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Date: Mon, 11 Mar 2024 17:15:26 +0000
Message-ID: <CZR39LW50A9F.1DWG2FYJ3OZP8@amazon.com>
CC: <jalliste@amazon.co.uk>, <mhiramat@kernel.org>,
	<akpm@linux-foundation.org>, <pmladek@suse.com>, <rdunlap@infradead.org>,
	<tsi@tuyoix.net>, <nphamcs@gmail.com>, <gregkh@linuxfoundation.org>,
	<linux-kernel@vger.kernel.org>, <kvm@vger.kernel.org>, <pbonzini@redhat.com>,
	<seanjc@google.com>, <paulmck@kernel.org>, <nsaenz@amazon.com>
Subject: Re: [RFC] cputime: Introduce option to force full dynticks
 accounting on NOHZ & NOHZ_IDLE CPUs
From: Nicolas Saenz Julienne <nsaenz@amazon.com>
To: <frederic@kernel.org>
References: <20240219175735.33171-1-nsaenz@amazon.com>
In-Reply-To: <20240219175735.33171-1-nsaenz@amazon.com>

Hi Frederic,

On Mon Feb 19, 2024 at 5:57 PM UTC, Nicolas Saenz Julienne wrote:
> Under certain extreme conditions, the tick-based cputime accounting may
> produce inaccurate data. For instance, guest CPU usage is sensitive to
> interrupts firing right before the tick's expiration. This forces the
> guest into kernel context, and has that time slice wrongly accounted as
> system time. This issue is exacerbated if the interrupt source is in
> sync with the tick, significantly skewing usage metrics towards system
> time.
>
> On CPUs with full dynticks enabled, cputime accounting leverages the
> context tracking subsystem to measure usage, and isn't susceptible to
> this sort of race conditions. However, this imposes a bigger overhead,
> including additional accounting and the extra dyntick tracking during
> user<->kernel<->guest transitions (RmW + mb).
>
> So, in order to get the best of both worlds, introduce a cputime
> configuration option that allows using the full dynticks accounting
> scheme on NOHZ & NOHZ_IDLE CPUs, while avoiding the expensive
> user<->kernel<->guest dyntick transitions.
>
> Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
> Signed-off-by: Jack Allister <jalliste@amazon.co.uk>
> ---

Would you be opposed to introducing a config option like this? Any
alternatives you might have in mind?

Nicolas