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

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Date:   Thu, 18 Apr 2019 11:14:59 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jeff Moyer <jmoyer@...hat.com>, 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 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 agree with not doing premature optimization in
principle, but this hack is minimally intrusive from a readability
perspective similar to likely()/unlikely() usage which also don't come
with numbers on a per patch basis.