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From: Mel Gorman <mgorman@techsingularity.net>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linux-MM <linux-mm@kvack.org>,
	Linux-RT-Users <linux-rt-users@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH 2/6] mm/page_alloc: Convert per-cpu list protection to local_lock
Date: Wed, 31 Mar 2021 21:42:10 +0100	[thread overview]
Message-ID: <20210331204210.GB3697@techsingularity.net> (raw)
In-Reply-To: <871rbv5iel.ffs@nanos.tec.linutronix.de>

On Wed, Mar 31, 2021 at 07:42:42PM +0200, Thomas Gleixner wrote:
> On Wed, Mar 31 2021 at 12:01, Mel Gorman wrote:
> > On Wed, Mar 31, 2021 at 11:55:56AM +0200, Thomas Gleixner wrote:
> > @@ -887,13 +887,11 @@ void cpu_vm_stats_fold(int cpu)
> >  
> >  		pzstats = per_cpu_ptr(zone->per_cpu_zonestats, cpu);
> >  
> > -		preempt_disable();
> >  		for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++)
> >  			if (pzstats->vm_stat_diff[i]) {
> >  				int v;
> >  
> > -				v = pzstats->vm_stat_diff[i];
> > -				pzstats->vm_stat_diff[i] = 0;
> > +				v = this_cpu_xchg(pzstats->vm_stat_diff[i], 0);
> 
> Confused. pzstats is not a percpu pointer. zone->per_cpu_zonestats is.
> 
> But @cpu is not necessarily the current CPU.
> 

I was drinking drain cleaner instead of coffee. The code was also broken
to begin with.

drain_pages() is draining pagesets of a local or dead CPU. For a local
CPU, disabling IRQs prevent an IRQ arriving during the drain, trying to
allocate a page and potentially corrupt the local pageset -- ok.

zone_pcp_reset is accessing a remote CPUs pageset, freeing the percpu
pointer and resetting it to boot_pageset. zone_pcp_reset calling
local_irq_save() does not offer any special protection against
drain_pages because there are two separate IRQs involved.

This particular patch may have no reason to touch zone_pcp_reset,
cpu_vm_stats_fold or drain_zonestat at all but I need to think about it
more tomorrow.

-- 
Mel Gorman
SUSE Labs


  parent reply	other threads:[~2021-03-31 20:42 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-29 12:06 [RFC PATCH 0/6] Use local_lock for pcp protection and reduce stat overhead Mel Gorman
2021-03-29 12:06 ` [PATCH 1/6] mm/page_alloc: Split per cpu page lists and zone stats Mel Gorman
2021-03-29 12:06 ` [PATCH 2/6] mm/page_alloc: Convert per-cpu list protection to local_lock Mel Gorman
2021-03-31  9:55   ` Thomas Gleixner
2021-03-31 11:01     ` Mel Gorman
2021-03-31 17:42       ` Thomas Gleixner
2021-03-31 17:46         ` Thomas Gleixner
2021-03-31 20:42         ` Mel Gorman [this message]
2021-03-29 12:06 ` [PATCH 3/6] mm/vmstat: Convert NUMA statistics to basic NUMA counters Mel Gorman
2021-03-29 12:06 ` [PATCH 4/6] mm/vmstat: Inline NUMA event counter updates Mel Gorman
2021-03-29 12:06 ` [PATCH 5/6] mm/page_alloc: Batch the accounting updates in the bulk allocator Mel Gorman
2021-03-29 12:06 ` [PATCH 6/6] mm/page_alloc: Reduce duration that IRQs are disabled for VM counters Mel Gorman
2021-03-30 18:51 ` [RFC PATCH 0/6] Use local_lock for pcp protection and reduce stat overhead Jesper Dangaard Brouer
2021-03-31  7:38   ` Mel Gorman
2021-03-31  8:17     ` Jesper Dangaard Brouer
2021-03-31  8:52 ` Mel Gorman
2021-03-31  9:51   ` Thomas Gleixner

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