netdev - Re: Page allocator order-0 optimizations merged

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Message-ID: <20170327133212.6azfgrariwocdzzd@techsingularity.net>
Date:   Mon, 27 Mar 2017 14:32:12 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Pankaj Gupta <pagupta@...hat.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>, netdev@...r.kernel.org,
        akpm@...ux-foundation.org, linux-mm <linux-mm@...ck.org>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: Page allocator order-0 optimizations merged

On Mon, Mar 27, 2017 at 02:39:47PM +0200, Jesper Dangaard Brouer wrote:
> On Mon, 27 Mar 2017 10:55:14 +0200
> Jesper Dangaard Brouer <brouer@...hat.com> wrote:
> 
> > A possible solution, would be use the local_bh_{disable,enable} instead
> > of the {preempt_disable,enable} calls.  But it is slower, using numbers
> > from [1] (19 vs 11 cycles), thus the expected cycles saving is 38-19=19.
> > 
> > The problematic part of using local_bh_enable is that this adds a
> > softirq/bottom-halves rescheduling point (as it checks for pending
> > BHs).  Thus, this might affects real workloads.
> 
> I implemented this solution in patch below... and tested it on mlx5 at
> 50G with manually disabled driver-page-recycling.  It works for me.
> 
> To Mel, that do you prefer... a partial-revert or something like this?
> 

If Tariq confirms it works for him as well, this looks far safer patch
than having a dedicate IRQ-safe queue. Your concern about the BH
scheduling point is valid but if it's proven to be a problem, there is
still the option of a partial revert.

-- 
Mel Gorman
SUSE Labs