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 12:20:55 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Aaron Lu <aaron.lu@...el.com>
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 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?

> > disabled. It gets even worse if it's from an interrupt context such as
> > jumbo frame allocation or a high-order slab allocation that is atomic.
> 
> My understanding is atomic allocation won't trigger compaction, no?
> 

No, they can't. I didn't check properly but be wary of any possibility
whereby interrupts can get delayed in zone_wait_cluster_alloc. I didn't
go back and check if it can -- partially because I'm more focused on the
lazy buddy aspect at the moment.

> > It may be necessary to consider instead minimising the number
> > of struct page update when merging to PCP and then either increasing the
> > size of the PCP or allowing it to exceed pcp->high for short periods of
> > time to batch the struct page updates.
> 
> I don't quite follow this part. It doesn't seem possible we can exceed
> pcp->high in allocation path, or are you talking about free path?
> 

I'm talking about the free path.

> And thanks a lot for the review!

My pleasure, hope it helps.

-- 
Mel Gorman
SUSE Labs