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Message-ID: <5d3059e5-168d-a039-5ea1-a7b787dadc97@linaro.org>
Date:   Thu, 19 Jan 2023 18:06:08 +0100
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Subject: Re: [PATCH v2 2/5] cpuidle: psci: Mark as PREEMPT_RT safe
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From:   Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
To:     Ulf Hansson <ulf.hansson@linaro.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kevin Hilman <khilman@kernel.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Sudeep Holla <sudeep.holla@arm.com>, linux-pm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Adrien Thierry <athierry@redhat.com>,
        Brian Masney <bmasney@redhat.com>,
        linux-rt-users@vger.kernel.org
References: <20221219151503.385816-1-krzysztof.kozlowski@linaro.org>
 <20221219151503.385816-3-krzysztof.kozlowski@linaro.org>
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 <26e431a5-9666-0c72-7b0b-1a6c1bfaec22@linaro.org>
In-Reply-To: <26e431a5-9666-0c72-7b0b-1a6c1bfaec22@linaro.org>
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On 19/01/2023 16:40, Krzysztof Kozlowski wrote:
> On 17/01/2023 16:27, Ulf Hansson wrote:
>> On Mon, 19 Dec 2022 at 16:15, Krzysztof Kozlowski
>> <krzysztof.kozlowski@linaro.org> wrote:
>>>
>>> The PSCI cpuidle power domain in power_off callback uses
>>> __this_cpu_write() so it is PREEMPT_RT safe.  This allows to use it in
>>> Realtime kernels and solves errors like:
>>>
>>>   BUG: scheduling while atomic: swapper/2/0/0x00000002
>>>   Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
>>>   Call trace:
>>>    dump_backtrace.part.0+0xe0/0xf0
>>>    show_stack+0x18/0x40
>>>    dump_stack_lvl+0x68/0x84
>>>    dump_stack+0x18/0x34
>>>    __schedule_bug+0x60/0x80
>>>    __schedule+0x628/0x800
>>>    schedule_rtlock+0x28/0x5c
>>>    rtlock_slowlock_locked+0x360/0xd30
>>>    rt_spin_lock+0x88/0xb0
>>>    genpd_lock_nested_spin+0x1c/0x30
>>>    genpd_power_off.part.0.isra.0+0x20c/0x2a0
>>>    genpd_runtime_suspend+0x150/0x2bc
>>>    __rpm_callback+0x48/0x170
>>>    rpm_callback+0x6c/0x7c
>>>    rpm_suspend+0x108/0x660
>>>    __pm_runtime_suspend+0x4c/0x8c
>>>    __psci_enter_domain_idle_state.constprop.0+0x54/0xe0
>>>    psci_enter_domain_idle_state+0x18/0x2c
>>>    cpuidle_enter_state+0x8c/0x4e0
>>>    cpuidle_enter+0x38/0x50
>>>    do_idle+0x248/0x2f0
>>>    cpu_startup_entry+0x24/0x30
>>>    secondary_start_kernel+0x130/0x154
>>>    __secondary_switched+0xb0/0xb4
>>>
>>> Cc: Adrien Thierry <athierry@redhat.com>
>>> Cc: Brian Masney <bmasney@redhat.com>
>>> Cc: linux-rt-users@vger.kernel.org
>>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
>>> ---
>>>  drivers/cpuidle/cpuidle-psci-domain.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/cpuidle/cpuidle-psci-domain.c b/drivers/cpuidle/cpuidle-psci-domain.c
>>> index c80cf9ddabd8..d15a91fb7048 100644
>>> --- a/drivers/cpuidle/cpuidle-psci-domain.c
>>> +++ b/drivers/cpuidle/cpuidle-psci-domain.c
>>> @@ -62,7 +62,8 @@ static int psci_pd_init(struct device_node *np, bool use_osi)
>>>         if (!pd_provider)
>>>                 goto free_pd;
>>>
>>> -       pd->flags |= GENPD_FLAG_IRQ_SAFE | GENPD_FLAG_CPU_DOMAIN;
>>> +       pd->flags |= GENPD_FLAG_IRQ_SAFE | GENPD_FLAG_RT_SAFE | \
>>> +                    GENPD_FLAG_CPU_DOMAIN;
>>
>> My main concern with this, is that it will affect the parent domains
>> too. Whether those would be able to use the GENPD_FLAG_RT_SAFE or not,
>> is a different story.
>>
>> In one way or the other, I think it would be better to limit the
>> GENPD_FLAG_RT_SAFE to be used only for PREEMPT_RT kernels.
> 
> I can do it... or maybe we should just drop the flags (RT and IRQ safe)
> when parent domain does not have it?

Actually, with next patch, I can skip this one entirely. This is needed
if PSCI cpuidle driver invokes runtime PM functions which eventually
puts PSCI cpuidle power domain into suspend/resume. If the former does
not happen, the domain driver won't be even called so my problem disappears.

Since I need patch 3/5 - effectively disabling PSCI cpuidle runtime PM -
we can drop this one, till we find a real user needing it.

Best regards,
Krzysztof