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 Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7DCCCCA0FE7 for ; Mon, 25 Aug 2025 14:34:50 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uqYH9-0004SG-Dr; Mon, 25 Aug 2025 10:34:24 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uqYH2-0004Qt-Qt for qemu-devel@nongnu.org; Mon, 25 Aug 2025 10:34:16 -0400 Received: from mgamail.intel.com ([198.175.65.13]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uqYGr-0001po-U9 for qemu-devel@nongnu.org; Mon, 25 Aug 2025 10:34:16 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1756132447; x=1787668447; h=date:from:to:cc:subject:message-id:references: mime-version:in-reply-to; bh=imi/f3yNw4/LFCttWtHo4cWvZgy9220sGCMMpL2c6gc=; b=gg7x/eYwh0bBCIAn0fCNJeGFVs3f8AypJtdFjBMYNDA8+LZNBS/kS0zL jujRCNTx5VLlPB/klR8wSUwwLiWxTdGwPj3Y7uqL/yUshisoQe1PihQZV nAJBAI9UdcYXEjr+d959BeEsEiZqTuQzSwkAxIRTvtDSAJLwo6OqAnSPX dLTR8iB8UpRp9UFT76ufy6m5nR6Z6idrSipOAi0eYCxnV8mMKIcxG6ckn M4qt7LZAKFX4iAP2jlN2yZNSkRvuPrecc2737Zw2hdPMxA+RNSGQ4aKdd ES8qNZj4TN7b4KKtRCi1eKY+YLfN2iacSeQVr47Xbl9xSjLhIbvjZOxAz w==; X-CSE-ConnectionGUID: r+pgT9kXTy6Ekt3EPSh8zA== X-CSE-MsgGUID: LdYqVEKRRDawI/vaMaUgDQ== X-IronPort-AV: E=McAfee;i="6800,10657,11533"; a="69446976" X-IronPort-AV: E=Sophos;i="6.18,213,1751266800"; d="scan'208";a="69446976" Received: from fmviesa007.fm.intel.com ([10.60.135.147]) by orvoesa105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Aug 2025 07:34:00 -0700 X-CSE-ConnectionGUID: IjA/+oOiRUSfWun/5ff1ZA== X-CSE-MsgGUID: WKl4KgC/Tm24SFmKQ59m1g== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.18,213,1751266800"; d="scan'208";a="168821675" Received: from liuzhao-optiplex-7080.sh.intel.com (HELO localhost) ([10.239.160.39]) by fmviesa007.fm.intel.com with ESMTP; 25 Aug 2025 07:33:58 -0700 Date: Mon, 25 Aug 2025 22:55:43 +0800 From: Zhao Liu To: Igor Mammedov Cc: qemu-devel@nongnu.org, Paolo Bonzini , Peter Xu , "Michael S. Tsirkin" , mtosatti@redhat.com Subject: Re: [PATCH v4 5/8] hpet: make main counter read lock-less Message-ID: References: <20250814160600.2327672-1-imammedo@redhat.com> <20250814160600.2327672-6-imammedo@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250814160600.2327672-6-imammedo@redhat.com> Received-SPF: pass client-ip=198.175.65.13; envelope-from=zhao1.liu@intel.com; helo=mgamail.intel.com X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Thu, Aug 14, 2025 at 06:05:57PM +0200, Igor Mammedov wrote: > Date: Thu, 14 Aug 2025 18:05:57 +0200 > From: Igor Mammedov > Subject: [PATCH v4 5/8] hpet: make main counter read lock-less > > Make access to main HPET counter lock-less. > > In unlikely event of an update in progress, readers will busy wait > untill update is finished. > > As result micro benchmark of concurrent reading of HPET counter > with large number of vCPU shows over 80% better (less) latency. > > Signed-off-by: Igor Mammedov > Reviewed-by: Peter Xu > --- > v3: > * make reader busy wait during update and reuse existing seqlock API > Peter Xu > --- > hw/timer/hpet.c | 26 ++++++++++++++++++++------ > 1 file changed, 20 insertions(+), 6 deletions(-) ... > - QEMU_LOCK_GUARD(&s->lock); > if (addr == HPET_COUNTER) { > - if (hpet_enabled(s)) { > - cur_tick = hpet_get_ticks(s); > - } else { > - cur_tick = s->hpet_counter; > - } > + unsigned version; > + > + /* > + * Write update is rare, so busywait here is unlikely to happen > + */ > + do { > + version = seqlock_read_begin(&s->state_version); > + if (unlikely(!hpet_enabled(s))) { is there any particular consideration for rearranging the order of the conditional branches here (and not directly using likely(hpet_enable()))? > + cur_tick = s->hpet_counter; > + } else { > + cur_tick = hpet_get_ticks(s); > + } > + } while (seqlock_read_retry(&s->state_version, version)); > trace_hpet_ram_read_reading_counter(addr & 4, cur_tick); > return cur_tick >> shift; > } Nice imprvoment! Reviewed-by: Zhao Liu