netdev - Re: Page allocator order-0 optimizations merged

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Date:   Tue, 28 Mar 2017 10:29:10 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Tariq Toukan <ttoukan.linux@...il.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Pankaj Gupta <pagupta@...hat.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>, brouer@...hat.com
Subject: Re: Page allocator order-0 optimizations merged

On Tue, 28 Mar 2017 10:32:19 +0300
Tariq Toukan <ttoukan.linux@...il.com> wrote:

> On 27/03/2017 4:32 PM, Mel Gorman wrote:
> > 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  
> 
> Great.
> I will test Jesper's patch today in the afternoon.

Good to hear :-)

> > 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.

I wanted to evaluate my own BH scheduling point concern, but I could
not, because I ran into a softirq acct regression (which I bisected
see[1]).  AFAIK this should not affect Tariq's multi-TCP-stream test
(netperf TCP stream testing works fine on my testlab).

[1] http://lkml.kernel.org/r/20170328101403.34a82fbf@redhat.com
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer