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

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Date:   Mon, 16 May 2022 11:53:11 +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 Fri, May 13, 2022 at 12:38:05PM -0700, Andrew Morton wrote:
> > 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.
> 
> OK.  I pasted in your para from the other email.  Current 0/n blurb:
> 
> Some setups, notably NOHZ_FULL CPUs, may be running realtime or
> latency-sensitive applications that cannot tolerate interference due to
> per-cpu drain work queued by __drain_all_pages().  Introduce a new
> mechanism to remotely drain the per-cpu lists.  It is made possible by
> remotely locking 'struct per_cpu_pages' new per-cpu spinlocks.  This has
> two advantages, the time to drain is more predictable and other unrelated
> tasks are not interrupted.
> 
> 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 cause a real-time task running
> on a NOHZ_FULL CPU to miss a deadline and at minimum, the draining is
> non-deterministic.
> 
> 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.
> 
> This series adjusts the locking.  A spinlock is added to struct
> per_cpu_pages to protect the list contents while local_lock_irq continues
> to prevent migration and IRQ reentry.  This allows a remote CPU to safely
> drain a remote per-cpu list.
> 
> This series is a partial series.  Follow-on work should allow the
> local_irq_save to be converted to a local_irq to avoid IRQs being
> disabled/enabled in most cases.  Consequently, there are some TODO
> comments highlighting the places that would change if local_irq was used. 
> However, there are enough corner cases that it deserves a series on its
> own separated by one kernel release and the priority right now is to avoid
> interference of high priority tasks.
> 

Looks good, thanks!

-- 
Mel Gorman
SUSE Labs