linux-kernel - Re: [RFC PATCH 1/7] blk-mq: Export blk_mq_hctx_has

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Date:   Tue, 23 Jul 2019 11:31:04 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Orson Zhai <orsonzhai@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-block@...r.kernel.org
Subject: Re: [RFC PATCH 1/7] blk-mq: Export blk_mq_hctx_has_pending() function

On Tue, Jul 23, 2019 at 11:12:57AM +0800, Baolin Wang wrote:
> Hi Ming,
> 
> On Mon, 22 Jul 2019 at 22:19, Ming Lei <ming.lei@...hat.com> wrote:
> >
> > On Mon, Jul 22, 2019 at 09:09:36PM +0800, Baolin Wang wrote:
> > > Some SD/MMC host controllers can support packed command or packed request,
> > > that means we can package several requests to host controller at one time
> > > to improve performence. And this patch set will introduce MMC packed function
> > > to support this feature by following patches.
> > >
> > > To support MMC packed function, the MMC layer need to know if there are
> > > requests are pending now in hardware queue to help to combine requests
> > > as much as possible. If we know there are requests pending in hardware
> > > queue, then we should not package requests to host controller immediately,
> > > instead we should collect more requests into MMC packed queue to be packed
> > > to host controller with packed condition.
> > >
> > > Thus export this function for MMC packed function.
> > >
> > > Signed-off-by: Baolin Wang <baolin.wang@...aro.org>
> > > ---
> > >  block/blk-mq.c         |    3 ++-
> > >  include/linux/blk-mq.h |    1 +
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > index b038ec6..5bd4ef9 100644
> > > --- a/block/blk-mq.c
> > > +++ b/block/blk-mq.c
> > > @@ -63,12 +63,13 @@ static int blk_mq_poll_stats_bkt(const struct request *rq)
> > >   * Check if any of the ctx, dispatch list or elevator
> > >   * have pending work in this hardware queue.
> > >   */
> > > -static bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx)
> > > +bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx)
> > >  {
> > >       return !list_empty_careful(&hctx->dispatch) ||
> > >               sbitmap_any_bit_set(&hctx->ctx_map) ||
> > >                       blk_mq_sched_has_work(hctx);
> > >  }
> > > +EXPORT_SYMBOL_GPL(blk_mq_hctx_has_pending);
> >
> > Just wondering why you don't use the 'last' field of 'struct blk_mq_queue_data',
> > which is passed to .queue_rq(), and supposed for implementing batch submission.
> 
> The 'last' field of 'struct blk_mq_queue_data' does not indicate the
> last request in the hardware queue, since we want to collect more
> requests from block layer as much as possible to be packed later.
> 
> And from blk_mq_do_dispatch_sched()--->blk_mq_dispatch_rq_list()--->
> queue_rq(), I always get 'bd.last = true', which is not useful to
> combine requests for MMC packed queue. Maybe I missed anything?

That is one flaw of current implementation, and we may improve it,
so other drivers(virtio-io, ...) can benefit from it too.


Thanks,
Ming