linux-kernel - Re: [PATCH v2 09/11] mm/vmstat: use cmpxchg loop in cpu_vm_stats

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Date:   Fri, 3 Mar 2023 12:17:12 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 09/11] mm/vmstat: use cmpxchg loop in cpu_vm_stats_fold

On Thu, Mar 02, 2023 at 04:19:50PM -0500, Peter Xu wrote:
> On Thu, Mar 02, 2023 at 10:55:09AM -0300, Marcelo Tosatti wrote:
> > >   (2) If someone can modify the dead cpu's vm_stat_diff,
> > 
> > The only context that can modify the cpu's vm_stat_diff are:
> > 
> > 1) The CPU itself (increases the counter).
> > 2) cpu_vm_stats_fold (from vmstat_shepherd kernel thread), from 
> > x -> 0 only.
> 
> I think I didn't continue reading so I didn't see cpu_vm_stats_fold() will
> be reused when commenting, sorry.
> 
> Now with a reworked (and SMP-safe) cpu_vm_stats_fold() and vmstats, I'm
> wondering the possibility of merging it with refresh_cpu_vm_stats() since
> they really look similar.

Seems like a possibility. However that might require replacing

v = this_cpu_xchg(pzstats->vm_stat_diff[i], 0);

with

pzstats = per_cpu_ptr(zone->per_cpu_zonestats, cpu);

Which would drop the this_cpu optimization described at 

7340a0b15280c9d902c7dd0608b8e751b5a7c403

Also you would not want the unified function to sync NUMA events 
(as it would be called from NOHZ entry and exit).

See f19298b9516c1a031b34b4147773457e3efe743b

> IIUC the new refresh_cpu_vm_stats() logically doesn't need the small
> preempt disabled sections, not anymore, 

What preempt disabled sections you refer to?

> if with a cpu_id passed over to
> cpu_vm_stats_fold(), which seems to be even a good side effect. But not
> sure I missed something.
> 
> -- 
> Peter Xu
> 
>