From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4447BC54FCB for ; Sun, 26 Apr 2020 13:38:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 12C692071C for ; Sun, 26 Apr 2020 13:38:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=brainfault-org.20150623.gappssmtp.com header.i=@brainfault-org.20150623.gappssmtp.com header.b="LDAiQz7M" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726151AbgDZNiR (ORCPT ); Sun, 26 Apr 2020 09:38:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57264 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725876AbgDZNiQ (ORCPT ); Sun, 26 Apr 2020 09:38:16 -0400 Received: from mail-wm1-x343.google.com (mail-wm1-x343.google.com [IPv6:2a00:1450:4864:20::343]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7DFA7C061A0F for ; Sun, 26 Apr 2020 06:38:16 -0700 (PDT) Received: by mail-wm1-x343.google.com with SMTP id g12so17203824wmh.3 for ; Sun, 26 Apr 2020 06:38:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=brainfault-org.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=AKeJsfVQBTmRJ5DMftJ0126mlRY+cjmcCtKvFup4ozY=; b=LDAiQz7MYSSfMUnc0M3PtEkFizLlN5hTiL5JRxWtoyeXmT71fiHPcnk4OsCeezFvM7 23TSEk2irysoDe0OEvORA1LkzD6X2g9oSmxvSpNlIyeIk6WHyTDSUZYeRSyGiJoks2Hm 4hgkQuJajNgYXWT02+aRTAZdsGQqY73d3WQj3OkutChq/qqyrIB/0GtMoEaGawVX4qQS 4AmWxNXtUVaNxC6rsxBFcz5OEZXJTVGstj5XUxIiWOA/DTm087YGDReBmK9zKwWEjPWR VI0IZXVURX+pc+DaMJ/NBNgUqojonhR8oDblVkkOcRRI3Jj4TwNmwcLscQzERxVWwhV5 w6WA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=AKeJsfVQBTmRJ5DMftJ0126mlRY+cjmcCtKvFup4ozY=; b=V5uTybqcuINggjAzJ3wSBG/qm9VsQqGywoRpdsCTuOVuAMczW/WrzEHF6w9n2708KX mk08JipO5XjTkJ/nKClcZrpOopztKjZpoSkASt9dkCN/t5ALUy6GD1wLLs/NFTViTiYU 1wjagwcb13Jscps3AzlLVm7ccXS0L8IhLJN6ezF8MaKq2Hdi2L0M2BPCi2U97+uaOgU2 hnoWxAtfdxUVXW/QU0tZW0fXcov4l92g5nqW+5DxRjPmPSiNlzmuxXs2Jmv3rZmCKa8p 6CycMftb8GK4UsceuraPbD6tIMPoOO/e84LaMBr3tCgyuXDXUsvQyfAVMdUSO3N5Caaz rHFQ== X-Gm-Message-State: AGi0PuYLJRn+bul4uwf48DaxuXJMLEsSNqGfIMToIYwcyx75F1Uvr83D eBcXAmOLw8WbtUL3ZlYWKH1XfQPYxxwCpQeVE8d9Og== X-Google-Smtp-Source: APiQypLYcPLROTkqB8LzsYssjIU4lEHWA65Pc9cTE+Y39E81HNfUzw/chX6hgTuZNKJOX8E7TCXt+lUBH8wLC3x+xqg= X-Received: by 2002:a1c:9d0d:: with SMTP id g13mr21978860wme.102.1587908294824; Sun, 26 Apr 2020 06:38:14 -0700 (PDT) MIME-Version: 1.0 References: <20200426110740.123638-1-zong.li@sifive.com> In-Reply-To: From: Anup Patel Date: Sun, 26 Apr 2020 19:08:03 +0530 Message-ID: Subject: Re: [PATCH] irqchip/sifive-plic: allow many cores to handle IRQs To: Zong Li Cc: Palmer Dabbelt , Paul Walmsley , "linux-kernel@vger.kernel.org List" , linux-riscv , David Abdurachmanov , Marc Zyngier Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org +Mark Z On Sun, Apr 26, 2020 at 6:49 PM Zong Li wrote: > > On Sun, Apr 26, 2020 at 8:47 PM Anup Patel wrote: > > > > On Sun, Apr 26, 2020 at 4:37 PM Zong Li wrote: > > > > > > Currently, driver forces the IRQs to be handled by only one core. This > > > patch provides the way to enable others cores to handle IRQs if needed, > > > so users could decide how many cores they wanted on default by boot > > > argument. > > > > > > Use 'irqaffinity' boot argument to determine affinity. If there is no > > > irqaffinity in dts or kernel configuration, use irq default affinity, > > > so all harts would try to claim IRQ. > > > > > > For example, add irqaffinity=0 in chosen node to set irq affinity to > > > hart 0. It also supports more than one harts to handle irq, such as set > > > irqaffinity=0,3,4. > > > > > > You can change IRQ affinity from user-space using procfs. For example, > > > you can make CPU0 and CPU2 serve IRQ together by the following command: > > > > > > echo 4 > /proc/irq//smp_affinity > > > > > > Signed-off-by: Zong Li > > > --- > > > drivers/irqchip/irq-sifive-plic.c | 21 +++++++-------------- > > > 1 file changed, 7 insertions(+), 14 deletions(-) > > > > > > diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c > > > index d0a71febdadc..bc1440d54185 100644 > > > --- a/drivers/irqchip/irq-sifive-plic.c > > > +++ b/drivers/irqchip/irq-sifive-plic.c > > > @@ -111,15 +111,12 @@ static inline void plic_irq_toggle(const struct cpumask *mask, > > > static void plic_irq_unmask(struct irq_data *d) > > > { > > > struct cpumask amask; > > > - unsigned int cpu; > > > struct plic_priv *priv = irq_get_chip_data(d->irq); > > > > > > cpumask_and(&amask, &priv->lmask, cpu_online_mask); > > > - cpu = cpumask_any_and(irq_data_get_affinity_mask(d), > > > - &amask); > > > - if (WARN_ON_ONCE(cpu >= nr_cpu_ids)) > > > - return; > > > - plic_irq_toggle(cpumask_of(cpu), d, 1); > > > + cpumask_and(&amask, &amask, irq_data_get_affinity_mask(d)); > > > + > > > + plic_irq_toggle(&amask, d, 1); > > > } > > > > > > static void plic_irq_mask(struct irq_data *d) > > > @@ -133,24 +130,20 @@ static void plic_irq_mask(struct irq_data *d) > > > static int plic_set_affinity(struct irq_data *d, > > > const struct cpumask *mask_val, bool force) > > > { > > > - unsigned int cpu; > > > struct cpumask amask; > > > struct plic_priv *priv = irq_get_chip_data(d->irq); > > > > > > cpumask_and(&amask, &priv->lmask, mask_val); > > > > > > if (force) > > > - cpu = cpumask_first(&amask); > > > + cpumask_copy(&amask, mask_val); > > > else > > > - cpu = cpumask_any_and(&amask, cpu_online_mask); > > > - > > > - if (cpu >= nr_cpu_ids) > > > - return -EINVAL; > > > + cpumask_and(&amask, &amask, cpu_online_mask); > > > > > > plic_irq_toggle(&priv->lmask, d, 0); > > > - plic_irq_toggle(cpumask_of(cpu), d, 1); > > > + plic_irq_toggle(&amask, d, 1); > > > > > > - irq_data_update_effective_affinity(d, cpumask_of(cpu)); > > > + irq_data_update_effective_affinity(d, &amask); > > > > > > return IRQ_SET_MASK_OK_DONE; > > > } > > > -- > > > 2.26.1 > > > > > > > I strongly oppose (NACK) this patch due to performance reasons. > > > > In PLIC, if we enable an IRQ X for N CPUs then when IRQ X occurs: > > 1) All N CPUs will take interrupt > > 2) All N CPUs will try to read PLIC CLAIM register > > 3) Only one of the CPUs will see IRQ X using the CLAIM register > > but other N - 1 CPUs will see no interrupt and return back to what > > they were doing. In other words, N - 1 CPUs will just waste CPU > > every time IRQ X occurs. > > > > Example1, one Application doing heavy network traffic will > > degrade performance of other applications because with every > > network RX/TX interrupt N-1 CPUs will waste CPU trying to > > process network interrupt. > > > > Example1, one Application doing heavy MMC/SD traffic will > > degrade performance of other applications because with every > > SPI read/write interrupt N-1 CPUs will waste CPU trying to > > process it. > > > > In fact, the current PLIC approach is actually a performance > > optimization. This implementation also works fine with in-kernel > > load-balancer and user space load balancer. > > > > Yes, it's exactly, I know what you pointed out. But the idea of this > patch is just providing a way that users could enable other cores if > they wanted, it could still enable only one core by this change. The > purpose here is thinking of flexible use, rather than limitation. > Maybe it would be a happy medium that we make the default case enable > only one core? It is a good open discussion. Making the default case as enable only one core is just a work-around. As-per my understanding, if we set affinity mask of N CPUs for IRQ X then it does not mean all N CPUs should receive IRQ X rather it means that exactly one of the N CPUs will receive IRQ X and the IRQ receiving CPU will be fixed (reflected by effective affinity returned by the driver). If we ignore above semantics and still provide a mechanism to target IRQ X to N CPUs then most likely someone will try and run into performance issues. Please don't go this path. The performance impact in case of Guest/VM is even worst because PLIC is trap-n-emulated by hypervisors as MMIO device. Regards, Anup