Re: [PATCH 00/12 v4] multiqueue for MMC/SD

From: Adrian Hunter <adrian.hunter@intel.com>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: "linux-mmc@vger.kernel.org" <linux-mmc@vger.kernel.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	linux-block <linux-block@vger.kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
	Arnd Bergmann <arnd@arndb.de>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	Paolo Valente <paolo.valente@linaro.org>,
	Avri Altman <Avri.Altman@sandisk.com>
Subject: Re: [PATCH 00/12 v4] multiqueue for MMC/SD
Date: Fri, 27 Oct 2017 15:59:57 +0300	[thread overview]
Message-ID: <f9e25794-7619-bc61-7e90-f09e73853deb@intel.com> (raw)
In-Reply-To: <CACRpkdb795LnNkpo=Jg_fnn1mJwnE0A+FXwj5NjcUqCDhp32Ww@mail.gmail.com>

On 27/10/17 14:25, Linus Walleij wrote:
> On Thu, Oct 26, 2017 at 9:27 PM, Hunter, Adrian <adrian.hunter@intel.com> wrote:
>> On Thu, Oct 26, 2017 at 3:34 PM, Adrian Hunter <adrian.hunter@intel.com>
> 
>>> What I mean is that the CQE code will likely look better on top of these
>>> refactorings.
>>>
>>> But as I say it is a matter of taste. I just love the looks of my own code as
>>> much as the next programmer so I can't judge that. Let's see what the
>>> reviewers say.
>>
>> It doesn't look ready.  There seems still to be 2 task switches between
>> each transfer.
> 
> IIUC there is a task in blk-mq submitting the requests, and that goes
> all the way to starting it on the host. Then as an interrupt comes back
> I kick the worker and that reports back to the block layer. So that
> is one task switch per request and then one switch back to the MQ
> layer.
> 
> But I am experimenting as we speak to get rid of the worker
> for all simple cases that don't require retune or BKOPS and that's
> pretty cool if it can be made to work :)

Well the CQE patches already do that.

> 
>> mmc_blk_rw_done_error() is still using the messy error
>> handling and doesn’t handle retries e.g. 'retry' is a local variable so it can't
>> count the number of retries now that there is no loop.
> 
> Right! Nice catch.
> 
> I will update the patch and put that in the struct mmc_async_req so
> it survives the retry rounds.
> 
>>>>> It sounds simple but I bet this drives a truck through Adrians patch
>>>>> series. Sorry. :(
>>>>
>>>> I waited a long time for your patches but I had to give up waiting
>>>> when Ulf belated insisted on blk-mq before CQE.  I am not sure what
>>>> you are expecting now it seems too late.
>>>
>>> Too late for what? It's just a patch set, I don't really have a deadline for this or
>>> anything. As I explained above I have been working on this all the time, the
>>> problem was that I was/am not smart enough to find that solution for host
>>> claiming by context.
>>
>> Too late to go before CQE.  All the blk-mq work is now in the CQE patchset.
> 
> You seem to have an either/or stance.
> 
> Mine if more of a both/and.
> 
> It is not even necessary to have one set of these patches on
> top of each other, they can also be mixed in some order.
> 
> A lot of factors influence this I think, like structure of code and
> maintainability, performance, block layer interaction semantics,
> etc etc.
> 
> We definately need input from Ulf and Bartlomiej (who was actually
> the first to work on MQ for MMC/SD).
> 
>>> The host claiming by context was merged a month ago and now I have
>>> understood that I can use that and rebased my patches on it. Slow learner, I
>>> guess.
>>>
>>> If you feel it is ungrateful that you have put in so much work and things are
>>> not getting merged, and you feel your patches deserve to be merged first
>>> (because of human nature reasons) I can empathize with that. It's sad that
>>> your patches are at v12. Also I see that patch 4 bears the signoffs of a
>>> significant team at CodeAurora, so they are likely as impatient.
>>
>> It is important that you understand that this has nothing to do with
>> "human nature reasons".
> 
> You do come across as a bit hard-headed.
> 
> But I think it is better to focus on the code per se.
> 
> I would suggest we go and review each others patch series to
> learn from each codebase what was done in a good way for the
> MMC/SD stack and what was not, you tossed out a nice review
> comment above for example.

I can make a few more comments about what else is broken.  Have you tried
suspend / resume?  At a glance, it looks like you are trying to use
blk_stop_queue() which is not a blk-mq function.

> 
>>  Linux distributions use upstream kernels.
>> ChromeOS  has an "upstream first" policy.  Delaying features for long
>> periods has real-world consequences.  When people ask, what kernel
>> should they use, we expect to reply, just use mainline.
> 
> We are in violent agreement.
> 
> I take it that you are working on ChromeOS context and that since
> they have this policy, they, through their influence over Intel as a
> supplier is putting heavy pressure on you personally to get this
> merged.
> 
> Is that correctly understood?

No.  We just expect to base everything on mainline.

> 
> That would explain your increasing pushing to get this
> upstream pretty well, especially if you have tech leads and
> managers hovering over your shoulder every week asking how
> the CQE upstream work is progressing.
> 
> It is indeed tough to juggle this with the pressure to "upstream
> first" the BFQ scheduler policy that we are working on in Linaro
> to increase interactivity. We need to enable this on devices
> pronto and that means migrating MMC/SD to MQ and MQ only.
> I have shared this motivation since the start, so it should come
> as no surprise.

IMHO BFQ is just another example of unnecessary delay.

> 
> So I also have some pressure to "Get This Feature In Now".

It has nothing to do with pressure.  It is about what is reasonable.
Features should go in as soon as they are ready.  Ideally queued up in the
same release cycle they are submitted.  If the code doesn't work right, then
it can't go in straight away, but fake reasons for delaying things needs to
stop.