From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI Date: Thu, 23 Mar 2023 18:41:29 +0100 Message-ID: <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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=zp6Ev2/qqLL7BvP6yTiOOfsUY6tQNFuhrgZ4Bqaos4k=; b=KmdYH6Kn0c6hzD 4OArTDUHMMic6Twfsct2yd8FwrHlTP/gE2o6V/86nxQHzyLVxnwKxrRe8z94X28IxZrXSplinobkz RkSSeYFIwjJahRnqzzLiUyIa9Opzf1DuIs/ELlKwN4Brr5lXS2fVLtVfVFZGp4Jd8+1XOCsINsQ4z Jfaua/PpvunaoYqaLarwCFVoAcQNhLd9FvF3Y150OitcHltnYAfhD/nYlC7aWyEta+mgZUL28o4VN eAgYmDA1Uj+CFq7/6KFbhnKCZVIUZszLjTkxlpwMD2xYkzUwCD8RxLqoy/yqT0BMkkb5Gu1gz2KDA jk7YPbIjnNsqEgzPLH1w==; DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=uknWkbKCt7r7d+Uri/V39eu4SUqBOmf1+zXqyPceR/Q=; b=dDxw8FpzxhqVLTwZLp+6PLqsZb sRJ3X8JsTwZ/JiVyuRpgwQqpktLFA6bvn6+Z5BUFJqFkJRGddW2MQ80jsQJfIgRgfreJe2ASji3Ok ubgdvdDoJEumb5zC8iamfg/Bj2JpUUoIIkbWbcU+eS2YQIKqSfzW3woiON0TO/xxXdEJFV3HgvHvO sXiZid3LBW1uxBADyBwicyQ2lIEhNgqEfn2o6vq+arvPTl9YSvFX++J11S113A+M+rXKiwhf9f2WT HAcSaWWs+/beDL4nUBnINhc313sBkIdpEH77HQ1cT1mXenwbLj9N8jiiOSWeGoraLKmJYcbpzRY9+ kwRmLaAw==; Content-Disposition: inline In-Reply-To: List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-riscv" Errors-To: linux-riscv-bounces+glpr-linux-riscv=m.gmane-mx.org@lists.infradead.org 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 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