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From: Jeff Moyer <jmoyer@redhat.com>
Subject: Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support
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Date: Mon, 22 Apr 2019 11:51:35 -0400
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        (Dan Williams's message of "Thu, 18 Apr 2019 11:14:59 -0700")
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To: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@infradead.org>, 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>, virtualization@lists.linux-foundation.org, KVM list <kvm@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Linux ACPI <linux-acpi@vger.kernel.org>, 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>, "Rafael J. Wysocki" <rjw@rjwysocki.net>, Len Brown <lenb@kernel.org>, Theodore Ts'o <tytso@mit.edu>, Andreas Dilger <adilger.kernel@dilger.ca>, "Darrick J. Wong" <darrick.wong@oracle.com>, lcapitulino@redhat.com, Kevin Wolf <kwolf@redhat.com>, Igor Mammedov <imammedo@redhat.com>, Nitesh Narayan Lal <nilal@redhat.com>, Rik van Riel <riel@surriel.com>, Stefan Hajnoczi <stefanha@redhat.com>, Andrea Arcangeli <aarcange@redhat.com>, David Hildenbrand <david@redhat.com>, david <david@fromorbit.com>, cohuck@redhat.com, Xiao Guangrong <xiaoguangrong.eric@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, kilobyte@angband.pl, yuval shaia <yuval.shaia@oracle.com>

Dan Williams <dan.j.williams@intel.com> writes:

> 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 don't think nvdimm_flush is a hot path.  Numbers of some
representative workload would prove one of us right.

-Jeff