Re: [PATCH 2/2] nvme: use blk-mq polling for uring commands

From: Kanchan Joshi <joshiiitr@gmail.com>
To: Keith Busch <kbusch@kernel.org>
Cc: Kanchan Joshi <joshi.k@samsung.com>,
	Keith Busch <kbusch@meta.com>,
	linux-block@vger.kernel.org, linux-nvme@lists.infradead.org,
	axboe@kernel.dk, hch@lst.de
Subject: Re: [PATCH 2/2] nvme: use blk-mq polling for uring commands
Date: Mon, 3 Apr 2023 18:12:55 +0530	[thread overview]
Message-ID: <CA+1E3r+OrN-sFQ5TGC238Km=KurOZBvx+1ZeLXA=tsrgZZo8Ag@mail.gmail.com> (raw)
In-Reply-To: <ZCRjGmPCeLvi2m39@kbusch-mbp.dhcp.thefacebook.com>

On Wed, Mar 29, 2023 at 9:41 PM Keith Busch <kbusch@kernel.org> wrote:
>
> On Wed, Mar 29, 2023 at 02:16:18PM +0530, Kanchan Joshi wrote:
> > On Tue, Mar 28, 2023 at 08:52:53AM -0600, Keith Busch wrote:
> > > > > +       else if (issue_flags & IO_URING_F_IOPOLL)
> > > > > +               ioucmd->flags |= IORING_URING_CMD_NOPOLL;
> > > >
> > > > If IO_URING_F_IOPOLL would have come here as part of "ioucmd->flags", we
> > > > could have just cleared that here. That would avoid the need of NOPOLL flag.
> > > > That said, I don't feel strongly about new flag too. You decide.
> > >
> > > IO_URING_F_IOPOLL, while named in an enum that sounds suspiciouly like it is
> > > part of ioucmd->flags, is actually ctx flags, so a little confusing. And we
> > > need to be a litle careful here: the existing ioucmd->flags is used with uapi
> > > flags.
> >
> > Indeed. If this is getting crufty, series can just enable polling on
> > no-payload requests. Reducing nvme handlers - for another day.
>
> Well something needs to be done about multipath since it's broken today: if the
> path changes between submission and poll, we'll consult the wrong queue for
> polling enabled. This could cause a missed polling opprotunity, polling a
> pointer that isn't a bio, or poll an irq enabled cq. All are bad.

I see, that is because "nvme_find_path" was used.
How about using top 4 bits of "ioucmd->flags" for an internal flag.
That means we can support max 28 flags for "sqe->uring_cmd_flags".
That perhaps is not too bad, given that we use one bit at the moment?