netdev - Re: Page allocator order-0 optimizations merged

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Message-ID: <20170323145133.twzt4f5ci26vdyut@techsingularity.net>
Date:   Thu, 23 Mar 2017 14:51:33 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Tariq Toukan <tariqt@...lanox.com>,
        "netdev@...r.kernel.org" <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 Thu, Mar 23, 2017 at 02:43:47PM +0100, Jesper Dangaard Brouer wrote:
> On Wed, 22 Mar 2017 23:40:04 +0000
> Mel Gorman <mgorman@...hsingularity.net> wrote:
> 
> > On Wed, Mar 22, 2017 at 07:39:17PM +0200, Tariq Toukan wrote:
> > > > > > This modification may slow allocations from IRQ context slightly
> > > > > > but the
> > > > > > main gain from the per-cpu allocator is that it scales better for
> > > > > > allocations from multiple contexts.  There is an implicit
> > > > > > assumption that
> > > > > > intensive allocations from IRQ contexts on multiple CPUs from a single
> > > > > > NUMA node are rare  
> > > Hi Mel, Jesper, and all.
> > > 
> > > This assumption contradicts regular multi-stream traffic that is naturally
> > > handled
> > > over close numa cores.  I compared iperf TCP multistream (8 streams)
> > > over CX4 (mlx5 driver) with kernels v4.10 (before this series) vs
> > > kernel v4.11-rc1 (with this series).
> > > I disabled the page-cache (recycle) mechanism to stress the page allocator,
> > > and see a drastic degradation in BW, from 47.5 G in v4.10 to 31.4 G in
> > > v4.11-rc1 (34% drop).
> > > I noticed queued_spin_lock_slowpath occupies 62.87% of CPU time.  
> > 
> > Can you get the stack trace for the spin lock slowpath to confirm it's
> > from IRQ context?
> 
> AFAIK allocations happen in softirq.  Argh and during review I missed
> that in_interrupt() also covers softirq.  To Mel, can we use a in_irq()
> check instead?
> 
> (p.s. just landed and got home)

Not built or even boot tested. I'm unable to run tests at the moment

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6cbde310abed..f82225725bc1 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2481,7 +2481,7 @@ void free_hot_cold_page(struct page *page, bool cold)
 	unsigned long pfn = page_to_pfn(page);
 	int migratetype;
 
-	if (in_interrupt()) {
+	if (in_irq()) {
 		__free_pages_ok(page, 0);
 		return;
 	}
@@ -2647,7 +2647,7 @@ static struct page *__rmqueue_pcplist(struct zone *zone, int migratetype,
 {
 	struct page *page;
 
-	VM_BUG_ON(in_interrupt());
+	VM_BUG_ON(in_irq());
 
 	do {
 		if (list_empty(list)) {
@@ -2704,7 +2704,7 @@ struct page *rmqueue(struct zone *preferred_zone,
 	unsigned long flags;
 	struct page *page;
 
-	if (likely(order == 0) && !in_interrupt()) {
+	if (likely(order == 0) && !in_irq()) {
 		page = rmqueue_pcplist(preferred_zone, zone, order,
 				gfp_flags, migratetype);
 		goto out;

-- 
Mel Gorman
SUSE Labs