linux-kernel - Re: [PATCH 3/5] mm/vmstat: use cmpxchg loop in cpu_vm_stats

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Date:   Mon, 6 Feb 2023 19:19:06 +0000
From:   Matthew Wilcox <willy@...radead.org>
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 3/5] mm/vmstat: use cmpxchg loop in cpu_vm_stats_fold

On Wed, Feb 01, 2023 at 04:50:16PM -0300, Marcelo Tosatti wrote:
> In preparation to switch vmstat shepherd to flush
> per-CPU counters remotely, use a cmpxchg loop 
> instead of a pair of read/write instructions.

FYI, try_cmpxchg() is preferred to plain cmpxchg() these days.
Apparently it generates better code on x86.

> -				v = pzstats->vm_stat_diff[i];
> -				pzstats->vm_stat_diff[i] = 0;
> +				do {
> +					v = pzstats->vm_stat_diff[i];
> +				} while (cmpxchg(&pzstats->vm_stat_diff[i], v, 0) != v);

I think this would be:

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