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

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Message-ID: <ZL+wgn76H1em9hZU@lothringen>
Date:   Tue, 25 Jul 2023 13:22:42 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Valentin Schneider <vschneid@...hat.com>
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 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...