linux-kernel - Re: [RFC v4 PATCH 3/5] mm/rmqueue_bulk: alloc without touching individual page structure

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Date:   Thu, 18 Oct 2018 21:21:28 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [RFC v4 PATCH 3/5] mm/rmqueue_bulk: alloc without touching
 individual page structure

On Thu, Oct 18, 2018 at 12:20:55PM +0100, Mel Gorman wrote:
> On Wed, Oct 17, 2018 at 10:23:27PM +0800, Aaron Lu wrote:
> > > RT has had problems with cpu_relax in the past but more importantly, as
> > > this delay for parallel compactions and allocations of contig ranges,
> > > we could be stuck here for very long periods of time with interrupts
> > 
> > The longest possible time is one CPU accessing pcp->batch number cold
> > cachelines. Reason:
> > When zone_wait_cluster_alloc() is called, we already held zone lock so
> > no more allocations are possible. Waiting in_progress to become zero
> > means waiting any CPU that increased in_progress to finish processing
> > their allocated pages. Since they will at most allocate pcp->batch pages
> > and worse case are all these page structres are cache cold, so the
> > longest wait time is one CPU accessing pcp->batch number cold cache lines.
> > 
> > I have no idea if this time is too long though.
> > 
> 
> But compact_zone calls zone_wait_and_disable_cluster_alloc so how is the
> disabled time there bound by pcp->batch?

My mistake, I misunderstood spin_lock_irqsave() and thought lock would
need be acquired before irq is disabled...

So yeah, your concern of possible excessive long irq disabled time here
is true.