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Date:   Wed, 17 Mar 2021 10:37:31 -0300
From:   Arnaldo Carvalho de Melo <acme@kernel.org>
To:     Alexander Monakov <amonakov@ispras.ru>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Kim Phillips <kim.phillips@amd.com>,
        Jiri Olsa <jolsa@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Tom Lendacky <thomas.lendacky@amd.com>, x86@kernel.org,
        lkml <linux-kernel@vger.kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Stanislav Kozina <skozina@redhat.com>,
        Michael Petlan <mpetlan@redhat.com>,
        Pierre Amadio <pamadio@redhat.com>, onatalen@redhat.com,
        darcari@redhat.com, "Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: unknown NMI on AMD Rome
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Em Wed, Mar 17, 2021 at 04:32:17PM +0300, Alexander Monakov escreveu:
> On Wed, 17 Mar 2021, Peter Zijlstra wrote:
> > On Wed, Mar 17, 2021 at 09:48:29AM +0100, Ingo Molnar wrote:
> > > > https://developer.amd.com/wp-content/resources/56323-PUB_0.78.pdf

> > >   1215 IBS (Instruction Based Sampling) Counter Valid Value
> > >   May be Incorrect After Exit From Core C6 (CC6) State

> > >   Description

> > >   If a core's IBS feature is enabled and configured to generate an interrupt, including NMI (Non-Maskable
> > >   Interrupt), and the IBS counter overflows during the entry into the Core C6 (CC6) state, the interrupt may be
> > >   issued, but an invalid value of the valid bit may be restored when the core exits CC6.
> > >   Potential Effect on System

> > >   The operating system may receive interrupts due to an IBS counter event, including NMI, and not observe an
> > >   valid IBS register. Console messages indicating "NMI received for unknown reason" have been observed on
> > >   Linux systems.

> > >   Suggested Workaround: None
> > >   Fix Planned: No fix planned

> > Should be simple enough to disable CC6 while IBS is in use. Kim, can you
> > please make that happen?

> Wouldn't that "magically" significantly speed up workloads running under
> 'perf top', in case they don't saturate the CPUs? Scheduling gets
> much snappier if the target CPU doesn't need to wake up from deep sleep :)

> Alternatively, would you consider adding the errata reference to the
> printk message when IBS is in use, and rate-limit it so it doesn't
> flood dmesg? Then the user will know what's going on, and may
> choose to temporarily disable C-states using the 'cpupower' tool.

Would be interesting as well to make 'perf top' realize that somehow
(looking at some cpu id, etc) and don't use IBS when C-states are being
used and/or warn the user about the situation, i.e. cycles:P can't be
used in this machine if C-states are enabled?

- Arnaldo