From: Sean Christopherson <seanjc@google.com>
To: Nicolas Saenz Julienne <nsaenz@amazon.com>
Cc: frederic@kernel.org, paulmck@kernel.org, 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
Subject: Re: [RFC] cputime: Introduce option to force full dynticks accounting on NOHZ & NOHZ_IDLE CPUs
Date: Tue, 20 Feb 2024 08:18:17 -0800 [thread overview]
Message-ID: <ZdTQyb23KJEYqbcw@google.com> (raw)
In-Reply-To: <20240219175735.33171-1-nsaenz@amazon.com>
On Mon, Feb 19, 2024, 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.
...
> NOTE: This wasn't tested in depth, and it's mostly intended to highlight
> the issue we're trying to solve. Also ccing KVM folks, since it's
> relevant to guest CPU usage accounting.
How bad is the synchronization issue on upstream kernels? We tried to address
that in commit 160457140187 ("KVM: x86: Defer vtime accounting 'til after IRQ handling").
I don't expect it to be foolproof, but it'd be good to know if there's a blatant
flaw and/or easily closed hole.
next prev parent reply other threads:[~2024-02-20 16:18 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-19 17:57 [RFC] cputime: Introduce option to force full dynticks accounting on NOHZ & NOHZ_IDLE CPUs Nicolas Saenz Julienne
2024-02-20 16:18 ` Sean Christopherson [this message]
2024-02-20 18:19 ` Nicolas Saenz Julienne
2024-02-21 16:24 ` Sean Christopherson
2024-02-21 17:11 ` Nicolas Saenz Julienne
2024-03-11 17:15 ` Nicolas Saenz Julienne
2024-03-12 23:13 ` Frederic Weisbecker
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