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From:   Valentin Schneider <vschneid@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org,
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        x86@kernel.org, "Paul E. McKenney" <paulmck@kernel.org>,
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        Thomas Gleixner <tglx@linutronix.de>,
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        Juri Lelli <juri.lelli@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        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 7/7] sched, smp: Trace smp callback causing an IPI
In-Reply-To: <20230322140434.GC2357380@hirez.programming.kicks-ass.net>
References: <20230307143558.294354-1-vschneid@redhat.com>
 <20230307143558.294354-8-vschneid@redhat.com>
 <20230322095329.GS2017917@hirez.programming.kicks-ass.net>
 <xhsmhmt45c703.mognet@vschneid.remote.csb>
 <20230322140434.GC2357380@hirez.programming.kicks-ass.net>
Date:   Thu, 23 Mar 2023 16:25:25 +0000
Message-ID: <xhsmhbkkjcu4q.mognet@vschneid.remote.csb>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 22/03/23 15:04, Peter Zijlstra wrote:
> @@ -798,14 +794,20 @@ static void smp_call_function_many_cond(
>  		}
>  
>  		/*
> +		 * Trace each smp_function_call_*() as an IPI, actual IPIs
> +		 * will be traced with func==generic_smp_call_function_single_ipi().
> +		 */
> +		trace_ipi_send_cpumask(cfd->cpumask_ipi, _RET_IP_, func);

I just got a trace pointing out this can emit an event even though no IPI
is sent if e.g. the cond_func predicate filters all CPUs in the argument
mask:

  ipi_send_cpumask:     cpumask= callsite=on_each_cpu_cond_mask+0x3c callback=flush_tlb_func+0x0

Maybe something like so on top?

---
diff --git a/kernel/smp.c b/kernel/smp.c
index ba5478814e677..1dc452017d000 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -791,6 +791,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 			}
 		}
 
+		if (!nr_cpus)
+			goto local;
 		/*
 		 * Trace each smp_function_call_*() as an IPI, actual IPIs
 		 * will be traced with func==generic_smp_call_function_single_ipi().
@@ -804,10 +806,10 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		 */
 		if (nr_cpus == 1)
 			send_call_function_single_ipi(last_cpu);
-		else if (likely(nr_cpus > 1))
+		else
 			send_call_function_ipi_mask(cfd->cpumask_ipi);
 	}
-
+local:
 	if (run_local && (!cond_func || cond_func(this_cpu, info))) {
 		unsigned long flags;