Paul, what if your tb wraps during the test?
> -----Original Message----- > From: Linuxppc-dev [mailto:linuxppc-dev- > bounces+heinz.wrobel=freescale....@lists.ozlabs.org] On Behalf Of Paul > Clarke > Sent: Tuesday, October 07, 2014 21:13 > To: linuxppc-dev@lists.ozlabs.org > Subject: [PATCH] powerpc: mitigate impact of decrementer reset > > 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. > > Signed-off-by: Paul A. Clarke <p...@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) { What if "now" and *next_tb are not on the same wrap count? They are both modulo values AFACS. Shouldn't this be right here more like a "if ((*next_tb - now) < 2^63)" style test to check for deltas within the range instead of absolute values? > + 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) > -- > 2.1.2.330.g565301e BR, Heinz _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev