Re: [PATCH] suppress needless timer reprogramming {tick-sched.c} - test/comment

[Tony Lindgren <tony@atomide.com>]

* Koen Kooi <k.kooi@student.utwente.nl> [080805 15:11]:
>
> Op 19 jun 2008, om 14:54 heeft Woodruff, Richard het volgende  
> geschreven:
>
>> Hi,
>>
>> I'll resend this to the omap list.  I had originally sent it with some 
>> pictures but I guess they were too big for the lists.  Individual 
>> should have got them.
>>
>> If anyone has the time this is a good to test with.  If you have  
>> dynamic tick enabled it can double performance of high rate interrupt 
>> things like MUSB.
>
> This patch has been working for me quite well these last few week, any  
> chance it can to into git?

This needs to go in via LKML, I think Thomas Gleixner is looking into it.

Tony


>
> regards,
>
> Koen
>
>
>> If anyone wants I'll re-send larger trace pictures individually.
>>
>>
>>> From: Woodruff, Richard, Monday, June 16, 2008 7:06 PM
>>>
>>> It simply does a check to see if the about to be reprogrammed 1 shot
>>> already is the last event programmed in the hardware.  If it is it  
>>> skips
>>> calling the hardware.
>>>
>>> In my device I get many interrupts from a high speed USB device in a
>>> very short period of time.  The system spends a lot of time
>>> reprogramming the hardware timer which is in a slower timing domain  
>>> as
>>> compared to the CPU.  This results in the CPU spending a huge amount 
>>> of
>>> time waiting for the timer posting to be done.  All of this
>>> reprogramming is useless as the wake up time has not changed.
>>>
>>> As measured using ETM trace this drops my reprogramming penalty from
>>> almost 60% CPU load down to 15% during high interrupt rate.  If you  
>>> like
>>> I can send traces to show this.
>>
>> Attached are some results on OMAP-ARM from USB-OTG:
>>
>> As host:
>>
>> root@OMAP3EVM /]# mount -t vfat /dev/sda1 /mnt/
>> [root@OMAP3EVM /]# time -p cp /mnt/OE.mtn.bz2 /dev/null
>> real 32.51
>> user 0.02
>> sys 1.05
>> [root@OMAP3EVM /]# umount /mnt/
>> [root@OMAP3EVM /]# mount -t vfat /dev/sda1 /mnt/
>> [root@OMAP3EVM /]# time -p cp /mnt/OE.mtn.bz2 /dev/null
>> real 17.92
>> user 0.05
>> sys 1.57
>>
>> As Client:
>>
>> Attached are some visuals as of client doing gadget zero tests.   
>> Pictures clearly show before a domination by timer reprogram and after 
>> not.
>>
>> Regards,
>> Richard W.
>>
>>
>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> index b854a89..ff6b967 100644
>> --- a/kernel/time/tick-sched.c
>> +++ b/kernel/time/tick-sched.c
>> @@ -254,6 +254,17 @@ void tick_nohz_stop_sched_tick(void)
>>        /* Schedule the tick, if we are at least one jiffie off */
>>        if ((long)delta_jiffies >= 1) {
>>
>> +               /*
>> +               * calculate the expiry time for the next timer wheel
>> +               * timer
>> +               */
>> +               expires = ktime_add_ns(last_update, tick_period.tv64 *
>> +                                  delta_jiffies);
>> +
>> +               /* Skip reprogram of event if its not changed */
>> +               if(ts->tick_stopped && ktime_equal(expires, dev- 
>> >next_event))
>> +                       goto out2;
>> +
>>                if (delta_jiffies > 1)
>>                        cpu_set(cpu, nohz_cpu_mask);
>>                /*
>> @@ -304,12 +315,7 @@ void tick_nohz_stop_sched_tick(void)
>>                        goto out;
>>                }
>>
>> -               /*
>> -                * calculate the expiry time for the next timer wheel
>> -                * timer
>> -                */
>> -               expires = ktime_add_ns(last_update, tick_period.tv64 *
>> -                                      delta_jiffies);
>> +               /* Mark expiries */
>>                ts->idle_expires = expires;
>>
>>                if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
>> @@ -328,6 +334,7 @@ void tick_nohz_stop_sched_tick(void)
>>                tick_do_update_jiffies64(ktime_get());
>>                cpu_clear(cpu, nohz_cpu_mask);
>>        }
>> +out2:
>>        raise_softirq_irqoff(TIMER_SOFTIRQ);
>> out:
>>        ts->next_jiffies = next_jiffies;
>> --
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>