Messages in this thread | | | Subject | Re: [RFC PATCH v3 1/1] trace,smp: Add tracepoints around remotelly called functions | From | Leonardo Brás <> | Date | Mon, 15 May 2023 14:56:53 -0300 |
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On Thu, 2023-05-11 at 11:56 +0100, Valentin Schneider wrote: > On 10/05/23 20:01, Leonardo Bras wrote: > > When running RT workloads in isolated CPUs, many cases of deadline misses > > are caused by remote CPU requests such as smp_call_function*(). > > > > For those cases, having the names of those functions running around the > > deadline miss moment could help (a lot) finding a target for the next > > improvements. > > > > Add tracepoints for acquiring the function name & csd before entry and > > after returning from the remote-cpu requested function. > > > > Also, add tracepoints on the remote cpus requesting them. > > Per the v1 thread, I got some context as to why we want this, but this > changelog is quite sparse on the subject. How about: > > """ > The recently added ipi_send_{cpu,cpumask} tracepoints allow finding sources > of IPIs targeting CPUs running latency-sensitive applications. > > For NOHZ_FULL CPUs, all IPIs are interference, and those tracepoints are > sufficient to find them and work on getting rid of them. In some setups > however, not *all* IPIs are to be suppressed, but long-running IPI > callbacks can still be problematic. > > Add a pair of tracepoints to mark the start and end of processing a CSD IPI > callback, similar to what exists for softirq, workqueue or timer callbacks. > """ > > And you can probably split the csd_queue_cpu TP into a separate commit, > with something along the lines of: > > """ > Add a tracepoint for when a CSD is queued to a remote CPU's > call_single_queue. This allows finding exactly which CPU queued a given CSD > when looking at a csd_function_{entry,exit} event, and also enables us to > accurately measure IPI delivery time with e.g. a synthetic event: > > $ echo 'hist:keys=cpu,csd.hex:ts=common_timestamp.usecs' >\ > /sys/kernel/tracing/events/smp/csd_queue_cpu/trigger > $ echo 'csd_latency unsigned int dst_cpu; unsigned long csd; u64 time' >\ > /sys/kernel/tracing/synthetic_events > $ echo \ > 'hist:keys=common_cpu,csd.hex:'\ > 'time=common_timestamp.usecs-$ts:'\ > 'onmatch(smp.csd_queue_cpu).trace(csd_latency,common_cpu,csd,$time)' >\ > /sys/kernel/tracing/events/smp/csd_function_entry/trigger > > $ trace-cmd record -e 'synthetic:csd_latency' hackbench > $ trace-cmd report > <...>-467 [001] 21.824263: csd_queue_cpu: cpu=0 callsite=try_to_wake_up+0x2ea func=sched_ttwu_pending csd=0xffff8880076148b8 > <...>-467 [001] 21.824280: ipi_send_cpu: cpu=0 callsite=try_to_wake_up+0x2ea callback=generic_smp_call_function_single_interrupt+0x0 > <...>-489 [000] 21.824299: csd_function_entry: func=sched_ttwu_pending csd=0xffff8880076148b8 > <...>-489 [000] 21.824320: csd_latency: dst_cpu=0, csd=18446612682193848504, time=36 > """
Sure, I will split the patch and add the commit msg as suggested.
> > > > > Signed-off-by: Leonardo Bras <leobras@redhat.com> > > Overall I like the CSD vs IPI split, it's a saner approach than logging it > all as IPIs, even if it does generate more events as we have to emit an > event for every csd queued (i.e. no _cpumask() variant is possible). > > Some nitpicks below, and one other thing: are we happy with the smp event > namespace, and with the new TP names? Should the namespace be csd instead, > to match the trace_<namespace>_* nomenclature?
I selected "smp.h" filename because those tracepoints are getting used on smp.c.
I think keeping a generic name is good for any future tracepoints people end up designing for smp.c, but I am open to discussion.
> > > --- > > > > Changes since RFCv2: > > - Fixed some spacing issues and trace calls > > > > Changes since RFCv1: > > - Implemented trace_csd_queue_cpu() as suggested by Valentin Schneider > > - Using EVENT_CLASS in order to avoid duplication > > - Introduced new helper: csd_do_func() > > - Name change from smp_call_function_* to csd_function_* > > - Rebased on top of torvalds/master > > > > include/trace/events/smp.h | 72 ++++++++++++++++++++++++++++++++++++++ > > kernel/smp.c | 41 +++++++++++++--------- > > 2 files changed, 96 insertions(+), 17 deletions(-) > > create mode 100644 include/trace/events/smp.h > > > > diff --git a/include/trace/events/smp.h b/include/trace/events/smp.h > > new file mode 100644 > > index 000000000000..c304318a0203 > > --- /dev/null > > +++ b/include/trace/events/smp.h > > @@ -0,0 +1,72 @@ > > +/* SPDX-License-Identifier: GPL-2.0 */ > > +#undef TRACE_SYSTEM > > +#define TRACE_SYSTEM smp > > + > > +#if !defined(_TRACE_SMP_H) || defined(TRACE_HEADER_MULTI_READ) > > +#define _TRACE_SMP_H > > + > > +#include <linux/tracepoint.h> > > + > > +TRACE_EVENT(csd_queue_cpu, > > + > > + TP_PROTO(const unsigned int cpu, > > + unsigned long callsite, > > + smp_call_func_t func, > > + call_single_data_t *csd), > > + > > + TP_ARGS(cpu, callsite, func, csd), > > + > > + TP_STRUCT__entry( > > + __field(unsigned int, cpu) > > + __field(void *, callsite) > > + __field(void *, func) > > + __field(void *, csd) > > + ), > > + > > + TP_fast_assign( > > + __entry->cpu = cpu; > > + __entry->callsite = (void *)callsite; > > + __entry->func = func; > > + __entry->csd = csd; > > + ), > > + > > + TP_printk("cpu=%u callsite=%pS func=%pS csd=%p", > ^^^^^^^^ > This can be func=%ps, offsets should always be 0 so not very useful (I know > I put %pS for the IPI TPs, that's a force of habit...) >
I agree, let's do as suggested.
> > + __entry->cpu, __entry->callsite, __entry->func, __entry->csd) > > +); > > > + > > +/* > > + * Tracepoints for a function which is called as an effect of smp_call_function.* > > + */ > > +DECLARE_EVENT_CLASS(csd_function, > > + > > + TP_PROTO(smp_call_func_t func, call_single_data_t *csd), > > + > > + TP_ARGS(func, csd), > > + > > + TP_STRUCT__entry( > > + __field(void *, func) > > + __field(void *, csd) > > + ), > > + > > + TP_fast_assign( > > + __entry->func = func; > > + __entry->csd = csd; > > + ), > > + > > + TP_printk("function %ps, csd = %p", __entry->func, __entry->csd) > > To match the style of the other TPs, that should be: > > TP_printk("func=%ps csd=%p", __entry->func, __entry->csd) >
Done!
Thanks for reviewing! Leo
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