From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 112172877DA; Mon, 12 Jan 2026 14:24:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768227890; cv=none; b=n+u6XjT5ZA8YIgAhilBWNKIHYE7CCHcdLCb4vOt+1bxmPcTuAXWi4c4ycNCWGgBXelEj+Me6YHKBEOp6oXif88bu0zWGtOEFUHwvm6yLODvqpCg1r8f93hSjluS2cCh44+zmNeBZxF8e71KUmoP+spHVLWO1g8shRBMPYD2nFwY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768227890; c=relaxed/simple; bh=nKwISBLQbXPRFbbtij4NMF8qRE2KpVlrmeJniXMy3/U=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=PZ/KL7rR4q71B+fh0hfrJPN4jZnxefUeF7UPGDZr92ytK6hiE+SNBybtQGK0juMDlKiogWczdUm45rmE+w3Vr5PnD4CTcLMt4eSLK9tWo+MZ1OgnCbfMN39IS1MhHdOBDp2aNKwHEnfSow0zKlcTCqXer3QTDLUmzNZnlbt2rs0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=sdnPQjM3; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="sdnPQjM3" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=x09LXl6Wi+RiDD5vGf5aEWGgjFQp0fh8zIuLdsf/OnU=; b=sdnPQjM3Qb3mOYJOGzDE0CMXrc Ld1/PSlM91gECsnXDpUMEDvyK53dahpTAlHtFcUfX2rgbJ2RvvLUWqshxzorIqfCRFaMdwD74cza/ COwInS17GEdSwSFIBZ7rVZlO3+9QQ+/QFCrnL4R0dzOegnIHqn5dDQfywXupLl7Cm7GFIvPIZo7vE otGGO+KRy2W/0lZU+pCTcC+q3fwV+ozzfS2Es+41uJ0JbNjO1nsdkK1Ep4P1frLXBFJ+CU2Sl7b8p /n0/NM5m7KmmKkCEvh6MjLTji2qMtP4jv5Ma1Ez5qhNewE46MEyXinsT7hAJqv3BGInaRikZV1uIs /zzXjP4A==; Received: from 2001-1c00-8d85-5700-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:5700:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vfIqW-00000003KlW-2mjj; Mon, 12 Jan 2026 14:24:40 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 48BEE302D3E; Mon, 12 Jan 2026 15:24:40 +0100 (CET) Date: Mon, 12 Jan 2026 15:24:40 +0100 From: Peter Zijlstra To: Vishal Chourasia Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@kernel.org, frederic@kernel.org, neeraj.upadhyay@kernel.org, joelagnelf@nvidia.com, josh@joshtriplett.org, boqun.feng@gmail.com, urezki@gmail.com, rostedt@goodmis.org, tglx@linutronix.de, sshegde@linux.ibm.com, srikar@linux.ibm.com Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug operations Message-ID: <20260112142440.GN830755@noisy.programming.kicks-ass.net> References: <20260112094332.66006-2-vishalc@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260112094332.66006-2-vishalc@linux.ibm.com> On Mon, Jan 12, 2026 at 03:13:33PM +0530, Vishal Chourasia wrote: > Bulk CPU hotplug operations—such as switching SMT modes across all > cores—require hotplugging multiple CPUs in rapid succession. On large > systems, this process takes significant time, increasing as the number > of CPUs grows, leading to substantial delays on high-core-count > machines. Analysis [1] reveals that the majority of this time is spent > waiting for synchronize_rcu(). > > Expedite synchronize_rcu() during the hotplug path to accelerate the > operation. Since CPU hotplug is a user-initiated administrative task, > it should complete as quickly as possible. > > Performance data on a PPC64 system with 400 CPUs: > > + ppc64_cpu --smt=1 (SMT8 to SMT1) > Before: real 1m14.792s > After: real 0m03.205s # ~23x improvement > > + ppc64_cpu --smt=8 (SMT1 to SMT8) > Before: real 2m27.695s > After: real 0m02.510s # ~58x improvement > But who cares? Its not like you'd *ever* do this, right?