linux-kernel - Re: [PATCH 00/13] convert block layer to bioset_init()/mempool

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Message-ID: <20180521233855.GI11495@kmo-pixel>
Date:   Mon, 21 May 2018 19:38:55 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, hch@...radead.org, colyli@...e.de,
        darrick.wong@...cle.com, clm@...com, bacik@...com,
        linux-xfs@...r.kernel.org, drbd-dev@...ts.linbit.com,
        linux-btrfs@...r.kernel.org, linux-raid@...r.kernel.org,
        neilb@...e.com
Subject: Re: [PATCH 00/13] convert block layer to bioset_init()/mempool_init()

On Mon, May 21, 2018 at 02:24:32PM -0400, Mike Snitzer wrote:
> Every single data structure change in this series should be reviewed for
> unforeseen alignment consequences.  Jens seemed to say that is
> worthwhile.  Not sure if he'll do it or we divide it up.  If we divide
> it up a temp topic branch should be published for others to inspect.
> 
> Could be alignment hasn't been a historic concern for a bunch of the
> data structures changed in this series.. if so then all we can do is fix
> up any obvious potential for false sharing.

Honestly, I almost never worry about alignment... the very few times I do care,
I use __cacheline_aligned_in_smp.

If alignment is a concern in any of those structs, there really ought to be a
comment indicating it. I very much doubt anything I touched was performance
sensitive enough for it to be an issue, though. And if there is a performance
impact, it should be oughtweighed by the reduced pointer chasing.

If you disagree, I don't mind leaving the device mapper patch out, it really
makes no difference to me. I could glance over for alignment issues but I feel
like my analysis would not be terribly valuable to you considering I've already
said my position on alignment is "meh, don't care" :)