linux-kernel - Re: [RFC PATCH v2 6/8] treewide: Trace IPIs sent via smp_send

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Message-ID: <xhsmhiljdy7zv.mognet@vschneid.remote.csb>
Date:   Thu, 17 Nov 2022 14:24:04 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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, Guo Ren <guoren@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.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>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC PATCH v2 6/8] treewide: Trace IPIs sent via
 smp_send_reschedule()

On 17/11/22 10:12, Peter Zijlstra wrote:
> On Wed, Nov 02, 2022 at 06:33:34PM +0000, Valentin Schneider wrote:
>
>> diff --git a/kernel/smp.c b/kernel/smp.c
>> index c4d561cf50d45..44fa4b9b1f46b 100644
>> --- a/kernel/smp.c
>> +++ b/kernel/smp.c
>> @@ -167,6 +167,14 @@ send_call_function_ipi_mask(const struct cpumask *mask)
>>      arch_send_call_function_ipi_mask(mask);
>>  }
>>
>> +void smp_send_reschedule(int cpu)
>> +{
>> +	/* XXX scheduler_ipi is inline :/ */
>> +	trace_ipi_send_cpumask(cpumask_of(cpu), _RET_IP_, NULL);
>> +	arch_smp_send_reschedule(cpu);
>> +}
>> +EXPORT_SYMBOL_GPL(smp_send_reschedule);
>
> Yeah, no.. I see some crazy archs do this, but no we're not exporting
> this in generic.

So the list is: ia64, powerpc, riscv
and they all seem to do it because of KVM:
  c4cb768f0277 ("[IA64] export smp_send_reschedule")
  de56a948b918 ("KVM: PPC: Add support for Book3S processors in hypervisor mode")
  d3d7a0ce020e ("RISC-V: Export kernel symbols for kvm")

Other archs get out of it either because their smp_send_reschedule() is
inline (e.g. x86), or because they don't allow building KVM as a module
(e.g. arm64).

If I can cobble the tracepoint+reschedule in an inline helper, then that
wouldn't require any new exports - I'm fighting a bit with the header maze
ATM, but hopefully I can get somewhere with this.