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

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Date:   Wed, 1 Mar 2023 17:57:08 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Marcelo Tosatti <mtosatti@...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, Feb 09, 2023 at 12:01:59PM -0300, Marcelo Tosatti wrote:
>  /*
> - * Fold the data for an offline cpu into the global array.
> + * Fold the data for a cpu into the global array.
>   * There cannot be any access by the offline cpu and therefore
>   * synchronization is simplified.
>   */
> @@ -906,8 +906,9 @@ void cpu_vm_stats_fold(int cpu)
>  			if (pzstats->vm_stat_diff[i]) {
>  				int v;
>  
> -				v = pzstats->vm_stat_diff[i];
> -				pzstats->vm_stat_diff[i] = 0;
> +				do {
> +					v = pzstats->vm_stat_diff[i];
> +				} while (!try_cmpxchg(&pzstats->vm_stat_diff[i], &v, 0));

IIUC try_cmpxchg will update "v" already, so I'd assume this'll work the
same:

        while (!try_cmpxchg(&pzstats->vm_stat_diff[i], &v, 0));

Then I figured, maybe it's easier to use xchg()?

I've no knowledge at all on cpu offline code, so sorry if this will be a
naive question.  But from what I understand this should not be touched by
anyone else.  Reasons:

  (1) cpu_vm_stats_fold() is only called in page_alloc_cpu_dead(), and the
      comment says:
  
	/*
	 * Zero the differential counters of the dead processor
	 * so that the vm statistics are consistent.
	 *
	 * This is only okay since the processor is dead and cannot
	 * race with what we are doing.
	 */
	cpu_vm_stats_fold(cpu);

      so.. I think that's what it says..

  (2) If someone can modify the dead cpu's vm_stat_diff, what guarantees it
      won't be e.g. boosted again right after try_cmpxchg() / xchg()
      returns?  What to do with the left-overs?

Thanks,

-- 
Peter Xu