linux-kernel - Re: [next-20120823] NOHZ: local_softirq

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Message-ID: <CA+icZUV+UMXGJTyfsUYBcbRVouVRv6pzvzUVUu7QEoAJKU0j7Q@mail.gmail.com>
Date:	Thu, 23 Aug 2012 20:55:09 +0200
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Paul McKenney <paul.mckenney@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	linux-next <linux-next@...r.kernel.org>
Subject: Re: [next-20120823] NOHZ: local_softirq_pending 200 on s/r

On Thu, Aug 23, 2012 at 7:14 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Thu, Aug 23, 2012 at 06:06:23PM +0200, Sedat Dilek wrote:
>> On Thu, Aug 23, 2012 at 5:43 PM, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>
> [ . . . ]
>
>> Applying this patch on top of my local GIT tree results in error-messages:
>>
>> [...]
>>   CC      kernel/rcupdate.o
>>   AS      arch/x86/platform/efi/efi_stub_64.o
>>   CC      arch/x86/kernel/irqinit.o
>>   LD      arch/x86/platform/efi/built-in.o
>> arch/x86/platform/efi/efi_64.o:(.rodata+0x0): multiple definition of
>> `softirq_stop_idle_mask'
>> arch/x86/platform/efi/efi.o:(.rodata+0x50): first defined here
>> make[5]: *** [arch/x86/platform/efi/built-in.o] Error 1
>> make[4]: *** [arch/x86/platform/efi] Error 2
>> make[3]: *** [arch/x86/platform] Error 2
>> make[3]: *** Waiting for unfinished jobs....
>> [...]
>>
>> $ LC_ALL=C grep softirq_stop_idle_mask -nr linux-next/arch/x86/platform/efi/
>> Binary file linux-next/arch/x86/platform/efi/efi.o matches
>> Binary file linux-next/arch/x86/platform/efi/efi_64.o matches
>
> Right...  Clearly I was spinning a bit too quickly on that one.  :-/
> So back to old tech: C-preprocessor macros.
>
> This one actually builds, boots, and passes mild rcutorture testing
> under KVM.
>

This fixes the issue with NOHZ - no messages in the logs.

There seems to remain an old PM issue (call-trace) I have seen before
this time-rcu-issue with apps needing to long to freeze.
So, this is another story.

I have added below my reported-by/tested-by.

- Sedat -

>                                                         Thanx, Paul
>
> -------------------------------------------------------------------------
>
> time: RCU permitted to stop idle entry via softirq
>
> The can_stop_idle_tick() function complains if a softirq vector is
> raised too late in the idle-entry process, presumably in order to
> prevent dangling softirq invocations from being delayed across the
> full idle period, which might be indefinitely long -- and if softirq
> was asserted any later than the call to this function, such a delay
> might well happen.
>
> However, RCU needs to be able to use softirq to stop idle entry in
> order to be able to drain RCU callbacks from the current CPU, which in
> turn enables faster entry into dyntick-idle mode, which in turn reduces
> power consumption.  Because RCU takes this action at a well-defined
> point in the idle-entry path, it is safe for RCU to take this approach.
>
> This commit therefore silences the error message that is sometimes
> produced when the going-idle CPU suddenly finds that it has an RCU_SOFTIRQ
> to process.  The error message will continue to be issued for other
> softirq vectors.
>

Reported-by: Sedat Dilek <sedat.dilek@...il.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>

> Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
>
> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> index c5f856a..5e4e617 100644
> --- a/include/linux/interrupt.h
> +++ b/include/linux/interrupt.h
> @@ -430,6 +430,8 @@ enum
>         NR_SOFTIRQS
>  };
>
> +#define SOFTIRQ_STOP_IDLE_MASK (~(1 << RCU_SOFTIRQ))
> +
>  /* map softirq index to softirq name. update 'softirq_to_name' in
>   * kernel/softirq.c when adding a new softirq.
>   */
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 024540f..4b1785a 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -436,7 +436,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
>         if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
>                 static int ratelimit;
>
> -               if (ratelimit < 10) {
> +               if (ratelimit < 10 &&
> +                   (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
>                         printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
>                                (unsigned int) local_softirq_pending());
>                         ratelimit++;
>

View attachment "dmesg_3.6.0-rc3-next20120823-2-iniza-generic_time-rcu-issue_for-paulmck.txt" of type "text/plain" (94193 bytes)