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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, 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 CFCC0C433B4 for ; Tue, 20 Apr 2021 13:35:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 925C661027 for ; Tue, 20 Apr 2021 13:35:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232413AbhDTNf2 (ORCPT ); Tue, 20 Apr 2021 09:35:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39390 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232673AbhDTNfM (ORCPT ); Tue, 20 Apr 2021 09:35:12 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 154B6C06174A for ; Tue, 20 Apr 2021 06:34:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; 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=vdBcp9f+Y+NjuSY4plP2jnk3fMAR2GgyVGNWh1RxQLs=; b=KckIqJHZiRqF1mfSqOHIlS+E6t U22gXybkRvT2ebT3gRDrbCsXZ9Nh1oqmgEghWOFGhkT151Su9LSywEm5Vgtpl2GLyPjS/MLCye0fu LfVyfL3/mbtvQt4t1t6aDcFpei7wvdx0aDAe1gUXEz62pRQRbGvBYHfUyHB2jyrkPRvEjFp40wnTx etzaO0R4OEgzUmBP+FfIV94YiSGEATc3u17mHQnPcmche84gIBfiuX9JQrwolpOWUVzrqv8eEtB9E 4rgIp4QUuaslVmh8ooTFPYePwuy3hfOSuyg/WkVeBaBwAVZL5pYPCw7G0fABDNgCLvPWRx2wVZUiI TTn/Zudg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lYqT1-00FDAj-Hy; Tue, 20 Apr 2021 13:31:26 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 3BC4C30018E; Tue, 20 Apr 2021 15:31:03 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 241342CD0A95B; Tue, 20 Apr 2021 15:31:03 +0200 (CEST) Date: Tue, 20 Apr 2021 15:31:03 +0200 From: Peter Zijlstra To: Luigi Rizzo Cc: linux-kernel , axboe@kernel.dk, paulmck@kernel.org Subject: Re: [PATCH] smp: add a best_effort version of smp_call_function_many() Message-ID: References: <20210419184455.2987243-1-lrizzo@google.com> <20210419191712.GB26214@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 20, 2021 at 12:41:08PM +0200, Luigi Rizzo wrote: > On Tue, Apr 20, 2021 at 11:14 AM Peter Zijlstra wrote: > > We mostly try and avoid using this stuff wherever possible. Only when > > no other choice is left do we send IPIs. > > > > NOHZ_FULL already relies on this and gets massively unhappy when a new > > user comes and starts to spray IPIs. > > I am curious, why is that -- is it because the new user is stealing > the shared csd's in cfd_data (see below), or some other reason ? The premise of NOHZ_FULL is that it will not be interrupted. There are users who are working on a mode where any interruption will cause a (fatal) signal. > > So no; mostly we send an IPI because we _HAVE_ to, not because giggles. > > > > That said; there's still some places left where we can avoid sending > > IPIs, but in all those cases correctness mandates we actually handle > > things and not randomly not do anything. > > My case too requires that the request is eventually handled, but with > this non-blocking IPI the caller has a better option than blocking: > it can either retry the multicast IPI at a later time if conditions allow, > or it can post a dedicated CSD (with the advantage that being my > requests idempotent, if the CSD is locked there is no need to retry > because it means the handler has not started yet). > > In fact, if we had the option to use dedicated CSDs for multicast IPI, > we wouldn't even need to retry because we'd know that the posted CSD > is for our call back and not someone else's. What are you doing that CSD contention is such a problem?