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=-14.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 E379DC433F5 for ; Mon, 6 Sep 2021 13:33:38 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id A1199606A5 for ; Mon, 6 Sep 2021 13:33:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org A1199606A5 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=xen.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.179912.326351 (Exim 4.92) (envelope-from ) id 1mNEkc-0007yL-PA; Mon, 06 Sep 2021 13:33:30 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 179912.326351; Mon, 06 Sep 2021 13:33:30 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1mNEkc-0007yE-MN; Mon, 06 Sep 2021 13:33:30 +0000 Received: by outflank-mailman (input) for mailman id 179912; Mon, 06 Sep 2021 13:33:30 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1mNEkc-0007y8-1C for xen-devel@lists.xenproject.org; Mon, 06 Sep 2021 13:33:30 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1mNEka-0004oI-Is; Mon, 06 Sep 2021 13:33:28 +0000 Received: from 54-240-197-236.amazon.com ([54.240.197.236] helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1mNEka-00057U-CS; Mon, 06 Sep 2021 13:33:28 +0000 X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xen.org; s=20200302mail; h=Content-Transfer-Encoding:Content-Type:In-Reply-To: MIME-Version:Date:Message-ID:From:References:Cc:To:Subject; bh=APik1CbMbgF6wJTSvB432A/CGZAYI0LBfC+hISwtP1Y=; b=63RDQMgBCf/ln7NWRC2xpZ64S2 KlkhERBFLjEhXiRn5exhoxEEX8ugn8IPls5vlAQxiNgdmUzMQi+aAwsFy8vcvksAtu+6QOpPgbzdC 31yt9Kr4ens/sAyQGPVSjIsZdaRtCyg+veQ5X6JdM68OI9loOmzCh3PCGTkOJfcr7chY=; Subject: Re: [PATCH 1/9] gnttab: defer allocation of maptrack frames table To: Jan Beulich Cc: Andrew Cooper , George Dunlap , Ian Jackson , Stefano Stabellini , Wei Liu , "xen-devel@lists.xenproject.org" References: <062bcbd3-420e-e1c0-3aa0-0dfb229e6ae9@suse.com> <7b8fba1a-767e-87f8-d0eb-5af5e5427bb4@xen.org> <60acd486-1641-6db4-4451-247edcc11522@suse.com> From: Julien Grall Message-ID: <26669864-09e2-a026-67f6-1f6dc48267db@xen.org> Date: Mon, 6 Sep 2021 14:33:26 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <60acd486-1641-6db4-4451-247edcc11522@suse.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Hi Jan, On 06/09/2021 14:29, Jan Beulich wrote: > On 06.09.2021 15:13, Julien Grall wrote: >> On 26/08/2021 11:09, Jan Beulich wrote: >>> --- a/xen/common/grant_table.c >>> +++ b/xen/common/grant_table.c >>> @@ -633,6 +633,34 @@ get_maptrack_handle( >>> if ( likely(handle != INVALID_MAPTRACK_HANDLE) ) >>> return handle; >>> >>> + if ( unlikely(!read_atomic(&lgt->maptrack)) ) >>> + { >>> + struct grant_mapping **maptrack = NULL; >>> + >>> + if ( lgt->max_maptrack_frames ) >>> + maptrack = vzalloc(lgt->max_maptrack_frames * sizeof(*maptrack)); >> >> While I understand that allocating with a lock is bad idea, I don't like >> the fact that multiple vCPUs racing each other would result to >> temporarily allocate more memory. >> >> If moving the call within the lock is not a solution, would using a loop >> with a cmpxchg() a solution to block the other vCPU? > > As with any such loop the question then is for how long to retry. No matter > what (static) loop bound you choose, if you exceed it you would return an > error to the caller for no reason. I was thinking to have an unbound loop. This would be no better no worth than the current implementation because of the existing lock. > > As to the value to store by cmpxchg() - were you thinking of some "alloc in > progress" marker? Yes. > You obviously can't store the result of the allocation > before actually doing the allocation, yet it is unnecessary allocations > that you want to avoid (i.e. to achieve your goal the allocation would need > to come after the cmpxchg()). A marker would further complicate the other > code here, even if (maybe) just slightly ... Right, the code will be a bit more complicated (although it would not be that bad if moved in a separate function...) but I feel it is better than the multiple vzalloc(). Cheers, -- Julien Grall