On Thu, Apr 26, 2012 at 3:48 PM, Shashidhara Shamaiah
[off-list ref] wrote:

Hi Arun,

My question is how will the scheduler run, if interrupts are disabled on the local CPU. As timer interrupt controls the CPU?

scheduler is nothing but a piece of code( __schedule()) which is just
like any other function.

If interrupts are disabled, you wont be getting the timer interrupts.
So time slice will never get elapsed, which in turn allows current
process to run for ever until it calls schedule again or exits.

But as we figured out that __scheduler is enabling the interrupts back
before we really schedules out. So this problem won't occur in our
Linux kernel.

Thanks,
Arun

Regards
Shashidhara

-----Original Message-----
From: Arun KS [mailto:getarunks at gmail.com]
Sent: Thursday, April 26, 2012 11:36 AM
To: Shashidhara Shamaiah
Subject: Re: side effects of calling interruptible_sleep_on_timeout()

Hi,

My code is,

/* disable interrupts and preemption */
spin_lock_irqsave(&lock, flags);
/* enable preemption, but interrupt still disabled */
spin_unlock(&lock);
/* Now schedule something else */
schedule_timeout(10 * HZ);

If you see the second step spin_unlock(&lock). This will enable the
preemtion back.

Arun

On Thu, Apr 26, 2012 at 11:26 AM, Shashidhara Shamaiah
[off-list ref] wrote:

quoted

Hi Arun,

I think if you run the code on a SMP machine, then the BUG() in schedule() will be triggered as spinlock_irqsave() disables preemption for a SMP.

Regards
Shashidhara

-----Original Message-----
From: Arun KS [mailto:getarunks at gmail.com]
Sent: Thursday, April 26, 2012 10:41 AM
To: Shashidhara Shamaiah
Subject: Re: side effects of calling interruptible_sleep_on_timeout()

Hello Shashidhara,

My machine is UP.

thanks,
Arun

On Thu, Apr 26, 2012 at 10:23 AM, Shashidhara Shamaiah
[off-list ref] wrote:

quoted

Hi Arun,

One more question regarding your experiment. Were you running your code on a UP or SMP machine?

Regards
Shashidhara

-----Original Message-----
From: kernelnewbies-bounces at kernelnewbies.org [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of Arun KS
Sent: Thursday, April 26, 2012 10:03 AM
To: Srivatsa S. Bhat
Cc: vinayak menon; Philipp Ittershagen; devendra rawat; kernelnewbies at kernelnewbies.org
Subject: Re: side effects of calling interruptible_sleep_on_timeout()

Hi Srivatsa,

On Wed, Apr 25, 2012 at 3:56 PM, Srivatsa S. Bhat
[off-list ref] wrote:

quoted

On 04/25/2012 03:36 AM, Philipp Ittershagen wrote:

quoted

Hi Devendra,

On Tue, Apr 24, 2012 at 03:24:23PM +0530, devendra rawat wrote:

quoted

? ?Hi,
? ?A switch driver is causing soft lockup on Montavista Linux Kernel
? ?2.6.10 system.
? ?While browsing through the code of the driver. I came across a snippet
? ?where after disabling the interrupts
? ?a call is made to interruptible_sleep_on_timeout().
? ?The code snippet is like
? ?cli();
? ?init_waitqueue_head(&queue);
? ? ? ? ? ?interruptible_sleep_on_timeout(&queue, USEC_TO_JIFFIES(usec));
? ? ? ? ? ?thread_check_signals();
? ?sti();
? ?I need to know the side effect of this sort of code, can it be
? ?responsible for the softlockup of the system ? Its a PowerPC based
? ?system.

you cannot call sleep functions after disabling interrupts, because no
interrupt will arrive for the scheduler to see the timeout and resume your
task.

Yes, that's right. Also, in general, sleeping inside atomic sections (eg.,
sections with interrupts disabled or preempt disabled) is wrong. There is a
config option in the kernel that you can use to enable
sleep-inside-atomic-section-checking (CONFIG_DEBUG_ATOMIC_SLEEP I believe),
which can help you pin-point such bugs easily.

I tired an experiment to check this.

/* disable interrupts and preemption */
spin_lock_irqsave(&lock, flags);
/* enable preemption, but interrupt still disabled */
spin_unlock(&lock);
/* Now schedule something else */
schedule_timeout(10 * HZ);

But this is not causing any harm. I m able to call schedule with
interrupt disabled and system works fine afterwards.

So when I looked inside the schedule() function, it checks only
whether preemption is disabled or not. schedule calls ?BUG() only if
preemption is disabled and not if interrupts are disabled.

And AFAIK there is no fuction inside the kernel which tells you that
interrupt are disabled.

So explantion why system works fine after calling a schedule with
interrupt disabled go here,

There is a raw_spin_lock_irq(&rq->lock) inside the __schedule() which
in turn calls local_irq_disable().

local_irq_disable/enable() functions are not nested. We dont have
reference counting.
One call to local_irq_enable is enough to enable multiple calls of
local_irq_disable().

So my inference is that if you call a schedule with interrupt disable
will not cause any problem. Because schedule function enable it back
before we really schedules out.
But call to schedule() with preemtion disabled will end up in famous
BUG scheduling while atomic.

NB: Kernel version used is 3.0.15

Thanks,
Arun

quoted

Regards,
Srivatsa S. Bhat


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