Messages in this thread | | | Date | Tue, 1 Dec 2015 21:41:09 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/7] nohz: New tick dependency mask |
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On Fri, Nov 13, 2015 at 03:22:04PM +0100, Frederic Weisbecker wrote: > The tick dependency is evaluated on every IRQ. This is a batch of checks > which determine whether it is safe to stop the tick or not. These checks > are often split in many details: posix cpu timers, scheduler, sched clock, > perf events. Each of which are made of smaller details: posix cpu > timer involves checking process wide timers then thread wide timers. Perf > involves checking freq events then more per cpu details. > > Checking these details asynchronously every time we update the full > dynticks state bring avoidable overhead and a messy layout. > > Lets introduce instead tick dependency masks: one for system wide > dependency (unstable sched clock), one for CPU wide dependency (sched, > perf), and task/signal level dependencies. The subsystems are responsible > of setting and clearing their dependency through a set of APIs that will > take care of concurrent dependency mask modifications and kick targets > to restart the relevant CPU tick whenever needed.
Maybe better explain why we need the per task and per signal thingy?
> +static void trace_tick_dependency(unsigned long dep) > +{ > + if (dep & TICK_POSIX_TIMER_MASK) { > + trace_tick_stop(0, "posix timers running\n"); > + return; > + } > + > + if (dep & TICK_PERF_EVENTS_MASK) { > + trace_tick_stop(0, "perf events running\n"); > + return; > + } > + > + if (dep & TICK_SCHED_MASK) { > + trace_tick_stop(0, "more than 1 task in runqueue\n"); > + return; > + } > + > + if (dep & TICK_CLOCK_UNSTABLE_MASK) > + trace_tick_stop(0, "unstable sched clock\n"); > +}
I would suggest ditching the strings and using the
> +static void kick_all_work_fn(struct work_struct *work) > +{ > + tick_nohz_full_kick_all(); > +} > +static DECLARE_WORK(kick_all_work, kick_all_work_fn); > + > +void __tick_nohz_set_dep_delayed(enum tick_dependency_bit bit, unsigned long *dep) > +{ > + unsigned long prev; > + > + prev = fetch_or(dep, BIT_MASK(bit)); > + if (!prev) { > + /* > + * We need the IPIs to be sent from sane process context.
Why ?
> + * The posix cpu timers are always set with irqs disabled. > + */ > + schedule_work(&kick_all_work); > + } > +} > + > +/* > + * Set a global tick dependency. Lets do the wide IPI kick asynchronously > + * for callers with irqs disabled.
This seems to suggest you can call this with IRQs disabled
> + */ > +void tick_nohz_set_dep(enum tick_dependency_bit bit) > +{ > + unsigned long prev; > + > + prev = fetch_or(&tick_dependency, BIT_MASK(bit)); > + if (!prev) > + tick_nohz_full_kick_all();
But that function seems implemented using smp_call_function_many() which cannot be called with IRQs disabled.
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