From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@kernel.org>
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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED,
	DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,
	SPF_HELO_NONE,SPF_PASS 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 00969C433ED
	for <linux-kernel@archiver.kernel.org>; Mon, 10 May 2021 02:07:02 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [23.128.96.18])
	by mail.kernel.org (Postfix) with ESMTP id CA95460FD8
	for <linux-kernel@archiver.kernel.org>; Mon, 10 May 2021 02:07:01 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S230131AbhEJCIE (ORCPT <rfc822;linux-kernel@archiver.kernel.org>);
        Sun, 9 May 2021 22:08:04 -0400
Received: from mail.synology.com ([211.23.38.101]:42444 "EHLO synology.com"
        rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP
        id S230038AbhEJCID (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Sun, 9 May 2021 22:08:03 -0400
Subject: Re: [PATCH v2] block: fix trace completion for chained bio
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=synology.com; s=123;
        t=1620612418; bh=aiIIs6RGEjdXhexQOGgZusHgo4xHZHZ4r2ou6nC6+hw=;
        h=Subject:From:To:Cc:References:Date:In-Reply-To;
        b=DxV6bSALuhocDfV7KPmmIyFWPXk9FYphOwyUZai2Ud06qqb4U8RlJK7DNXRcsF5M7
         MBS43dQnZGYA09VLuTYjcYJUPU+6JIiekowRnqJO/e30zo389dmUG9kesAk8aE0gOY
         ealUGSaVxDfZ+bwPZnqXabzscCUSbeeZxQn7WcXc=
From:   Edward Hsieh <edwardh@synology.com>
To:     NeilBrown <neilb@suse.de>, axboe@kernel.dk, neilb@suse.com
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        s3t@synology.com, bingjingc@synology.com, cccheng@synology.com
References: <1614741726-28074-1-git-send-email-edwardh@synology.com>
 <87zgyudgss.fsf@notabene.neil.brown.name>
 <fb8620bf-f4e9-5787-ab79-6e0a5d79d26d@synology.com>
Message-ID: <10f3e198-f34c-47e9-608a-e5f84e3379a1@synology.com>
Date:   Mon, 10 May 2021 10:06:57 +0800
MIME-Version: 1.0
In-Reply-To: <fb8620bf-f4e9-5787-ab79-6e0a5d79d26d@synology.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 8bit
X-Synology-MCP-Status: no
X-Synology-Spam-Flag: no
X-Synology-Spam-Status: score=0, required 6, WHITELIST_FROM_ADDRESS 0
X-Synology-Virus-Status: no
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


On 4/23/2021 4:04 PM, Edward Hsieh wrote:
> On 3/23/2021 5:22 AM, NeilBrown wrote:
>> On Wed, Mar 03 2021, edwardh wrote:
>>
>>> From: Edward Hsieh <edwardh@synology.com>
>>>
>>> For chained bio, trace_block_bio_complete in bio_endio is currently 
>>> called
>>> only by the parent bio once upon all chained bio completed.
>>> However, the sector and size for the parent bio are modified in 
>>> bio_split.
>>> Therefore, the size and sector of the complete events might not match 
>>> the
>>> queue events in blktrace.
>>>
>>> The original fix of bio completion trace <fbbaf700e7b1> ("block: trace
>>> completion of all bios.") wants multiple complete events to correspond
>>> to one queue event but missed this.
>>>
>>> md/raid5 read with bio cross chunks can reproduce this issue.
>>>
>>> To fix, move trace completion into the loop for every chained bio to 
>>> call.
>>
>> Thanks.  I think this is correct as far as tracing goes.
>> However the code still looks a bit odd.
>>
>> The comment for the handling of bio_chain_endio suggests that the *only*
>> purpose for that is to avoid deep recursion.  That suggests it should be
>> at the end of the function.
>> As it is blk_throtl_bio_endio() and bio_unint() are only called on the
>> last bio in a chain.
>> That seems wrong.
>>
>> I'd be more comfortable if the patch moved the bio_chain_endio()
>> handling to the end, after all of that.
>> So the function would end.
>>
>> if (bio->bi_end_io == bio_chain_endio) {
>>     bio = __bio_chain_endio(bio);
>>     goto again;
>> } else if (bio->bi_end_io)
>>     bio->bi_end_io(bio);
>>
>> Jens:  can you see any reason why that functions must only be called on
>> the last bio in the chain?
>>
>> Thanks,
>> NeilBrown
>>
> 
> Hi Neil and Jens,
> 
>  From the commit message, bio_uninit is put here for bio allocated in
> special ways (e.g., on stack), that will not be release by bio_free. For
> chained bio, __bio_chain_endio invokes bio_put and release the
> resources, so it seems that we don't need to call bio_uninit for chained
> bio.
> 
> The blk_throtl_bio_endio is used to update the latency for the throttle
> group. I think the latency should only be updated after the whole bio is
> finished?
> 
> To make sense for the "tail call optimization" in the comment, I'll
> suggest to wrap the whole statement with an else. What do you think?
> 
> if (bio->bi_end_io == bio_chain_endio) {
>      bio = __bio_chain_endio(bio);
>      goto again;
> } else {
>      blk_throtl_bio_endio(bio);
>      /* release cgroup info */
>      bio_uninit(bio);
>      if (bio->bi_end_io)
>          bio->bi_end_io(bio);
> }
> 
> Thanks,
> Edward Hsieh

Hi Neil and Jens,

Any feedback on this one?

Thank you,
Edward Hsieh