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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" Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI In-Reply-To: <20230323174129.GA2753619@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> <20230322140434.GC2357380@hirez.programming.kicks-ass.net> <20230323174129.GA2753619@hirez.programming.kicks-ass.net> Date: Thu, 23 Mar 2023 18:31:37 +0000 Message-ID: MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-sh@vger.kernel.org On 23/03/23 18:41, Peter Zijlstra wrote: > 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. > Duh, glanced over that. > 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. > Yeah, that makes sense to me. Just one tiny suggestion below. > 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) With your change to cfd->cpumask, we could ditch nr_queued and make this if (!cpumask_empty(cfd->cpumask)) since cfd->cpumask now only contains CPUs that have had a CSD queued. > + trace_ipi_send_cpumask(cfd->cpumask, _RET_IP_, func); > > /* > * Choose the most efficient way to send an IPI. Note that the