Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Date:   Fri, 13 May 2022 15:23:30 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 0/6] Drain remote per-cpu directly v3
Message-ID: <20220513142330.GI3441@techsingularity.net>
References: <20220512085043.5234-1-mgorman@techsingularity.net>
 <20220512124325.751781bb88ceef5c37ca653e@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-15
Content-Disposition: inline
In-Reply-To: <20220512124325.751781bb88ceef5c37ca653e@linux-foundation.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

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@techsingularity.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