linux-kernel - Re: [PATCH RFC v2 1/4] block/mq-deadline: Revert "block/mq-deadline: Fix the tag reservation code"

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <9809ecdd-9190-cbd9-4397-5c1f718a9f15@huaweicloud.com>
Date: Wed, 18 Dec 2024 09:16:25 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Bart Van Assche <bvanassche@....org>, Yu Kuai <yukuai1@...weicloud.com>,
 axboe@...nel.dk, akpm@...ux-foundation.org, ming.lei@...hat.com,
 yang.yang@...o.com, osandov@...com, paolo.valente@...aro.org
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 yi.zhang@...wei.com, yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC v2 1/4] block/mq-deadline: Revert "block/mq-deadline:
 Fix the tag reservation code"

Hi,

在 2024/12/18 5:39, Bart Van Assche 写道:
> On 12/16/24 6:40 PM, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> This reverts commit 39823b47bbd40502632ffba90ebb34fff7c8b5e8.
>>
>> 1) Set min_shallow_depth to 1 will end up setting wake_batch to 1,
>>     and this will cause performance degradation in some high concurrency
>>     test, for both IO bandwidth and cpu usage.
>>
>>     async_depth can be changed by sysfs, and the minimal value is 1. This
>>     is why min_shallow_depth is set to 1 at initialization to make sure
>>     functional is correct if async_depth is set to 1. However, sacrifice
>>     performance in the default scenario is not acceptable.
>>
>> 2) dd_to_word_depth() is supposed to scale down async_depth, however, 
>> user
>>     can set low nr_requests and sb->depth can be less than 1 << 
>> sb->shift,
>>     then dd_to_word_depth() will end up scale up async_depth.
> 
> Although this patch fixes a performance regression, it breaks the
> async_depth functionality. If we are going to break that functionality
> temporarily, I propose to do something like this:

Yes, I'll split this patch, and merge the async_depth changes into the
last patch.

Thanks,
Kuai

> 
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index 20a8a3afb88b..4cc7b5db4669 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -487,37 +487,12 @@ static struct request *dd_dispatch_request(struct 
> blk_mq_hw_ctx *hctx)
>       return rq;
>   }
> 
> -/*
> - * 'depth' is a number in the range 1..INT_MAX representing a number of
> - * requests. Scale it with a factor (1 << bt->sb.shift) / 
> q->nr_requests since
> - * 1..(1 << bt->sb.shift) is the range expected by sbitmap_get_shallow().
> - * Values larger than q->nr_requests have the same effect as 
> q->nr_requests.
> - */
> -static int dd_to_word_depth(struct blk_mq_hw_ctx *hctx, unsigned int 
> qdepth)
> -{
> -    struct sbitmap_queue *bt = &hctx->sched_tags->bitmap_tags;
> -    const unsigned int nrr = hctx->queue->nr_requests;
> -
> -    return ((qdepth << bt->sb.shift) + nrr - 1) / nrr;
> -}
> -
>   /*
>    * Called by __blk_mq_alloc_request(). The shallow_depth value set by 
> this
>    * function is used by __blk_mq_get_tag().
>    */
>   static void dd_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
>   {
> -    struct deadline_data *dd = data->q->elevator->elevator_data;
> -
> -    /* Do not throttle synchronous reads. */
> -    if (op_is_sync(opf) && !op_is_write(opf))
> -        return;
> -
> -    /*
> -     * Throttle asynchronous requests and writes such that these requests
> -     * do not block the allocation of synchronous requests.
> -     */
> -    data->shallow_depth = dd_to_word_depth(data->hctx, dd->async_depth);
>   }
> 
>   /* Called by blk_mq_update_nr_requests(). */
> @@ -525,11 +500,8 @@ static void dd_depth_updated(struct blk_mq_hw_ctx 
> *hctx)
>   {
>       struct request_queue *q = hctx->queue;
>       struct deadline_data *dd = q->elevator->elevator_data;
> -    struct blk_mq_tags *tags = hctx->sched_tags;
> 
>       dd->async_depth = q->nr_requests;
> -
> -    sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, 1);
>   }
> 
>   /* Called by blk_mq_init_hctx() and blk_mq_init_sched(). */
> 
> .
>