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Date:   Thu, 18 Apr 2019 12:10:44 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     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>,
        Christoph Hellwig <hch@...radead.org>,
        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

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

> On Fri, Apr 12, 2019 at 6:12 AM Jeff Moyer <jmoyer@...hat.com> wrote:
>>
>> Jan Kara <jack@...e.cz> writes:
>>
>> > On Thu 11-04-19 07:51:48, Dan Williams wrote:
>> >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta <pagupta@...hat.com> wrote:
>> >> > +       } else {
>> >> > +               if (nd_region->flush(nd_region))
>> >> > +                       rc = -EIO;
>> >>
>> >> Given the common case wants to be fast and synchronous I think we
>> >> should try to avoid retpoline overhead by default. So something like
>> >> this:
>> >>
>> >> if (nd_region->flush == generic_nvdimm_flush)
>> >>     rc = generic_nvdimm_flush(...);
>> >
>> > 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/

The technique is fine, but that doesn't mean it should be applied
everywhere.  Is *this* code path really going to benefit from the
optimization?

-Jeff

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