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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 93AFCC43460 for ; Tue, 20 Apr 2021 04:54:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 603B5613AF for ; Tue, 20 Apr 2021 04:54:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229650AbhDTEzX (ORCPT ); Tue, 20 Apr 2021 00:55:23 -0400 Received: from mail-1.ca.inter.net ([208.85.220.69]:51185 "EHLO mail-1.ca.inter.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229593AbhDTEzW (ORCPT ); Tue, 20 Apr 2021 00:55:22 -0400 Received: from localhost (offload-3.ca.inter.net [208.85.220.70]) by mail-1.ca.inter.net (Postfix) with ESMTP id 5141C2EA03C; Tue, 20 Apr 2021 00:54:51 -0400 (EDT) Received: from mail-1.ca.inter.net ([208.85.220.69]) by localhost (offload-3.ca.inter.net [208.85.220.70]) (amavisd-new, port 10024) with ESMTP id uI-VVjCe-ir8; Tue, 20 Apr 2021 00:35:11 -0400 (EDT) Received: from [192.168.48.23] (host-45-58-219-4.dyn.295.ca [45.58.219.4]) (using TLSv1 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) (Authenticated sender: dgilbert@interlog.com) by mail-1.ca.inter.net (Postfix) with ESMTPSA id 15ED42EA00A; Tue, 20 Apr 2021 00:54:50 -0400 (EDT) Reply-To: dgilbert@interlog.com Subject: Re: [bug report] shared tags causes IO hang and performance drop To: Bart Van Assche , John Garry , Ming Lei Cc: Kashyap Desai , linux-block@vger.kernel.org, linux-scsi@vger.kernel.org, "Martin K. Petersen" , Jens Axboe References: <9a6145a5-e6ac-3d33-b52a-0823bfc3b864@huawei.com> <87ceccf2-287b-9bd1-899a-f15026c9e65b@huawei.com> <3e76ffc7-1d71-83b6-ef5b-3986e947e372@huawei.com> <217e4cc1-c915-0e95-1d1c-4a11496080d6@huawei.com> <89ebc37c-21d6-c57e-4267-cac49a3e5953@huawei.com> From: Douglas Gilbert Message-ID: <993c3ae5-a7e2-aa6d-a6f3-147f06e9d015@interlog.com> Date: Tue, 20 Apr 2021 00:54:49 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-CA Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 2021-04-19 11:22 p.m., Bart Van Assche wrote: > On 4/19/21 8:06 PM, Douglas Gilbert wrote: >> I have always suspected under extreme pressure the block layer (or scsi >> mid-level) does strange things, like an IO hang, attempts to prove that >> usually lead back to my own code :-). But I have one example recently >> where upwards of 10 commands had been submitted (blk_execute_rq_nowait()) >> and the following one stalled (all on the same thread). Seconds later >> those 10 commands reported DID_TIME_OUT, the stalled thread awoke, and >> my dd variant went to its conclusion (reporting 10 errors). Following >> copies showed no ill effects. >> >> My weapons of choice are sg_dd, actually sgh_dd and sg_mrq_dd. Those last >> two monitor for stalls during the copy. Each submitted READ and WRITE >> command gets its pack_id from an incrementing atomic and a management >> thread in those copies checks every 300 milliseconds that that atomic >> value is greater than the previous check. If not, dump the state of the >> sg driver. The stalled request was in busy state with a timeout of 1 >> nanosecond which indicated that blk_execute_rq_nowait() had not been >> called. So the chief suspect would be blk_get_request() followed by >> the bio setup calls IMO. >> >> So it certainly looked like an IO hang, not a locking, resource nor >> corruption issue IMO. That was with a branch off MKP's >> 5.13/scsi-staging branch taken a few weeks back. So basically >> lk 5.12.0-rc1 . > > Hi Doug, > > If it would be possible to develop a script that reproduces this hang and > if that script can be shared I will help with root-causing and fixing this > hang. Possible, but not very practical: 1) apply supplied 83 patches to sg driver 2) apply pending patch to scsi_debug driver 3) find a stable kernel platform (perhaps not lk 5.12.0-rc1) 4) run supplied scripts for three weeks 5) dig through the output and maybe find one case (there were lots of EAGAINs from blk_get_request() but they are expected when thrashing the storage layers) My basic testing strategy may be useful for others: sg_dd iflag=random bs=512 of=/dev/sg6 sg_dd if=/dev/sg6 bs=512 of=/dev/sg7 sg_dd --verify if=/dev/sg6 bs=512 of=/dev/sg7 If the copy works, so should the verify (compare). The sg_dd utility in sg3_utils release 1.46 is needed to support iflag=random in the first line and the --verify in the third line. If the backing LLD is scsi_debug, then per_host_store=1 is needed. Best not to use SSDs. The above pattern will work just as well for /dev/sd* device nodes, but iflag= and oflag= lists must contain the sgio flag. Then ioctl(/dev/sd*, SG_IO, ...) is used for IO. The limitations of the third line could be bypassed with something like: cmp /dev/sd6 /dev/sd7 If real disks are used, all user data will be trashed. Doug Gilbert