linux-kernel - Re: [PATCH] cpuidle:powernv: Add the CPUIDLE_FLAG

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Date:   Tue, 3 Jul 2018 19:36:16 +0530
From:   Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     akshay.adiga@...ux.vnet.ibm.com,
        Nicholas Piggin <npiggin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpuidle:powernv: Add the CPUIDLE_FLAG_POLLING for snooze

* Gautham R Shenoy <ego@...ux.vnet.ibm.com> [2018-07-03 10:54:16]:

> From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
> 
> In the situations where snooze is the only cpuidle state due to
> firmware not exposing any platform idle states, the idle CPUs will
> remain in snooze for a long time with interrupts disabled causing the
> Hard-lockup detector to complain.

snooze_loop() will spin in SMT low priority with interrupt enabled. We
have local_irq_enable() before we get into the snooze loop.
Since this is a polling state, we should wakeup without an interrupt
and hence we set TIF_POLLING_NRFLAG as well.

 
> watchdog: CPU 51 detected hard LOCKUP on other CPUs 59
> watchdog: CPU 51 TB:535296107736, last SMP heartbeat TB:527472229239 (15281ms ago)
> watchdog: CPU 59 Hard LOCKUP
> watchdog: CPU 59 TB:535296252849, last heartbeat TB:526554725466 (17073ms ago)

hmm.. not sure why watchdog will complain, maybe something more is
going on.
 
> Fix this by adding CPUIDLE_FLAG_POLLING flag to the state, so that the
> cpuidle governor will do the right thing, such as not stopping the
> tick if it is going to put the idle cpu to snooze.
> 
> Reported-by: Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> ---
>  drivers/cpuidle/cpuidle-powernv.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/cpuidle/cpuidle-powernv.c b/drivers/cpuidle/cpuidle-powernv.c
> index d29e4f0..b73041b 100644
> --- a/drivers/cpuidle/cpuidle-powernv.c
> +++ b/drivers/cpuidle/cpuidle-powernv.c
> @@ -156,6 +156,7 @@ static int stop_loop(struct cpuidle_device *dev,
>  	{ /* Snooze */
>  		.name = "snooze",
>  		.desc = "snooze",
> +		.flags = CPUIDLE_FLAG_POLLING,
>  		.exit_latency = 0,
>  		.target_residency = 0,
>  		.enter = snooze_loop },

Adding the CPUIDLE_FLAG_POLLING is good and enables more optimization.
But the reason that we spin with interrupt disabled does not seem
right.

--Vaidy