linux-kernel - Re: [PATCH 08/11] x86/tsc: Avoid synchronizing TSCs with multiple CPUs in parallel

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Message-ID: <YbIkMyNeb72R4Ma6@hirez.programming.kicks-ass.net>
Date:   Thu, 9 Dec 2021 16:43:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        hushiyuan@...wei.com, luolongjun@...wei.com, hejingxian@...wei.com
Subject: Re: [PATCH 08/11] x86/tsc: Avoid synchronizing TSCs with multiple
 CPUs in parallel

On Thu, Dec 09, 2021 at 03:09:35PM +0000, David Woodhouse wrote:
> diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
> index 50a4515fe0ad..4ee247d89a49 100644
> --- a/arch/x86/kernel/tsc_sync.c
> +++ b/arch/x86/kernel/tsc_sync.c
> @@ -202,6 +202,7 @@ bool tsc_store_and_check_tsc_adjust(bool bootcpu)
>   * Entry/exit counters that make sure that both CPUs
>   * run the measurement code at once:
>   */
> +static atomic_t tsc_sync_cpu = ATOMIC_INIT(-1);
>  static atomic_t start_count;
>  static atomic_t stop_count;
>  static atomic_t skip_test;
> @@ -326,6 +327,8 @@ void check_tsc_sync_source(int cpu)
>  		atomic_set(&test_runs, 1);
>  	else
>  		atomic_set(&test_runs, 3);
> +
> +	atomic_set(&tsc_sync_cpu, cpu);
>  retry:
>  	/*
>  	 * Wait for the target to start or to skip the test:
> @@ -407,6 +410,10 @@ void check_tsc_sync_target(void)
>  	if (unsynchronized_tsc())
>  		return;
>  
> +	/* Wait for this CPU's turn */
> +	while (atomic_read(&tsc_sync_cpu) != cpu)
> +		cpu_relax();
> +
>  	/*
>  	 * Store, verify and sanitize the TSC adjust register. If
>  	 * successful skip the test.

This new atomic_t seems superfluous, there isn't any actual atomic
operation used.