linux-kernel - Re: [PATCH] cpuidle:powernv: Make the snooze timeout dynamic.

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Message-Id: <87k1rd28t0.fsf@linux.vnet.ibm.com>
Date:   Tue, 05 Jun 2018 13:47:55 +1000
From:   Stewart Smith <stewart@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Michael Neuling <mikey@...ling.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cpuidle:powernv: Make the snooze timeout dynamic.

Michael Ellerman <mpe@...erman.id.au> writes:
> "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> writes:
>
>> From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
>>
>> The commit 78eaa10f027c ("cpuidle: powernv/pseries: Auto-promotion of
>> snooze to deeper idle state") introduced a timeout for the snooze idle
>> state so that it could be eventually be promoted to a deeper idle
>> state. The snooze timeout value is static and set to the target
>> residency of the next idle state, which would train the cpuidle
>> governor to pick the next idle state eventually.
>>
>> The unfortunate side-effect of this is that if the next idle state(s)
>> is disabled, the CPU will forever remain in snooze, despite the fact
>> that the system is completely idle, and other deeper idle states are
>> available.
>
> That sounds like a bug, I'll add?
>
> Fixes: 78eaa10f027c ("cpuidle: powernv/pseries: Auto-promotion of snooze to deeper idle state")
> Cc: stable@...r.kernel.org # v4.2+

Yes, it's a bug - we had a customer bug because we lacked this that
meant we had to do firmware changes rather than just tweaking what stop
states were used.

-- 
Stewart Smith
OPAL Architect, IBM.