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SubjectRe: [PATCH v7 1/4] mmc: Add MMC host software queue support
Hi Ulf,

On Fri, Nov 22, 2019 at 6:42 PM Baolin Wang <baolin.wang7@gmail.com> wrote:
>
> Hi Arnd,
>
> On Fri, Nov 22, 2019 at 6:32 PM Arnd Bergmann <arnd@arndb.de> wrote:
> >
> > On Mon, Nov 18, 2019 at 11:43 AM Baolin Wang <baolin.wang7@gmail.com> wrote:
> > >
> > > From: Baolin Wang <baolin.wang@linaro.org>
> > >
> > > Now the MMC read/write stack will always wait for previous request is
> > > completed by mmc_blk_rw_wait(), before sending a new request to hardware,
> > > or queue a work to complete request, that will bring context switching
> > > overhead, especially for high I/O per second rates, to affect the IO
> > > performance.
> > >
> > > Thus this patch introduces MMC software queue interface based on the
> > > hardware command queue engine's interfaces, which is similar with the
> > > hardware command queue engine's idea, that can remove the context
> > > switching. Moreover we set the default queue depth as 32 for software
> > > queue, which allows more requests to be prepared, merged and inserted
> > > into IO scheduler to improve performance, but we only allow 2 requests
> > > in flight, that is enough to let the irq handler always trigger the
> > > next request without a context switch, as well as avoiding a long latency.
> > >
> > > From the fio testing data in cover letter, we can see the software
> > > queue can improve some performance with 4K block size, increasing
> > > about 16% for random read, increasing about 90% for random write,
> > > though no obvious improvement for sequential read and write.
> > >
> > > Moreover we can expand the software queue interface to support MMC
> > > packed request or packed command in future.
> > >
> > > Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
> > > Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
> >
> > Overall, this looks like enough of a win that I think we should just
> > use the current version for the moment, while still working on all the
> > other improvements.
> >
> > My biggest concern is the naming of "software queue", which is
> > a concept that runs against the idea of doing all the heavy lifting,
> > in particular the queueing in bfq.
> >
> > Then again, it does not /actually/ do much queuing at all, beyond
> > preparing a single request so it can fire it off early. Even with the
> > packed command support added in, there is not really any queuing
> > beyond what it has to do anyway.
>
> Yes. But can not find any better name until now and 'software queue'
> was suggested by Adrian.
>
> >
> > Using the infrastructure that was added for cqe seems like a good
> > compromise, as this already has a way to hand down multiple
> > requests to the hardware and is overall more modern than the
> > existing support.
> >
> > I still think we should do all the other things I mentioned in my
> > earlier reply today, but they can be done as add-ons:
> >
> > - remove all blocking calls from the queue_rq() function:
> > partition-change, retune, etc should become non-blocking
> > operations that return busy in the queue_rq function.
> >
> > - get bfq to send down multiple requests all the way into
> > the device driver, so we don't have to actually queue them
> > here at all to do packed commands
> >
> > - add packed command support
> >
> > - submit cmds from hardirq context if this is advantageous,
> > and move everything else in the irq handler into irqthread
> > context in order to remove all other workqueue and softirq
> > processing from the request processing path.
> >
> > If we can agree on this as the rough plan for the future,
> > feel free to add my
>
> Yes, I agree with your plan. Thast's what we should do in future.
>
> >
> > Reviewed-by: Arnd Bergmann <arnd@arndb.de>
>
> Thanks for your reviewing and good suggestion.
>
> Ulf,
>
> I am not sure if there is any chance to merge this patch set into
> V5.5, I've tested for a long time and did not find any resession.
> Thanks.

Could you apply this patchset if no objection from your side? Or do
you need me to rebase and resend? Thanks.

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