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Date: Mon, 25 Mar 2024 21:17:38 -0400
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Subject: Re: [PATCH v2] x86/tsc: Use topology_max_packages() to get package
 number
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To: Feng Tang <feng.tang@intel.com>, Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
 Dave Hansen <dave.hansen@intel.com>, "H . Peter Anvin" <hpa@zytor.com>,
 Peter Zijlstra <peterz@infradead.org>, x86@kernel.org, paulmck@kernel.org,
 rui.zhang@intel.com, linux-kernel@vger.kernel.org
Cc: Dave Hansen <dave.hansen@linux.intel.com>
References: <20240325030928.4190408-1-feng.tang@intel.com>
From: Waiman Long <longman@redhat.com>
In-Reply-To: <20240325030928.4190408-1-feng.tang@intel.com>
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On 3/24/24 23:09, Feng Tang wrote:
> Commit b50db7095fe0 ("x86/tsc: Disable clocksource watchdog for TSC
> on qualified platorms") was introduced to solve problem that
> sometimes TSC clocksource is wrongly judged as unstable by watchdog
> like 'jiffies', HPET, etc.
>
> In it, the hardware package number is a key factor for judging whether
> to disable the watchdog for TSC, and 'nr_online_nodes' was chosen due
> to it is available in early boot phase before registering 'tsc-early'
> clocksource, where all non-boot CPUs are not brought up yet.
>
> Dave and Rui pointed out there are many cases in which 'nr_online_nodes'
> is cheated and not accurate, like:
>
> * numa emulation (numa=fake=8 etc.)
> * numa=off
> * platforms with CPU-less HBM nodes, CPU-less Optane memory nodes.
> * SNC (sub-numa cluster) mode enabled
> * 'maxcpus=' cmdline setup, where chopped CPUs could be onlined later
> * 'nr_cpus=', 'possible_cpus=' cmdline setup, where chopped CPUs can
>    not be onlined after boot
>
> Thomas' recent patchset of refactoring x86 topology code improves
> topology_max_packages(), by making it more accurate and available in
> early boot phase, which works well in most of the above cases.
>
> The only exceptions are 'nr_cpus=' and 'possible_cpus=' setup.  And
> the reason is, during topology setup, the boot CPU iterates through
> all enumerated APIC ids and either accepts or rejects the APIC id.
> For accepted ids, it figures out which bits of the id map to the
> package number.  It tracks which package numbers have been seen in a
> bitmap.  topology_max_packages() just returns the number of bits set
> in that bitmap.
>
> 'nr_cpus=' and 'possible_cpus=' can cause more APIC ids to be rejected
> and can artificially lower the number of bits in the package bitmap
> and thus topology_max_packages().  This means that, for example, a
> system with 8 physical packages might reject all the CPUs on 6 of those
> packages and be left with only 2 packages and 2 bits set in the package
> bitmap. It needs the TSC watchdog, but would disable it anyway.  This
> isn't ideal, but this only happens for debug-oriented options. This is
> fixable by tracking the package numbers for rejected CPUs.  But it's
> not worth the trouble for debugging.
>
> So use topology_max_packages() to replace nr_online_nodes().
>
> Reported-by: Dave Hansen <dave.hansen@linux.intel.com>
> Closes: https://lore.kernel.org/lkml/a4860054-0f16-6513-f121-501048431086@intel.com/
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
>
> Hi all,
>
> For warning about possible compromise due to 'nr_cpus=' and 'possible_cpus=',
> another alternative could be checking whether these has been setup in cmdline
> inside tsc.c and warn there.
>
> Changelog:
>
>    Since v1:
>
>    * Use Dave's detailed elaboration about 'nr_cpus=', 'possible_cpus='
>      possibly compromising '__max_logical_packages' in commit log
>    * Fix typos and inaccuracy pointed out by Rui and Longman
>
>   arch/x86/kernel/cpu/topology.c | 5 ++++-
>   arch/x86/kernel/tsc.c          | 7 ++-----
>   2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
> index 3259b1d4fefe..2db03b00e29b 100644
> --- a/arch/x86/kernel/cpu/topology.c
> +++ b/arch/x86/kernel/cpu/topology.c
> @@ -460,8 +460,11 @@ void __init topology_init_possible_cpus(void)
>   	pr_info("Num. threads per package: %3u\n", __num_threads_per_package);
>   
>   	pr_info("Allowing %u present CPUs plus %u hotplug CPUs\n", assigned, disabled);
> -	if (topo_info.nr_rejected_cpus)
> +	if (topo_info.nr_rejected_cpus) {
>   		pr_info("Rejected CPUs %u\n", topo_info.nr_rejected_cpus);
> +		if (__max_logical_packages <= 4)
> +			pr_warn("TSC might be buggered due to the rejected CPUs\n");
> +	}

People may sometimes use kernel option like "panic_on_warn=1" to cause a 
crash dump on warning. Maybe we should just use pr_info() as the 
presence of rejected CPUs are unlikely to be a real problem from the TSC 
stability point of view. To emphasize it a bit more, we could add a 
"WARNING: " prefix, for example.

Cheers,
Longman


>   
>   	init_cpu_present(cpumask_of(0));
>   	init_cpu_possible(cpumask_of(0));
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 5a69a49acc96..d00f88f16eb1 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -1252,15 +1252,12 @@ static void __init check_system_tsc_reliable(void)
>   	 *  - TSC which does not stop in C-States
>   	 *  - the TSC_ADJUST register which allows to detect even minimal
>   	 *    modifications
> -	 *  - not more than two sockets. As the number of sockets cannot be
> -	 *    evaluated at the early boot stage where this has to be
> -	 *    invoked, check the number of online memory nodes as a
> -	 *    fallback solution which is an reasonable estimate.
> +	 *  - not more than four packages
>   	 */
>   	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
>   	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
>   	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
> -	    nr_online_nodes <= 4)
> +	    topology_max_packages() <= 4)
>   		tsc_disable_clocksource_watchdog();
>   }
>