Re: [PATCH V2] arm64: psci: Reduce waiting time of cpu_psci_cpu_kill()

From: Sudeep Holla <sudeep.holla@arm.com>
To: Yunfeng Ye <yeyunfeng@huawei.com>
Cc: "kstewart@linuxfoundation.org" <kstewart@linuxfoundation.org>,
	"ard.biesheuvel@linaro.org" <ard.biesheuvel@linaro.org>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	David Laight <David.Laight@ACULAB.COM>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"wuyun.wu@huawei.com" <wuyun.wu@huawei.com>,
	"will@kernel.org" <will@kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH V2] arm64: psci: Reduce waiting time of cpu_psci_cpu_kill()
Date: Wed, 16 Oct 2019 16:32:21 +0100	[thread overview]
Message-ID: <20191016153221.GA8978@bogus> (raw)
In-Reply-To: <9df267db-e647-a81d-16bb-b8bfb06c2624@huawei.com>

On Wed, Oct 09, 2019 at 12:45:16PM +0800, Yunfeng Ye wrote:
> If psci_ops.affinity_info() fails, it will sleep 10ms, which will not
> take so long in the right case. Use usleep_range() instead of msleep(),
> reduce the waiting time, and give a chance to busy wait before sleep.
>
> Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
> ---
> V1->V2:
> - use usleep_range() instead of udelay() after waiting for a while
>
>  arch/arm64/kernel/psci.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kernel/psci.c b/arch/arm64/kernel/psci.c
> index c9f72b2..99b3122 100644
> --- a/arch/arm64/kernel/psci.c
> +++ b/arch/arm64/kernel/psci.c
> @@ -82,6 +82,7 @@ static void cpu_psci_cpu_die(unsigned int cpu)
>  static int cpu_psci_cpu_kill(unsigned int cpu)
>  {
>  	int err, i;
> +	unsigned long timeout;
>
>  	if (!psci_ops.affinity_info)
>  		return 0;
> @@ -91,16 +92,24 @@ static int cpu_psci_cpu_kill(unsigned int cpu)
>  	 * while it is dying. So, try again a few times.
>  	 */
>
> -	for (i = 0; i < 10; i++) {
> +	i = 0;
> +	timeout = jiffies + msecs_to_jiffies(100);
> +	do {
>  		err = psci_ops.affinity_info(cpu_logical_map(cpu), 0);
>  		if (err == PSCI_0_2_AFFINITY_LEVEL_OFF) {
>  			pr_info("CPU%d killed.\n", cpu);
>  			return 0;
>  		}
>
> -		msleep(10);
> -		pr_info("Retrying again to check for CPU kill\n");

You dropped this message, any particular reason ?

> -	}
> +		/* busy-wait max 1ms */
> +		if (i++ < 100) {
> +			cond_resched();
> +			udelay(10);
> +			continue;

Why can't it be simple like loop of 100 * msleep(1) instead of loop of
10 * msleep(10). The above initial busy wait for 1 ms looks too much
optimised for your setup where it takes 50-500us, what if it take just
over 1 ms ?

We need more generic solution.

--
Regards,
Sudeep

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