linux-kernel - Re: Loadavg accounting error on arm64

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Message-ID: <20201116124657.GA3121392@hirez.programming.kicks-ass.net>
Date:   Mon, 16 Nov 2020 13:46:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Will Deacon <will@...nel.org>, Davidlohr Bueso <dave@...olabs.net>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Loadavg accounting error on arm64

On Mon, Nov 16, 2020 at 09:10:54AM +0000, Mel Gorman wrote:
> Similarly, it's not clear why the arm64 implementation
> does not call smp_acquire__after_ctrl_dep in the smp_load_acquire
> implementation. Even when it was introduced, the arm64 implementation
> differed significantly from the arm implementation in terms of what
> barriers it used for non-obvious reasons.

This is because ARM64's smp_cond_load_acquire() implementation uses
smp_load_aquire() directly, as opposed to the generic version that uses
READ_ONCE().

This is because ARM64 has a load-acquire instruction, which is highly
optimized, and generally considered cheaper than the smp_rmb() from
smp_acquire__after_ctrl_dep().

Or so I've been led to believe.