[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <27e4a2b4557ae24e673b6fb85b34fe0e32efa06f.camel@redhat.com>
Date: Mon, 15 May 2023 14:56:53 -0300
From: Leonardo BrĂ¡s <leobras@...hat.com>
To: Valentin Schneider <vschneid@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Juergen Gross <jgross@...e.com>,
Yury Norov <yury.norov@...il.com>,
Chen Zhongjin <chenzhongjin@...wei.com>,
Zhen Lei <thunder.leizhen@...wei.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Nadav Amit <namit@...are.com>,
Daniel Bristot de Oliveira <bristot@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 1/1] trace,smp: Add tracepoints around remotelly
called functions
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@...hat.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
Powered by blists - more mailing lists