[PATCH] powerpc: mitigate impact of decrementer reset

[Benjamin Herrenschmidt]

On Tue, 2014-10-07 at 14:13 -0500, Paul Clarke wrote:
> The POWER ISA defines an always-running decrementer which can be used
> to schedule interrupts after a certain time interval has elapsed.
> The decrementer counts down at the same frequency as the Time Base,
> which is 512 MHz.  The maximum value of the decrementer is 0x7fffffff.
> This works out to a maximum interval of about 4.19 seconds.
> 
> If a larger interval is desired, the kernel will set the decrementer
> to its maximum value and reset it after it expires (underflows)
> a sufficient number of times until the desired interval has elapsed.
> 
> The negative effect of this is that an unwanted latency spike will
> impact normal processing at most every 4.19 seconds.  On an IBM
> POWER8-based system, this spike was measured at about 25-30
> microseconds, much of which was basic, opportunistic housekeeping
> tasks that could otherwise have waited.
> 
> This patch short-circuits the reset of the decrementer, exiting after
> the decrementer reset, but before the housekeeping tasks if the only
> need for the interrupt is simply to reset it.  After this patch,
> the latency spike was measured at about 150 nanoseconds.

Doesn't this break the irq_work stuff ? We trigger it with a set_dec(1);
and your patch will probably cause it to be skipped...

Cheers,
Ben.

> Signed-off-by: Paul A. Clarke <pc at us.ibm.com>
> ---
>   arch/powerpc/kernel/time.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 368ab37..962a06b 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -528,6 +528,7 @@ void timer_interrupt(struct pt_regs * regs)
>   {
>   	struct pt_regs *old_regs;
>   	u64 *next_tb = &__get_cpu_var(decrementers_next_tb);
> +	u64 now;
> 
>   	/* Ensure a positive value is written to the decrementer, or else
>   	 * some CPUs will continue to take decrementer exceptions.
> @@ -550,6 +551,18 @@ void timer_interrupt(struct pt_regs * regs)
>   	 */
>   	may_hard_irq_enable();
> 
> +	/* If this is simply the decrementer expiring (underflow) due to
> +	 * the limited size of the decrementer, and not a set timer,
> +	 * reset (if needed) and return
> +	 */
> +	now = get_tb_or_rtc();
> +	if (now < *next_tb) {
> +		now = *next_tb - now;
> +		if (now <= DECREMENTER_MAX)
> +			set_dec((int)now);
> +		__get_cpu_var(irq_stat).timer_irqs_others++;
> +		return;
> +	}
> 
>   #if defined(CONFIG_PPC32) && defined(CONFIG_PPC_PMAC)
>   	if (atomic_read(&ppc_n_lost_interrupts) != 0)