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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 18 Apr 2019 11:14:59 -0700
Message-ID: <CAPcyv4jPOF_+R6U17J_+88+X8LC-LSPCugA6gbKxuphuuFB2gg@mail.gmail.com>
Subject: Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Jeff Moyer <jmoyer@redhat.com>, Jan Kara <jack@suse.cz>,
        Pankaj Gupta <pagupta@redhat.com>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
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        Qemu Developers <qemu-devel@nongnu.org>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        Ross Zwisler <zwisler@kernel.org>,
        Vishal L Verma <vishal.l.verma@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
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        Len Brown <lenb@kernel.org>, "Theodore Ts'o" <tytso@mit.edu>,
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        Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
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        yuval shaia <yuval.shaia@oracle.com>
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On Thu, Apr 18, 2019 at 9:18 AM Christoph Hellwig <hch@infradead.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.