Re: [PATCH v5 1/7] trace: Add trace_ipi_send_cpumask()

From: Peter Zijlstra <peterz@infradead.org>
To: Valentin Schneider <vschneid@redhat.com>
Cc: linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-snps-arc@lists.infradead.org,
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	x86@kernel.org, Steven Rostedt <rostedt@goodmis.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
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	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Marc Zyngier <maz@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Russell King <linux@armlinux.org.uk>,
	Nicholas Piggin <npiggin@gmail.com>, Guo Ren <guoren@kernel.org>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v5 1/7] trace: Add trace_ipi_send_cpumask()
Date: Wed, 22 Mar 2023 10:39:55 +0100	[thread overview]
Message-ID: <20230322093955.GR2017917@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20230307143558.294354-2-vschneid@redhat.com>

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@redhat.com>
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.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.

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