Messages in this thread | | | Date | Mon, 28 Feb 2022 21:00:26 +0100 | Subject | Re: [PATCH] tracing/osnoise: Force quiescent states while tracing | From | Daniel Bristot de Oliveira <> |
| |
On 2/28/22 15:14, Nicolas Saenz Julienne wrote: > At the moment running osnoise on an isolated CPU and a PREEMPT_RCU > kernel might have the side effect of extending grace periods too much. > This will eventually entice RCU to schedule a task on the isolated CPU > to end the overly extended grace period, adding unwarranted noise to the > CPU being traced in the process. > > So, check if we're the only ones running on this isolated CPU and that > we're on a PREEMPT_RCU setup. If so, let's force quiescent states in > between measurements. > > Non-PREEMPT_RCU setups don't need to worry about this as osnoise main > loop's cond_resched() will go though a quiescent state for them. > > Note that this same exact problem is what extended quiescent states were > created for. But adapting them to this specific use-case isn't trivial > as it'll imply reworking entry/exit and dynticks/context tracking code. > > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com> > --- > kernel/trace/trace_osnoise.c | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c > index 870a08da5b48..4928358f6e88 100644 > --- a/kernel/trace/trace_osnoise.c > +++ b/kernel/trace/trace_osnoise.c > @@ -21,7 +21,9 @@ > #include <linux/uaccess.h> > #include <linux/cpumask.h> > #include <linux/delay.h> > +#include <linux/tick.h> > #include <linux/sched/clock.h> > +#include <linux/sched/isolation.h> > #include <uapi/linux/sched/types.h> > #include <linux/sched.h> > #include "trace.h" > @@ -1295,6 +1297,7 @@ static int run_osnoise(void) > struct osnoise_sample s; > unsigned int threshold; > u64 runtime, stop_in; > + unsigned long flags; > u64 sum_noise = 0; > int hw_count = 0; > int ret = -1; > @@ -1386,6 +1389,22 @@ static int run_osnoise(void) > osnoise_stop_tracing(); > } > > + /* > + * Check if we're the only ones running on this nohz_full CPU > + * and that we're on a PREEMPT_RCU setup. If so, let's fake a > + * QS since there is no way for RCU to know we're not making > + * use of it. > + * > + * Otherwise it'll be done through cond_resched(). > + */ > + if (IS_ENABLED(CONFIG_PREEMPT_RCU) && > + !housekeeping_cpu(raw_smp_processor_id(), HK_FLAG_MISC) &&
Does this restrict to only isolcpus cpus? what if this CPU was isolated via other methods?
> + tick_nohz_tick_stopped()) { > + local_irq_save(flags);
This code is always with interrupts enabled, so local_irq_disable()/enable() should be enough (and faster).
> + rcu_momentary_dyntick_idle(); > + local_irq_restore(flags); > + }
Question, if we set this once, we could avoid setting it on every loop unless we have a preemption from another thread, right?
> /* > * For the non-preemptive kernel config: let threads runs, if > * they so wish.
/me keeps playing with this patch.
-- Daniel
| |