linux-kernel - Re: [PATCH 0/6] Drain remote per-cpu directly v3

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Message-ID: <20220513142330.GI3441@techsingularity.net>
Date:   Fri, 13 May 2022 15:23:30 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 0/6] Drain remote per-cpu directly v3

On Thu, May 12, 2022 at 12:43:25PM -0700, Andrew Morton wrote:
> On Thu, 12 May 2022 09:50:37 +0100 Mel Gorman <mgorman@...hsingularity.net> wrote:
> 
> > Changelog since v2
> > o More conversions from page->lru to page->[pcp_list|buddy_list]
> > o Additional test results in changelogs
> > 
> > Changelog since v1
> > o Fix unsafe RT locking scheme
> > o Use spin_trylock on UP PREEMPT_RT
> > 
> > This series has the same intent as Nicolas' series "mm/page_alloc: Remote
> > per-cpu lists drain support" -- avoid interference of a high priority
> > task due to a workqueue item draining per-cpu page lists. While many
> > workloads can tolerate a brief interruption, it may be cause a real-time
> 
> s/may be/may/
> 
> > task runnning on a NOHZ_FULL CPU to miss a deadline and at minimum,
> 
> s/nnn/nn/
> 

Correct.

> > the draining in non-deterministic.
> 
> s/n/s/;)
> 

Think that one is ok. At least spell check did not complain.

> > Currently an IRQ-safe local_lock protects the page allocator per-cpu lists.
> > The local_lock on its own prevents migration and the IRQ disabling protects
> > from corruption due to an interrupt arriving while a page allocation is
> > in progress. The locking is inherently unsafe for remote access unless
> > the CPU is hot-removed.
> 
> I don't understand the final sentence here.  Which CPU and why does
> hot-removing it make the locking safe?
> 

The sentence can be dropped because it adds little and is potentially
confusing. The PCP being safe to access remotely is specific to the
context of the CPU being hot-removed and there are other special corner
cases like zone_pcp_disable that modifies a per-cpu structure remotely
but not in a way that causes corruption.

-- 
Mel Gorman
SUSE Labs