linux-ext4 - Re: [PATCH v5 1/6] libnvdimm: nd

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Date:   Mon, 22 Apr 2019 12:44:21 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
        Pankaj Gupta <pagupta@...hat.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        KVM list <kvm@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Qemu Developers <qemu-devel@...gnu.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Ross Zwisler <zwisler@...nel.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        lcapitulino@...hat.com, Kevin Wolf <kwolf@...hat.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Nitesh Narayan Lal <nilal@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        david <david@...morbit.com>, cohuck@...hat.com,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kilobyte@...band.pl,
        yuval shaia <yuval.shaia@...cle.com>
Subject: Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support

On Mon, Apr 22, 2019 at 8:59 AM Jeff Moyer <jmoyer@...hat.com> wrote:
>
> Dan Williams <dan.j.williams@...el.com> writes:
>
> > On Thu, Apr 18, 2019 at 9:18 AM Christoph Hellwig <hch@...radead.org> wrote:
> >>
> >> On Thu, Apr 18, 2019 at 09:05:05AM -0700, Dan Williams wrote:
> >> > > > I'd either add a comment about avoiding retpoline overhead here or just
> >> > > > make ->flush == NULL mean generic_nvdimm_flush(). Just so that people don't
> >> > > > get confused by the code.
> >> > >
> >> > > Isn't this premature optimization?  I really don't like adding things
> >> > > like this without some numbers to show it's worth it.
> >> >
> >> > I don't think it's premature given this optimization technique is
> >> > already being deployed elsewhere, see:
> >> >
> >> > https://lwn.net/Articles/774347/
> >>
> >> For one this one was backed by numbers, and second after feedback
> >> from Linux we switched to the NULL pointer check instead.
> >
> > Ok I should have noticed the switch to NULL pointer check. However,
> > the question still stands do we want everyone to run numbers to
> > justify this optimization, or make it a new common kernel coding
> > practice to do:
> >
> >     if (!object->op)
> >         generic_op(object);
> >     else
> >         object->op(object);
> >
> > ...in hot paths?
>
> I don't think nvdimm_flush is a hot path.  Numbers of some
> representative workload would prove one of us right.

I'd rather say that the if "if (!op) do_generic()" pattern is more
readable in the general case, saves grepping for who set the op in the
common case. The fact that it has the potential to be faster is gravy
at that point.