linux-kernel - Re: [PATCH 08/17] blkcg: shoot down blkio

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Message-ID: <20120123193335.GK12652@google.com>
Date:	Mon, 23 Jan 2012 11:33:35 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org,
	Lennart Poettering <mzxreary@...inter.de>
Subject: Re: [PATCH 08/17] blkcg: shoot down blkio_groups on elevator switch

On Mon, Jan 23, 2012 at 10:43:36AM -0800, Tejun Heo wrote:
> Yeah, this is much more arguable.  I don't think it would be too
> complex to keep per-policy granularity even w/ unified blkg managed by
> blkcg core (we'll just need to point to separately allocated
> per-policy data from the unified blkg and clear them selectively).
> I'm just not convinced of its necessity.  With initial config out of
> the way, elvs and blkcg policies don't get molested all that often.
> 
> I'll see how complex it actually gets.  If it isn't too much
> complexity, yeah, why not...

Hmmm... while this isn't terribly complex, it involves considerable
amount of churn as core layer doesn't currently know what policies are
bound to which queues how - we'll have to add some part of that before
shootdown change, use it there, and then later replace it with proper
per-queue thing.  The conversion is already painful enough without
adding another chunk to juggle around.  Given that clearing all on pol
change isn't too crazy, how about the following?

* For now, clear unconditionally on pol/elv change.

* But structure things such that policy specific data are allocated
  separately and on pol/elv change only those policy specific part is
  flushed.

* Later, if deemed necessary, make the clearing of pol-specific part
  selective.

-- 
tejun
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