linux-kernel - Re: [PATCH V13 03/10] mmc: block: Add blk-mq support

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Date:   Thu, 16 Nov 2017 08:19:28 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Bough Chen <haibo.chen@....com>,
        Alex Lemberg <alex.lemberg@...disk.com>,
        Mateusz Nowak <mateusz.nowak@...el.com>,
        Yuliy Izrailov <Yuliy.Izrailov@...disk.com>,
        Jaehoon Chung <jh80.chung@...sung.com>,
        Dong Aisheng <dongas86@...il.com>,
        Das Asutosh <asutoshd@...eaurora.org>,
        Zhangfei Gao <zhangfei.gao@...il.com>,
        Sahitya Tummala <stummala@...eaurora.org>,
        Harjani Ritesh <riteshh@...eaurora.org>,
        Venu Byravarasu <vbyravarasu@...dia.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V13 03/10] mmc: block: Add blk-mq support

On 15 November 2017 at 14:07, Adrian Hunter <adrian.hunter@...el.com> wrote:
> On 15/11/17 12:55, Ulf Hansson wrote:
>> Linus, Adrian,
>>
>> Apologize for sidetracking the discussion, just wanted to add some
>> minor comments.
>>
>> [...]
>>
>>>
>>>>> But what I think is nice in doing it around
>>>>> each request is that since mmc_put_card() calls mmc_release_host()
>>>>> contains this:
>>>>>
>>>>> if (--host->claim_cnt) { (...)
>>>>>
>>>>> So there is a counter inside the claim/release host functions
>>>>> and now there is another counter keeping track if the in-flight
>>>>> requests as well and it gives me the feeling we are maintaining
>>>>> two counters when we only need one.
>>>>
>>>> The gets / puts also get runtime pm for the card.  It is a lot a messing
>>>> around for the sake of a quick check for the number of requests inflight -
>>>> which is needed anyway for CQE.
>>
>> Actually the get / puts for runtime PM of the card is already done
>> taking the host->claim_cnt into account.
>
> We do pm_runtime_get_sync(&card->dev) always i.e.
>
> void mmc_get_card(struct mmc_card *card, struct mmc_ctx *ctx)
> {
>         pm_runtime_get_sync(&card->dev);
>         __mmc_claim_host(card->host, ctx, NULL);
> }

You are absolutely correct, so let's leave the inflight counter in.
Apologize for the noise!

I was thinking about the runtime PM of the host dev, which is managed
by mmc_claim|release host().

[...]

>>>> Well I saw 3% drop in sequential read performance, improving to 1% when an
>>>> unbound workqueue was used.
>>
>> Can you please make this improvement as a standalone patch on top of
>> the mq patch?
>
> Unfortunately it was just a hack to the blk-mq core - the block layer does
> not have an unbound workqueue.  I have not had time to consider a proper
> fix.  It will have to wait, but I agree 3% is not enough to delay going forward.

I see, thanks!

[...]

Kind regards
Uffe