linux-kernel - Re: [RFC PATCH] mm: move pcp and lru-pcp drainging into vmstat

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Date:   Wed, 8 Feb 2017 12:31:13 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, Vlastimil Babka <vbabka@...e.cz>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm: move pcp and lru-pcp drainging into vmstat_wq

On Wed, Feb 08, 2017 at 01:03:55PM +0100, Michal Hocko wrote:
> > I don't object to it being actually moved. I have a slight concern that
> > it could somehow starve a vmstat update while frequent drains happen
> > during reclaim though which potentially compounds the problem. It could
> > be offset by a variety of other factors but if it ever is an issue,
> > it'll show up and the paths that really matter check the vmstats
> > directly instead of waiting for an update.
> 
> vmstat updates can tolared delays, that's we we are using deferable
> scheduling in the first place so I am not really worried about that. Any
> user which needs a better precision should use *_snapshot API.
> 

Agreed, we already had cases where deferred vmstat updates had problems
and were resolved by using _snapshot. It's a slight concern only and I'd
be surprised if the _snapshot usage didn't cover it.

> > The altering of the return value in setup_vmstat was mildly surprising as
> > it increases the severity of registering the vmstat callback for memory
> > hotplug so maybe split that out and appears unrelated.
> 
> not sure I understand. What do you mean?
> 

This hunk

@@ -1763,9 +1762,11 @@ static int vmstat_cpu_dead(unsigned int cpu)

 static int __init setup_vmstat(void)
 {
-#ifdef CONFIG_SMP
-       int ret;
+       int ret = 0;
+
+       vmstat_wq = alloc_workqueue("vmstat", WQ_FREEZABLE|WQ_MEM_RECLAIM, 0);

+#ifdef CONFIG_SMP
        ret = cpuhp_setup_state_nocalls(CPUHP_MM_VMSTAT_DEAD, "mm/vmstat:dead",
                                        NULL, vmstat_cpu_dead);
        if (ret < 0)
@@ -1789,7 +1790,7 @@ static int __init setup_vmstat(void)
        proc_create("vmstat", S_IRUGO, NULL, &proc_vmstat_file_operations);
        proc_create("zoneinfo", S_IRUGO, NULL, &proc_zoneinfo_file_operations);
 #endif
-       return 0;
+       return ret;


A failed register of vmstat_cpu_dead is returning the failure code in an
init function now. Chances are it'll never hit but it didn't seem related
to the patches general intent.

> > It also feels like vmstat is now a misleading name for something that
> > handles vmstat, lru drains and per-cpu drains but that's cosmetic.
> 
> yeah a better name sounds like a good thing. mm_nonblock_wq?
> 

it's not always non-blocking. Maybe mm_percpu_wq to describev a workqueue
that handles a variety of MM-related per-cpu updates?

-- 
Mel Gorman
SUSE Labs