linux-kernel - Re: [patch 2/2] MM: allow per-cpu vmstat_threshold and vmstat

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Date:   Tue, 2 May 2017 13:15:27 -0400
From:   Luiz Capitulino <lcapitulino@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Rik van Riel <riel@...hat.com>,
        Linux RT Users <linux-rt-users@...r.kernel.org>,
        cl@...ux.com, cmetcalf@...lanox.com
Subject: Re: [patch 2/2] MM: allow per-cpu vmstat_threshold and
 vmstat_worker configuration

On Tue, 2 May 2017 13:52:00 -0300
Marcelo Tosatti <mtosatti@...hat.com> wrote:

> > I have several questions about the tunables:
> > 
> >  - What does the vmstat_threshold value mean? What are the implications
> >    of changing this value? What's the difference in choosing 1, 2, 3
> >    or 500?  
> 
> Its the maximum value for a vmstat statistics counter to hold. After
> that value, the statistics are transferred to the global counter:
> 
> void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
>                                 long delta)
> {
>         struct per_cpu_nodestat __percpu *pcp = pgdat->per_cpu_nodestats;
>         s8 __percpu *p = pcp->vm_node_stat_diff + item;
>         long x;
>         long t;
> 
>         x = delta + __this_cpu_read(*p);
> 
>         t = __this_cpu_read(pcp->stat_threshold);
> 
>         if (unlikely(x > t || x < -t)) {
>                 node_page_state_add(x, pgdat, item);
>                 x = 0;
>         }
>         __this_cpu_write(*p, x);
> }
> EXPORT_SYMBOL(__mod_node_page_state);
> 
> BTW, there is a bug there, should change that to:
> 
>         if (unlikely(x >= t || x <= -t)) {
> 
> Increasing the threshold value does two things:
> 	1) It decreases the number of inter-processor accesses.
> 	2) It increases how much the global counters stay out of
> 	   sync relative to actual current values.

OK, but I'm mostly concerned with the sysadmin who will have
to change the tunable. So, I think it's a good idea to improve
the doc to contain that information.

> >  - If the purpose of having vmstat_threshold is to allow disabling
> >    the vmstat kworker, why can't the kernel pick a value automatically?  
> 
> Because it might be acceptable for the user to accept a small 
> out of syncedness of the global counters in favour of performance
> (one would have to analyze the situation).
> 
> Setting vmstat_threshold == 1 means the global counter is always
> in sync with the page counter state of the pCPU.

IMHO, if vmstat_threshold == 1 is the required setting for
disabling the vmstat kworker then I'd go with only one tunable
for now. But that's just a suggestion.

> 
> >  - What are the implications of disabling the vmstat kworker? Will vm
> >    stats still be collected someway or will it be completely off for
> >    the CPU?  
> 
> It will not be necessary to collect vmstats because at every modification
> of the vm statistics, pCPUs with vmstat_threshold=1 transfer their 
> values to the global counters (that is, there is no queueing of statistics
> locally to improve performance).

Ah, OK. Got this now. I'll give this patch a try. But I think we want
to hear from Christoph (who worked on reducing the vmstat interruptions
in the past).

> > Also, shouldn't this patch be split into two?  
> 
> First add one sysfs file, then add another sysfs file, you mean?

Yes, one tunable per patch.