[PATCH] Run timers as softirqs, not tasklets

[PATCH] Run timers as softirqs, not tasklets

[Matthew Wilcox]

Seems to me that the timer code is attempting to replicate the softirq
characteristics at the tasklet level, which is a little pointless.  This
patch converts timers to be a first-class softirq citizen.

Ingo, was there a reason you didn't do it this way to begin with?

diff -u linux-2.5.47-pci/include/linux/interrupt.h linux-2.5.47-pci/include/linux/interrupt.h
--- linux-2.5.47-pci/include/linux/interrupt.h	2002-11-16 22:28:40.000000000 -0500
+++ linux-2.5.47-pci/include/linux/interrupt.h	2002-11-17 11:23:25.000000000 -0500
@@ -45,6 +45,7 @@
 enum
 {
 	HI_SOFTIRQ=0,
+	TIMER_SOFTIRQ,
 	NET_TX_SOFTIRQ,
 	NET_RX_SOFTIRQ,
 	SCSI_SOFTIRQ,
--- linux-2.5.47/kernel/timer.c	2002-11-14 10:52:17.000000000 -0500
+++ linux-2.5.47-pci/kernel/timer.c	2002-11-17 11:27:30.000000000 -0500
@@ -66,9 +66,6 @@ typedef struct tvec_t_base_s tvec_base_t
 /* Fake initialization */
 static DEFINE_PER_CPU(tvec_base_t, tvec_bases) = { SPIN_LOCK_UNLOCKED };
 
-/* Fake initialization needed to avoid compiler breakage */
-static DEFINE_PER_CPU(struct tasklet_struct, timer_tasklet) = { NULL };
-
 static void check_timer_failed(timer_t *timer)
 {
 	static int whine_count;
@@ -766,9 +763,9 @@ rwlock_t xtime_lock __cacheline_aligned_
 unsigned long last_time_offset;
 
 /*
- * This function runs timers and the timer-tq in softirq context.
+ * This function runs timers and the timer-tq in bottom half context.
  */
-static void run_timer_tasklet(unsigned long data)
+static void run_timer_softirq(struct softirq_action *h)
 {
 	tvec_base_t *base = &per_cpu(tvec_bases, smp_processor_id());
 
@@ -781,7 +778,7 @@ static void run_timer_tasklet(unsigned l
  */
 void run_local_timers(void)
 {
-	tasklet_hi_schedule(&per_cpu(timer_tasklet, smp_processor_id()));
+	raise_softirq(TIMER_SOFTIRQ);
 }
 
 /*
@@ -1140,7 +1137,6 @@ static void __devinit init_timers_cpu(in
 	}
 	for (j = 0; j < TVR_SIZE; j++)
 		INIT_LIST_HEAD(base->tv1.vec + j);
-	tasklet_init(&per_cpu(timer_tasklet, cpu), run_timer_tasklet, 0UL);
 }
 	
 static int __devinit timer_cpu_notify(struct notifier_block *self, 
@@ -1167,4 +1163,5 @@ void __init init_timers(void)
 	timer_cpu_notify(&timers_nb, (unsigned long)CPU_UP_PREPARE,
 				(void *)(long)smp_processor_id());
 	register_cpu_notifier(&timers_nb);
+	open_softirq(TIMER_SOFTIRQ, run_timer_softirq, NULL);
 }

-- 
Revolutions do not require corporate support.

Re: [PATCH] Run timers as softirqs, not tasklets

[Dipankar Sarma]

On Sun, Nov 17, 2002 at 06:20:11PM +0100, Matthew Wilcox wrote:
> 
> Seems to me that the timer code is attempting to replicate the softirq
> characteristics at the tasklet level, which is a little pointless.  This
> patch converts timers to be a first-class softirq citizen.
> 
> Ingo, was there a reason you didn't do it this way to begin with?
> 

I wrote that part of smptimers to run the per-CPU lists from per-CPU
tasklets while porting Ingo's code to 2.5 and Ingo just included it.
At that time, it didn't seem necessary to use up a softirq vector 
when it could be easily done using tasklets.

However it should work fine with softirqs too.

Thanks
Dipankar

Re: [PATCH] Run timers as softirqs, not tasklets

[Ingo Molnar]

On Sun, 17 Nov 2002, Matthew Wilcox wrote:

> Seems to me that the timer code is attempting to replicate the softirq
> characteristics at the tasklet level, which is a little pointless.  
> This patch converts timers to be a first-class softirq citizen.

i agree with your patch.

> Ingo, was there a reason you didn't do it this way to begin with?

because there was an interim state of the timer code in where we still had
a global timer context (ie. a timer tasklet). Only later did it get
converted to completely unsynchronized per-CPU tasklets. Which indeed is
what softirqs are :-)

	Ingo

Re: [PATCH] Run timers as softirqs, not tasklets

[Ingo Molnar]

On Mon, 18 Nov 2002, Dipankar Sarma wrote:

> I wrote that part of smptimers to run the per-CPU lists from per-CPU
> tasklets while porting Ingo's code to 2.5 and Ingo just included it. At
> that time, it didn't seem necessary to use up a softirq vector when it
> could be easily done using tasklets.

i think a separate timer softirq is justified, timers are important
enough.

	Ingo

Re: [PATCH] Run timers as softirqs, not tasklets

[george anzinger]

Ingo Molnar wrote:
> 
> On Mon, 18 Nov 2002, Dipankar Sarma wrote:
> 
> > I wrote that part of smptimers to run the per-CPU lists from per-CPU
> > tasklets while porting Ingo's code to 2.5 and Ingo just included it. At
> > that time, it didn't seem necessary to use up a softirq vector when it
> > could be easily done using tasklets.
> 
> i think a separate timer softirq is justified, timers are important
> enough.
> 
>         Ingo
> 
So then, is there any reason to not put them ahead of
HI_SOFTIRQ?  I.e.:

 enum
 {
        TIMER_SOFTIRQ=0,
      	HI_SOFTIRQ
        NET_TX_SOFTIRQ,
        NET_RX_SOFTIRQ,
        SCSI_SOFTIRQ,
-- 
George Anzinger   george@mvista.com
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml

Re: [PATCH] Run timers as softirqs, not tasklets

[Matthew Wilcox]

On Mon, Nov 18, 2002 at 02:29:10PM -0800, george anzinger wrote:
> So then, is there any reason to not put them ahead of
> HI_SOFTIRQ?  I.e.:
> 
>  enum
>  {
>         TIMER_SOFTIRQ=0,
>  	HI_SOFTIRQ
>         NET_TX_SOFTIRQ,
>         NET_RX_SOFTIRQ,
>         SCSI_SOFTIRQ,

because then there would be no way to make a tasklet run before the timers?
turn it around.  convince us that timers should run first.

-- 
Revolutions do not require corporate support.