Messages in this thread Patch in this message | | | From | "Zhivich, Michael" <> | Subject | Re: [PATCH] softirq: don't push timer softirq handling to ksoftirqd | Date | Fri, 16 Nov 2018 18:46:36 +0000 |
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On 11/15/18, 12:17 PM, "John Stultz" <john.stultz@linaro.org> wrote:
On Thu, Nov 15, 2018 at 9:07 AM, Michael Zhivich <mzhivich@akamai.com> wrote: > Require TIMER_SOFTIRQ to be handled immediately instead of delaying until > ksoftirqd runs, thus preventing problems with reading clocksources that > wrap often (e.g. acpi_pm). > > If acpi_pm is used as the clocksource watchdog, and machine is under heavy > load, the time period for the watchdog check may be significantly longer > than the requested 0.5 seconds. If the watchdog check is delayed by 2 > seconds (observed behavior), then acpi_pm time delta will be > > 2.5 sec * 3579545 ticks/sec = 8948863 = 0x888c3f > > which will be treated as negative (since acpi_pm is only 24-bits wide) and > truncated to 0. This behavior will cause tsc to be incorrectly declared > unstable in clocksource_watchdog(), as it no longer agrees with acpi_pm. > If the clocksource watchdog check is delayed by more than 4.7 sec, then the > acpi_pm clocksource will wrap altogether and produce incorrect time delta. > > The likely cause of this delay is that timer interrupts are serviced in > ksoftirqd when the machine is very busy. > > Per Linus' comment in commit 3c53776e29f8 ("Mark HI and TASKLET softirq > synchronous"): > ... > We should probably also consider the timer softirqs to be synchronous > and not be delayed to ksoftirqd (since they were the issue with the > earlier watchdog problems), but that should be done as a separate patch. > ... > > Signed-off-by: Michael Zhivich <mzhivich@akamai.com> > --- > kernel/softirq.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/softirq.c b/kernel/softirq.c > index d28813306b2c..6d517ce0fba8 100644 > --- a/kernel/softirq.c > +++ b/kernel/softirq.c > @@ -82,7 +82,8 @@ static void wakeup_softirqd(void) > * right now. Let ksoftirqd handle this at its own rate, to get fairness, > * unless we're doing some of the synchronous softirqs. > */ > -#define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ)) > +#define SOFTIRQ_NOW_MASK \ > + ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ) | (1 << TIMER_SOFTIRQ)) > static bool ksoftirqd_running(unsigned long pending) > { > struct task_struct *tsk = __this_cpu_read(ksoftirqd); Thanks so much for sending this along! Sorry I didn't get back to your mail earlier this week, I've been at Plumbers.
So while this does try to attack the reliability issue w/ the clocksource watchdog being delayed, I worry this will have to many side-effects elsewhere.
Would a more focused fix be to move the clocksource watchdog from a normal timer to a hrtimer?
thanks -john
Hi John,
That's an interesting idea - it would get clocksource watchdog out of ksoftirqd. However, clocksource watchdog iterates over available CPUs to check the TSC on each core (see add_timer_on() call in clocksource_watchdog()). I'm not seeing an API to start an hrtimer on a specific CPU - is this possible and I'm missing it? Or would something like this have to be added to hrtimer?
Thanks, ~ Michael
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