linux-kernel - Re: [RFC PATCH v2 15/20] context-tracking: Introduce work deferral infrastructure

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Message-ID: <xhsmhwmyorvis.mognet@vschneid.remote.csb>
Date:   Tue, 25 Jul 2023 14:05:47 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [RFC PATCH v2 15/20] context-tracking: Introduce work deferral
 infrastructure

On 25/07/23 13:22, Frederic Weisbecker wrote:
> On Tue, Jul 25, 2023 at 11:10:31AM +0100, Valentin Schneider wrote:
>> I have reasons! I just swept them under the rug and didn't mention them :D
>> Also looking at the config dependencies again I got it wrong, but
>> nevertheless that means I get to ramble about it.
>>
>> With NO_HZ_IDLE, we get CONTEXT_TRACKING_IDLE, so we get these
>> transitions:
>>
>>   ct_idle_enter()
>>     ct_kernel_exit()
>>       ct_state_inc_clear_work()
>>
>>   ct_idle_exit()
>>     ct_kernel_enter()
>>       ct_work_flush()
>>
>> Now, if we just make CONTEXT_TRACKING_WORK depend on CONTEXT_TRACKING_IDLE
>> rather than CONTEXT_TRACKING_USER, we get to leverage the IPI deferral for
>> NO_HZ_IDLE kernels - in other words, we get to keep idle CPUs idle longer.
>>
>> It's a completely different argument than reducing interference for
>> NOHZ_FULL userspace applications and I should have at the very least
>> mentioned it in the cover letter, but it's the exact same backing
>> mechanism.
>>
>> Looking at it again, I'll probably make the CONTEXT_IDLE thing a separate
>> patch with a proper changelog.
>
> Ok should that be a seperate Kconfig? This indeed can bring power improvement
> but at the cost of more overhead from the sender. A balance to be measured...

Yep agreed, I'll make that an optional config.