From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Date: Thu, 23 Mar 2023 17:41:29 +0000 Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI Message-Id: <20230323174129.GA2753619@hirez.programming.kicks-ass.net> List-Id: References: <20230307143558.294354-1-vschneid@redhat.com> <20230307143558.294354-8-vschneid@redhat.com> <20230322095329.GS2017917@hirez.programming.kicks-ass.net> <20230322140434.GC2357380@hirez.programming.kicks-ass.net> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Valentin Schneider 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" , Steven Rostedt , Thomas Gleixner , Sebastian Andrzej Siewior , Juri Lelli , Daniel Bristot de Oliveira , Marcelo Tosatti , Frederic Weisbecker , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , Marc Zyngier , Mark Rutland , Russell King , Nicholas Piggin , Guo Ren , "David S. Miller" 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