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=-3.9 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 66DE4C4363A for ; Tue, 27 Oct 2020 17:07:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0E3C420809 for ; Tue, 27 Oct 2020 17:07:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="Ji7B4Rl3"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="PhI3vz0u" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1816578AbgJ0RHt (ORCPT ); Tue, 27 Oct 2020 13:07:49 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:47936 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1815760AbgJ0RFU (ORCPT ); Tue, 27 Oct 2020 13:05:20 -0400 From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1603818316; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=PeTo+nqMytKpidhwafrElTV5yZ+u300kHK1bdYzrdw8=; b=Ji7B4Rl3j/bc5QszNJTBOXLS9DKhUIp6eGC7m9JGmL8+p5X62B1SEvaHz7e+wgCUYQacNQ YFvA6g7fMDF/u62uz3zYY6HpVs3BC7jjdMMlNMxzRWF/gijDwYRHLbVtJgHobvVhFbX3S2 ZXohGjJqkBaaNGSs9yenVrglBiiYyQkWUUBVudZdseS/6dKgtgjKCKSQsENO/Vh0C+YEi2 nDiAFLX+TzFguzKH2rhNNtVA3OpSREOFqRAMv8hdqHvFOi6oLd4ajsuCHlDbDounIaBuMd xB49J9fSQ4Yrm6CBoAVAWxm1HnQ/rEXJudYHAn5vGjaDZs2K+HDcmgLWdoUI7g== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1603818316; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=PeTo+nqMytKpidhwafrElTV5yZ+u300kHK1bdYzrdw8=; b=PhI3vz0uvNTzw98BkAOjrW1UAkn5pjXfEy8Q6hYwgepBkyoKHxqO7V12yyaL7t3UQjEe16 GiF2Tcxqc2CtypAg== To: Christoph Hellwig , Sebastian Andrzej Siewior Cc: Christoph Hellwig , David Runge , linux-rt-users@vger.kernel.org, Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Peter Zijlstra , Daniel Wagner Subject: Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT In-Reply-To: <20201027160742.GA19073@infradead.org> References: <20201021175059.GA4989@hmbx> <20201023110400.bx3uzsb7xy5jtsea@linutronix.de> <20201023112130.GA23790@infradead.org> <20201023135219.mzzl76eqqy6tqwhe@linutronix.de> <20201027092606.GA20805@infradead.org> <20201027101102.cvczdb3mkvtoguo5@linutronix.de> <20201027160742.GA19073@infradead.org> Date: Tue, 27 Oct 2020 18:05:15 +0100 Message-ID: <87eelj1tx0.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 27 2020 at 16:07, Christoph Hellwig wrote: > On Tue, Oct 27, 2020 at 11:11:02AM +0100, Sebastian Andrzej Siewior wrote: >> Right. I found this David Runge's log: > > True, ->bi_end_io instances can do a lot of things as long as they > are hardirq safe. > > And in the end the IPI case isn't the super fast path anyway, as it > means we don't use a queue per CPU. > > Is there a way to raise a softirq and preferably place it on a given > CPU without our IPI dance? That should be a win-win situation for > everyone. Not really. Softirq pending bits are strictly per cpu and we don't have locking or atomics to set them remotely. Even if we had that, then you'd still need a mechanism to make sure that the remote CPU actually processes them. So you'd still need an IPI of some sorts. Thanks, tglx