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Tue, 25 Sep 2018 20:17:38 +0000 Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by userv0022.oracle.com (8.14.4/8.14.4) with ESMTP id w8PKHbtS022232 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 25 Sep 2018 20:17:37 GMT Received: from abhmp0014.oracle.com (abhmp0014.oracle.com [141.146.116.20]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id w8PKHaTQ003432; Tue, 25 Sep 2018 20:17:37 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 25 Sep 2018 13:17:36 -0700 Subject: Re: [PATCH 1/6] xenbus: prepare data structures and parameter for xenwatch multithreading To: Dongli Zhang , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org Cc: jgross@suse.com, paul.durrant@citrix.com, wei.liu2@citrix.com, konrad.wilk@oracle.com, roger.pau@citrix.com, srinivas.eeda@oracle.com References: <1536910456-13337-1-git-send-email-dongli.zhang@oracle.com> <1536910456-13337-2-git-send-email-dongli.zhang@oracle.com> <04d7dee9-3011-512a-09b0-0e8dcbdd99d6@oracle.com> <797673a8-fa7e-0bfc-332e-4e0190c8d1ed@oracle.com> <68418036-ae16-b58c-71d8-bb177fb30b51@oracle.com> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9027 signatures=668707 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1809250198 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/25/18 1:14 AM, Dongli Zhang wrote: > > So far we have: (1) domain hash table, (2) domain list (where duplicate entries > may exist) and (3) purge list. > > Can I assume you would like to discard the domain list and only keep domain hash > table and purge list? Yes, that's what I was thinking. > > The purpose of the domain list is to facilitate the unregister_xenbus_watch() to > scan the pending events for all otherend id. Should we remove it? Xen hypervisor > used both a hash table and a list to manage struct domain. Your concern is that searching for a pending event in the hash is not especially efficient. Since you are hashing on domain_id, then unregister_single_mtwatch() should be pretty fast if searching in the hash table (in fact, it's faster than traversing the domain list). unregister_all_mtwatch() will indeed take longer since you will be checking empty buckets. Right? How often do you expect will we call unregister_all_mtwatch() compared to unregister_single_mtwatch()? > > > About the duplicate entries in the domain list, can we change the flow like below: > > 1. Suppose the thread status is DOWN. To avoid having duplicate entries on the > domain list, instead of keeping the deprecated thread on domain list (until all > its events get processed), we move it to the purge list immediately. > > We will tag this deprecated thread so that the purge list would not purge it > unless all events belong to such thread get processed. We cannot purge it > immediately because the worker thread (to purge the purge list) would hang if > the deprecated thread is already stuck. > > In this flow, we may have duplicate entries on purge list, but not domain list. > > 2. During unregister_xenbus_watch(), we need to scan both the domain list and > purge list to remove pending events for the watch. In previous design, we only > scan domain lit. > > > One option is we allow the deprecated thread to resurrect and we would not move > the thread to purge list immediately when the thread is deprecated. > > Suppose when thread for domid=9 is needed, we would not create new one if the > deprecated one for domid=9 is still in domain list. Instead, we would resurrect > it, change its status and reuse it again. In this way, we would not have > duplicate entries on the domain list. > > I like the 1st option. I do not like to resurrect a deprecated thread again. > Would you please let me know how you think about it? Yes, I also think (1) is better --- you are walking the whole purge list anyway. -boris