linux-kernel - Re: [PATCH v5 1/7] trace: Add trace_ipi_send

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Message-ID: <20230322093955.GR2017917@hirez.programming.kicks-ass.net>
Date:   Wed, 22 Mar 2023 10:39:55 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-csky@...r.kernel.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
        openrisc@...ts.librecores.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Nicholas Piggin <npiggin@...il.com>,
        Guo Ren <guoren@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v5 1/7] trace: Add trace_ipi_send_cpumask()

On Tue, Mar 07, 2023 at 02:35:52PM +0000, Valentin Schneider wrote:
> trace_ipi_raise() is unsuitable for generically tracing IPI sources due to
> its "reason" argument being an uninformative string (on arm64 all you get
> is "Function call interrupts" for SMP calls).
> 
> Add a variant of it that exports a target cpumask, a callsite and a callback.
> 
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>
> Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  include/trace/events/ipi.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/include/trace/events/ipi.h b/include/trace/events/ipi.h
> index 0be71dad6ec03..b1125dc27682c 100644
> --- a/include/trace/events/ipi.h
> +++ b/include/trace/events/ipi.h
> @@ -35,6 +35,28 @@ TRACE_EVENT(ipi_raise,
>  	TP_printk("target_mask=%s (%s)", __get_bitmask(target_cpus), __entry->reason)
>  );
>  
> +TRACE_EVENT(ipi_send_cpumask,
> +
> +	TP_PROTO(const struct cpumask *cpumask, unsigned long callsite, void *callback),
> +
> +	TP_ARGS(cpumask, callsite, callback),
> +
> +	TP_STRUCT__entry(
> +		__cpumask(cpumask)
> +		__field(void *, callsite)
> +		__field(void *, callback)
> +	),
> +
> +	TP_fast_assign(
> +		__assign_cpumask(cpumask, cpumask_bits(cpumask));
> +		__entry->callsite = (void *)callsite;
> +		__entry->callback = callback;
> +	),
> +
> +	TP_printk("cpumask=%s callsite=%pS callback=%pS",
> +		  __get_cpumask(cpumask), __entry->callsite, __entry->callback)
> +);

Would it make sense to add a variant like: ipi_send_cpu() that records a
single cpu instead of a cpumask. A lot of sites seems to do:
cpumask_of(cpu) for that first argument, and it seems to me it is quite
daft to have to memcpy a full multi-word cpumask in those cases.

Remember, nr_possible_cpus > 64 is quite common these days.