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=-7.0 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS autolearn=no 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 EF023C433E0 for ; Tue, 4 Aug 2020 01:21:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EFD312072A for ; Tue, 4 Aug 2020 01:21:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729331AbgHDBVm (ORCPT ); Mon, 3 Aug 2020 21:21:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49122 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726756AbgHDBVm (ORCPT ); Mon, 3 Aug 2020 21:21:42 -0400 Received: from shards.monkeyblade.net (shards.monkeyblade.net [IPv6:2620:137:e000::1:9]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 54E14C06174A; Mon, 3 Aug 2020 18:21:42 -0700 (PDT) Received: from localhost (unknown [IPv6:2601:601:9f00:477::3d5]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client did not present a certificate) (Authenticated sender: davem-davemloft) by shards.monkeyblade.net (Postfix) with ESMTPSA id 51B341278B058; Mon, 3 Aug 2020 18:04:56 -0700 (PDT) Date: Mon, 03 Aug 2020 18:21:40 -0700 (PDT) Message-Id: <20200803.182140.1686117972461864999.davem@davemloft.net> To: olteanv@gmail.com Cc: kuba@kernel.org, netdev@vger.kernel.org, claudiu.manoil@nxp.com, ioana.ciornei@nxp.com, Jiafei.Pan@nxp.com, yangbo.lu@nxp.com, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Subject: Re: [PATCH net-next 1/2] dpaa2-eth: use napi_schedule to be compatible with PREEMPT_RT From: David Miller In-Reply-To: <20200803201009.613147-1-olteanv@gmail.com> References: <20200803201009.613147-1-olteanv@gmail.com> X-Mailer: Mew version 6.8 on Emacs 26.3 Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.12 (shards.monkeyblade.net [149.20.54.216]); Mon, 03 Aug 2020 18:04:56 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Vladimir Oltean Date: Mon, 3 Aug 2020 23:10:08 +0300 > From: Jiafei Pan > > The driver calls napi_schedule_irqoff() from a context where, in RT, > hardirqs are not disabled, since the IRQ handler is force-threaded. > > In the call path of this function, __raise_softirq_irqoff() is modifying > its per-CPU mask of pending softirqs that must be processed, using > or_softirq_pending(). The or_softirq_pending() function is not atomic, > but since interrupts are supposed to be disabled, nobody should be > preempting it, and the operation should be safe. > > Nonetheless, when running with hardirqs on, as in the PREEMPT_RT case, > it isn't safe, and the pending softirqs mask can get corrupted, > resulting in softirqs being lost and never processed. > > To have common code that works with PREEMPT_RT and with mainline Linux, > we can use plain napi_schedule() instead. The difference is that > napi_schedule() (via __napi_schedule) also calls local_irq_save, which > disables hardirqs if they aren't already. But, since they already are > disabled in non-RT, this means that in practice we don't see any > measurable difference in throughput or latency with this patch. > > Signed-off-by: Jiafei Pan > Signed-off-by: Vladimir Oltean Applied.