linux-kernel - Re: [PATCH] cpu/hotplug: Provide dynamic range for prepare stage

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Message-ID: <44896445.2407.1484168818195.JavaMail.zimbra@efficios.com>
Date:   Wed, 11 Jan 2017 21:06:58 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        rostedt <rostedt@...dmis.org>,
        lttng-dev <lttng-dev@...ts.lttng.org>
Subject: Re: [PATCH] cpu/hotplug: Provide dynamic range for prepare stage

----- On Jan 10, 2017, at 8:01 AM, Thomas Gleixner tglx@...utronix.de wrote:

> Mathieu reported that the LTTNG modules are broken as of 4.10-rc1 due to
> the removal of the cpu hotplug notifiers.
> 
> Usually I don't care much about out of tree modules, but LTTNG is widely
> used in distros. There are two ways to solve that:
> 
> 1) Reserve a hotplug state for LTTNG
> 
> 2) Add a dynamic range for the prepare states.
> 
> While #1 is the simplest solution, #2 is the proper one as we can convert
> in tree users, which do not care about ordering, to the dynamic range as
> well.
> 
> Add a dynamic range which allows LTTNG to request states in the prepare
> stage.

I prepared a lttng-modules commit [1] that uses CPUHP_BP_PREPARE_DYN on 4.10
kernels, and it works fine through my cpu hotplug stress-testing. It's a very
good cleanup! Thanks Thomas for this nice API.

I'll be able to merge my commit into lttng-modules as soon as your patch
lands into Linux mainline.

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Tested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

Thanks,

Mathieu

[1] https://github.com/compudj/lttng-modules-dev/commit/f02214ac2e9a0f3d8d0270c0ac702c3143cdf120

> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> ---
> 
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index 20bfefbe7594..d936a0021839 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -74,6 +74,8 @@ enum cpuhp_state {
> 	CPUHP_ZCOMP_PREPARE,
> 	CPUHP_TIMERS_DEAD,
> 	CPUHP_MIPS_SOC_PREPARE,
> +	CPUHP_BP_PREPARE_DYN,
> +	CPUHP_BP_PREPARE_DYN_END		= CPUHP_BP_PREPARE_DYN + 20,
> 	CPUHP_BRINGUP_CPU,
> 	CPUHP_AP_IDLE_DEAD,
> 	CPUHP_AP_OFFLINE,
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index f75c4d031eeb..c47506357519 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1302,10 +1302,24 @@ static int cpuhp_cb_check(enum cpuhp_state state)
>  */
> static int cpuhp_reserve_state(enum cpuhp_state state)
> {
> -	enum cpuhp_state i;
> +	enum cpuhp_state i, end;
> +	struct cpuhp_step *step;
> 
> -	for (i = CPUHP_AP_ONLINE_DYN; i <= CPUHP_AP_ONLINE_DYN_END; i++) {
> -		if (!cpuhp_ap_states[i].name)
> +	switch (state) {
> +	case CPUHP_AP_ONLINE_DYN:
> +		step = cpuhp_ap_states + CPUHP_AP_ONLINE_DYN;
> +		end = CPUHP_AP_ONLINE_DYN_END;
> +		break;
> +	case CPUHP_BP_PREPARE_DYN:
> +		step = cpuhp_bp_states + CPUHP_BP_PREPARE_DYN;
> +		end = CPUHP_BP_PREPARE_DYN_END;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	for (i = state; i <= end; i++, step++) {
> +		if (!step->name)
> 			return i;
> 	}
> 	WARN(1, "No more dynamic states available for CPU hotplug\n");
> @@ -1323,7 +1337,7 @@ static int cpuhp_store_callbacks(enum cpuhp_state state,
> const char *name,
> 
> 	mutex_lock(&cpuhp_state_mutex);
> 
> -	if (state == CPUHP_AP_ONLINE_DYN) {
> +	if (state == CPUHP_AP_ONLINE_DYN || state == CPUHP_BP_PREPARE_DYN) {
> 		ret = cpuhp_reserve_state(state);
> 		if (ret < 0)
>  			goto out;

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com