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,
linux-arm-kernel@lists.infradead.org, linux-csky@vger.kernel.org,
linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org,
loongarch@lists.linux.dev, linux-mips@vger.kernel.org,
openrisc@lists.librecores.org, linux-parisc@vger.kernel.org,
linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org,
x86@kernel.org, "Paul E. McKenney" <paulmck@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
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
Date: Thu, 23 Mar 2023 18:41:29 +0100 [thread overview]
Message-ID: <20230323174129.GA2753619@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <xhsmhbkkjcu4q.mognet@vschneid.remote.csb>
On Thu, Mar 23, 2023 at 04:25:25PM +0000, Valentin Schneider wrote:
> 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;
Hmm, this isn't right. You can get nr_cpus==0 even though it did add
some to various lists but never was first.
But urgh, even if we were to say count nr_queued we'd never get the mask
right, because we don't track which CPUs have the predicate matched,
only those we need to actually send an IPI to :/
Ooh, I think we can clear those bits from cfd->cpumask, arguably that's
a correctness fix too, because the 'run_remote && wait' case shouldn't
wait on things we didn't queue.
Hmm?
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -728,9 +728,9 @@ static void smp_call_function_many_cond(
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;
bool run_remote = false;
bool run_local = false;
- int nr_cpus = 0;
lockdep_assert_preemption_disabled();
@@ -772,8 +772,10 @@ static void smp_call_function_many_cond(
for_each_cpu(cpu, cfd->cpumask) {
call_single_data_t *csd = per_cpu_ptr(cfd->csd, cpu);
- if (cond_func && !cond_func(cpu, info))
+ if (cond_func && !cond_func(cpu, info)) {
+ __cpumask_clear_cpu(cpu, cfd->cpumask);
continue;
+ }
csd_lock(csd);
if (wait)
@@ -789,13 +791,15 @@ static void smp_call_function_many_cond(
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().
*/
- trace_ipi_send_cpumask(cfd->cpumask_ipi, _RET_IP_, func);
+ if (nr_queued)
+ trace_ipi_send_cpumask(cfd->cpumask, _RET_IP_, func);
/*
* Choose the most efficient way to send an IPI. Note that the
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next prev parent reply other threads:[~2023-03-23 17:41 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-07 14:35 [PATCH v5 0/7] Generic IPI sending tracepoint Valentin Schneider
2023-03-07 14:35 ` [PATCH v5 1/7] trace: Add trace_ipi_send_cpumask() Valentin Schneider
2023-03-22 9:39 ` Peter Zijlstra
2023-03-22 10:30 ` Peter Zijlstra
2023-03-22 11:24 ` Valentin Schneider
2023-03-07 14:35 ` [PATCH v5 2/7] sched, smp: Trace IPIs sent via send_call_function_single_ipi() Valentin Schneider
2023-03-07 14:35 ` [PATCH v5 3/7] smp: Trace IPIs sent via arch_send_call_function_ipi_mask() Valentin Schneider
2023-03-07 14:35 ` [PATCH v5 4/7] irq_work: Trace self-IPIs sent via arch_irq_work_raise() Valentin Schneider
2023-03-07 14:35 ` [PATCH v5 5/7] treewide: Trace IPIs sent via smp_send_reschedule() Valentin Schneider
2023-03-07 14:35 ` [PATCH v5 6/7] smp: reword smp call IPI comment Valentin Schneider
2023-03-07 14:35 ` [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI Valentin Schneider
2023-03-22 9:53 ` Peter Zijlstra
2023-03-22 12:20 ` Valentin Schneider
2023-03-22 14:04 ` Peter Zijlstra
2023-03-22 17:01 ` Valentin Schneider
2023-03-22 17:22 ` Peter Zijlstra
2023-03-22 18:22 ` Valentin Schneider
2023-03-22 23:14 ` Peter Zijlstra
2023-03-23 16:25 ` Valentin Schneider
2023-03-23 17:41 ` Peter Zijlstra [this message]
2023-03-23 18:31 ` Valentin Schneider
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