From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH v2 4/6] block: switch to per-cpu in-flight
	counters
Date: Wed, 5 Dec 2018 13:18:33 -0500
Message-ID: <20181205181832.GA10058@redhat.com>
References: <20181130222226.77216-1-snitzer@redhat.com>
	<20181130222226.77216-5-snitzer@redhat.com>
	<d6a741cd-ea75-19d0-13e3-eb1321e617b4@kernel.dk>
	<20181205174942.GA9838@redhat.com>
	<eb184c39-9d60-0fa3-0d27-7af2f78d601e@kernel.dk>
	<20181205180347.GA9966@redhat.com>
	<779ca97e-3353-5fd0-9899-73de8a0b2cad@kernel.dk>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Return-path: <dm-devel-bounces@redhat.com>
Content-Disposition: inline
In-Reply-To: <779ca97e-3353-5fd0-9899-73de8a0b2cad@kernel.dk>
List-Unsubscribe: <https://www.redhat.com/mailman/options/dm-devel>,
	<mailto:dm-devel-request@redhat.com?subject=unsubscribe>
List-Archive: <https://www.redhat.com/archives/dm-devel>
List-Post: <mailto:dm-devel@redhat.com>
List-Help: <mailto:dm-devel-request@redhat.com?subject=help>
List-Subscribe: <https://www.redhat.com/mailman/listinfo/dm-devel>,
	<mailto:dm-devel-request@redhat.com?subject=subscribe>
Sender: dm-devel-bounces@redhat.com
Errors-To: dm-devel-bounces@redhat.com
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org, dm-devel@redhat.com, Mikulas Patocka <mpatocka@redhat.com>
List-Id: dm-devel.ids

On Wed, Dec 05 2018 at  1:04pm -0500,
Jens Axboe <axboe@kernel.dk> wrote:

> On 12/5/18 11:03 AM, Mike Snitzer wrote:
> > On Wed, Dec 05 2018 at 12:54pm -0500,
> > Jens Axboe <axboe@kernel.dk> wrote:
> > 
> >> On 12/5/18 10:49 AM, Mike Snitzer wrote:
> >>> On Wed, Dec 05 2018 at 12:30pm -0500,
> >>> Jens Axboe <axboe@kernel.dk> wrote:
> >>>
> >>>> There's also no need to pass in the cpu, if we're not running with
> >>>> preempt disabled already we have a problem. 
> >>>
> >>> Why should this be any different than the part_stat_* interfaces?
> >>> __part_stat_add(), part_stat_read(), etc also use
> >>> per_cpu_ptr((part)->dkstats, (cpu) accessors.
> >>
> >> Maybe audit which ones actually need it? To answer the specific question,
> >> it's silly to pass in the cpu, if we're pinned already. That's true
> >> both programatically, but also for someone reading the code.
> > 
> > I understand you'd like to avoid excess interface baggage.  But seems to
> > me we'd be better off being consistent, when extending the percpu
> > portion of block core stats, and then do an incremental to clean it all
> > up.
> 
> The incremental should be done first in that case, it'd be silly to
> introduce something only to do a cleanup right after.

OK, all existing code for these percpu stats should follow the pattern:

  int cpu = part_stat_lock();

  <do percpu diskstats stuff>

  part_stat_unlock();

part_stat_lock() calls get_cpu() which does preempt_disable().  So to
your point: yes we have preempt disabled.  And yes we _could_ just use
smp_processor_id() in callers rather than pass 'cpu' to them.

Is that what you want to see?

Mike