netdev - Re: in_irq_or_nmi() and RFC patch

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Date:   Thu, 30 Mar 2017 11:46:50 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Pankaj Gupta <pagupta@...hat.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tariq Toukan <tariqt@...lanox.com>, netdev@...r.kernel.org,
        akpm@...ux-foundation.org, linux-mm <linux-mm@...ck.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        brouer@...hat.com
Subject: Re: in_irq_or_nmi() and RFC patch

On Thu, 30 Mar 2017 09:35:02 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Mar 30, 2017 at 09:12:23AM +0200, Jesper Dangaard Brouer wrote:
> > On Thu, 30 Mar 2017 08:49:58 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> >   
> > > On Wed, Mar 29, 2017 at 09:44:41PM +0200, Jesper Dangaard Brouer wrote:  
> > > > @@ -2481,7 +2481,11 @@ void free_hot_cold_page(struct page *page, bool cold)
> > > >  	unsigned long pfn = page_to_pfn(page);
> > > >  	int migratetype;
> > > >  
> > > > -	if (in_interrupt()) {
> > > > +	/*
> > > > +	 * Exclude (hard) IRQ and NMI context from using the pcplists.
> > > > +	 * But allow softirq context, via disabling BH.
> > > > +	 */
> > > > +	if (in_irq() || irqs_disabled()) {    
> > > 
> > > Why do you need irqs_disabled() ?   
> > 
> > Because further down I call local_bh_enable(), which calls
> > __local_bh_enable_ip() which triggers a warning during early boot on:
> > 
> >   WARN_ON_ONCE(in_irq() || irqs_disabled());
> > 
> > It looks like it is for supporting CONFIG_TRACE_IRQFLAGS.  
> 
> Ah, no. Its because when you do things like:
> 
> 	local_irq_disable();
> 	local_bh_enable();
> 	local_irq_enable();
> 
> you can loose a pending softirq.
> 
> Bugger.. that irqs_disabled() is something we could do without.

Yes, I really don't like adding this irqs_disabled() check here.

> I'm thinking that when tglx finishes his soft irq disable patches for
> x86 (same thing ppc also does) we can go revert all these patches.
> 
> Thomas, see:
> 
>   https://lkml.kernel.org/r/20170301144845.783f8cad@redhat.com

The summary is Mel and I found a way to optimized the page allocator,
by avoiding a local_irq_{save,restore} operation, see commit
374ad05ab64d ("mm, page_alloc: only use per-cpu allocator for irq-safe
requests")  [1] https://git.kernel.org/davem/net-next/c/374ad05ab64d696

But Tariq discovered that this caused a regression for 100Gbit/s NICs,
as the patch excluded softirq from using the per-cpu-page (PCP) lists.
As DMA RX page-refill happens in softirq context.

Now we are trying to re-enable allowing softirq to use the PCP.
My proposal is: https://lkml.kernel.org/r/20170329214441.08332799@redhat.com
The alternative is to revert this optimization.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer