in ftrace_now (cpu=3) at kernel/trace/trace.c:77
#4  0x404656ec in trace_hardirqs_off () at kernel/trace/trace_irqsoff.c:207
#5  0x40413020 in _spin_lock_irqsave (lock=0x3) at kernel/spinlock.c:82
#6  0x40451b2c in clocksource_get_next () at kernel/time/clocksource.c:254
#7  0x3ffd08ac in update_wall_time () at kernel/time/timekeeping.c:182
#8  0x4043bcd8 in do_timer (ticks=0) at kernel/timer.c:1125
#9  0x404169f8 in timer_tick (irq=<value optimized out>, 
    dummy=<value optimized out>) at arch/ubicom32/kernel/time.c:126
#10 0x3ffcefb0 in handle_IRQ_event (irq=<value optimized out>, 
    action=0x406fe6f4) at kernel/irq/handle.c:142
#11 0x3ffcee20 in __do_IRQ (irq=<value optimized out>)
    at kernel/irq/handle.c:239
#12 0x3ffcfcc8 in do_IRQ (irq=47089, regs=<value optimized out>)
    at arch/ubicom32/kernel/irq.c:250
#13 0x4041018c in sys_call_table ()
#14 0x4044f558 in sched_clock () at kernel/sched_clock.c:40
#15 0x00000008 in ?? ()




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