Date:   Wed, 10 May 2023 19:18:54 -0300
From:   Leonardo Bras Soares Passos <leobras@...hat.com>
To:     Valentin Schneider <vschneid@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yury Norov <yury.norov@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Nadav Amit <namit@...are.com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...nel.org>
Subject: Re: [RFC PATCH 1/1] smp: Add tracepoints for functions called with smp_call_function*()

On Wed, May 10, 2023 at 5:27 PM Leonardo Brás <leobras@...hat.com> wrote:
>
> On Thu, 2023-05-04 at 12:59 +0100, Valentin Schneider wrote:
> > +Daniel
> >
> > On 03/05/23 16:59, Peter Zijlstra wrote:
> > > On Wed, Apr 19, 2023 at 12:45:08AM -0300, Leonardo Brás wrote:
> > > >
> > > > IIUC the last commits add tracepoints that are collected in the
> > > > requesting CPU, at the moment of scheduling the IPI, which are also useful in
> > > > some scenarios.
> > > >
> > > > On my scenario, it could help a little, since it makes possible to filter what
> > > > all other cpus are scheduling on the requested cpu. OTOH it could be also be
> > > > misleading, as the requested cpu could be running something that was scheduled
> > > > way before.
> > > >
> > > > The change I propose does exactly what my scenario need: track exactly which
> > > > function was running at given time in the requested CPU. With this info, we can
> > > > check which (if any) remotely requested function was running on given time
> > > > window.
> > >
> > > I was thinking you could simply (graph)-trace
> > > __flush_smp_call_function_queue() with a max_graph_depth or so (Steve
> > > says that ought to work).
> > >
> > > But even that might be too specific, your use case sounds more like what
> > > we have the irq-off latency tracer for, and that thing will immediately
> > > tell you what functions were being ran.
> > >
> > > > (An unrelated thing I just thought: We could even use the commits you pointed
> > > > together with my proposed change in order to measure how long does it take for a
> > > > requested function to run / complete in the requested cpu)
> > >
> > > I don't think you could actually do that; the send tracepoints Valentin
> > > added don't log the csd address, this means you cannot distinguish
> > > two CSDs with the same function send from different CPUs.
> > >
> > > To do this you'd need to add the csd address to the the ipi_send
> > > tracepoints and your own (possibly replacing info -- because I don't
> > > think that's too useful).
> > >
> > > Valentin -- is any of this something you'd also find useful?
> >
> > Conceptually yeah, however:
> >
> > With isolcpus + NOHZ_FULL, *any* IPI sent to an isolated CPU is game over,
> > you interrupt the userspace task and you eat the full meal that is
> > NOHZ_FULL kernel entry. Pretty much any such IPI will show up when
> > evaluating your setup with rteval/rlta/whatever, so having an event at
> > IPI reception is redundant.
> >
> > IIUC Leonardo's looking at a setup with isolcpus but not necessarily
> > NOHZ_FULL, so he wants to look at *which* IPIs (if any) are pushing the
> > isolated task over its deadline/period. I would argue any IPI received
> > there is at risk of doing that, so it's the same fight as with NOHZ_FULL.
> >
> > With that said, I suppose this could still be helpful for e.g. osnoise to
> > hook into and point the finger at which CPU/context sent the problematic
> > IPI. Or more generally, as Leonardo suggested, to measure CSD IPI delivery
> > times.
> >
> > One thing though is that trace_ipi_send_cpu*() is not used solely for
> > CSD's, cf. irq_work_raise() or smp_send_reschedule(). We might want to
> > split that into e.g. trace_csd_queue_cpu*() + trace_ipi_send*().
> >
> > Something like so...
> >
> > ---
> > diff --git a/include/trace/events/smp.h b/include/trace/events/smp.h
> > new file mode 100644
> > index 0000000000000..8fc725a2b45b9
> > --- /dev/null
> > +++ b/include/trace/events/smp.h
> > @@ -0,0 +1,69 @@
> > +/* 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",
> > +               __entry->cpu, __entry->callsite, __entry->func, __entry->csd)
> > +);
>
> This is for the caller side, right?
>
> > +
> > +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("func=%pS csd=%p", __entry->func, __entry->csd)
> > +);
> > +
> > +DEFINE_EVENT(csd_function, csd_function_entry,
> > +     TP_PROTO(smp_call_func_t func, call_single_data_t *csd),
> > +     TP_ARGS(func, csd)
> > +);
> > +
> > +DEFINE_EVENT(csd_function, csd_function_exit,
> > +     TP_PROTO(smp_call_func_t func, call_single_data_t *csd),
> > +     TP_ARGS(func, csd)
> > +);
>
> Oh, this is what event_class is for. Thanks for the example :)
>
> > +
> > +#endif /* _TRACE_SMP_H */
> > +
> > +/* This part must be outside protection */
> > +#include <trace/define_trace.h>
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index ab3e5dad6cfe9..7d28db303e9bc 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -27,6 +27,9 @@
> >  #include <linux/jump_label.h>
> >
> >  #include <trace/events/ipi.h>
> > +#define CREATE_TRACE_POINTS
> > +#include <trace/events/smp.h>
> > +#undef CREATE_TRACE_POINTS
> >
> >  #include "smpboot.h"
> >  #include "sched/smp.h"
> > @@ -121,6 +124,14 @@ send_call_function_ipi_mask(struct cpumask *mask)
> >       arch_send_call_function_ipi_mask(mask);
> >  }
> >
> > +static __always_inline void
> > +csd_do_func(smp_call_func_t func, void *info, call_single_data_t *csd)
> > +{
> > +     trace_csd_function_entry(func, csd);
> > +     func(info);
> > +     trace_csd_function_exit(func, csd);
> > +}
> > +
>
> Good one, a helper to avoid calling those traces everywhere.
>
> Honest question:
> Since info == csd->info and func == csd->func, we could just pass csd, right?
> I suppose the suggestion on the 3-argument version is to use the values already
> fetched from memory instead of fetching them again. Is that correct?
>
> >  #ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
> >
> >  static DEFINE_STATIC_KEY_MAYBE(CONFIG_CSD_LOCK_WAIT_DEBUG_DEFAULT, csdlock_debug_enabled);
> > @@ -329,7 +340,7 @@ void __smp_call_single_queue(int cpu, struct llist_node *node)
> >        * even if we haven't sent the smp_call IPI yet (e.g. the stopper
> >        * executes migration_cpu_stop() on the remote CPU).
> >        */
> > -     if (trace_ipi_send_cpu_enabled()) {
> > +     if (trace_csd_queue_cpu_enabled()) {
> >               call_single_data_t *csd;
> >               smp_call_func_t func;
> >
> > @@ -337,7 +348,7 @@ void __smp_call_single_queue(int cpu, struct llist_node *node)
> >               func = CSD_TYPE(csd) == CSD_TYPE_TTWU ?
> >                       sched_ttwu_pending : csd->func;
> >
> > -             trace_ipi_send_cpu(cpu, _RET_IP_, func);
> > +             trace_csd_queue_cpu(cpu, _RET_IP_, func, csd);
> >       }
> >
> >       /*
> > @@ -375,7 +386,7 @@ static int generic_exec_single(int cpu, struct __call_single_data *csd)
> >               csd_lock_record(csd);
> >               csd_unlock(csd);
> >               local_irq_save(flags);
> > -             func(info);
> > +             csd_do_func(func, info, NULL);
> >               csd_lock_record(NULL);
> >               local_irq_restore(flags);
> >               return 0;
> > @@ -477,7 +488,7 @@ static void __flush_smp_call_function_queue(bool warn_cpu_offline)
> >                       }
> >
> >                       csd_lock_record(csd);
> > -                     func(info);
> > +                     csd_do_func(func, info, csd);
> >                       csd_unlock(csd);
> >                       csd_lock_record(NULL);
> >               } else {
> > @@ -508,7 +519,7 @@ static void __flush_smp_call_function_queue(bool warn_cpu_offline)
> >
> >                               csd_lock_record(csd);
> >                               csd_unlock(csd);
> > -                             func(info);
> > +                             csd_do_func(func, info, csd);
> >                               csd_lock_record(NULL);
> >                       } else if (type == CSD_TYPE_IRQ_WORK) {
> >                               irq_work_single(csd);
> > @@ -522,8 +533,10 @@ static void __flush_smp_call_function_queue(bool warn_cpu_offline)
> >       /*
> >        * Third; only CSD_TYPE_TTWU is left, issue those.
> >        */
> > -     if (entry)
> > -             sched_ttwu_pending(entry);
> > +     if (entry) {
> > +             csd = llist_entry(entry, typeof(*csd), node.llist);
> > +             csd_do_func(sched_ttwu_pending, entry, csd);
> > +     }
> >  }
> >
> >
> > @@ -624,7 +637,7 @@ EXPORT_SYMBOL(smp_call_function_single);
> >
> >  /**
> >   * smp_call_function_single_async() - Run an asynchronous function on a
> > - *                            specific CPU.
> > + *                            specific CPU.
> >   * @cpu: The CPU to run on.
> >   * @csd: Pre-allocated and setup data structure
> >   *
> > @@ -728,7 +741,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
> >       int cpu, last_cpu, this_cpu = smp_processor_id();
> >       struct call_function_data *cfd;
> >       bool wait = scf_flags & SCF_WAIT;
> > -     int nr_cpus = 0, nr_queued = 0;
> > +     int nr_cpus = 0;
> >       bool run_remote = false;
> >       bool run_local = false;
> >
> > @@ -786,21 +799,16 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
> >                       csd->node.src = smp_processor_id();
> >                       csd->node.dst = cpu;
> >  #endif
> > +
> > +                     trace_csd_queue_cpu(cpu, _RET_IP_, func, csd);
> > +
> >                       if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu))) {
> >                               __cpumask_set_cpu(cpu, cfd->cpumask_ipi);
> >                               nr_cpus++;
> >                               last_cpu = cpu;
> >                       }
> > -                     nr_queued++;
> >               }
> >
> > -             /*
> > -              * Trace each smp_function_call_*() as an IPI, actual IPIs
> > -              * will be traced with func==generic_smp_call_function_single_ipi().
> > -              */
> > -             if (nr_queued)
> > -                     trace_ipi_send_cpumask(cfd->cpumask, _RET_IP_, func);
> > -
> >               /*
> >                * Choose the most efficient way to send an IPI. Note that the
> >                * number of CPUs might be zero due to concurrent changes to the
> > @@ -816,7 +824,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
> >               unsigned long flags;
> >
> >               local_irq_save(flags);
> > -             func(info);
> > +             csd_do_func(func, info, NULL);
> >               local_irq_restore(flags);
> >       }
> >
> >
>
> I will rebase my patch on top of tip/smp/core and apply the suggested changes.
> A v2 will be send shortly.

RFC-v2 sent:
https://lore.kernel.org/lkml/20230510221513.93297-1-leobras@redhat.com/

>
> Thank you all for reviewing!
> Leo
>
>

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