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Date:   Thu, 5 May 2022 11:32:04 -0700
From:   "Luck, Tony" <tony.luck@intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Peter Zijlstra <peterz@infradead.org>, hdegoede@redhat.com,
        markgross@kernel.org, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
        corbet@lwn.net, gregkh@linuxfoundation.org,
        andriy.shevchenko@linux.intel.com, jithu.joseph@intel.com,
        ashok.raj@intel.com, rostedt@goodmis.org, dan.j.williams@intel.com,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        platform-driver-x86@vger.kernel.org, patches@lists.linux.dev,
        ravi.v.shankar@intel.com
Subject: Re: [PATCH v5 07/10] platform/x86/intel/ifs: Add scan test support
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On Thu, May 05, 2022 at 11:01:27AM +0200, Thomas Gleixner wrote:
> On Thu, May 05 2022 at 10:28, Peter Zijlstra wrote:
> > On Thu, May 05, 2022 at 01:15:07AM +0200, Thomas Gleixner wrote:
> >> We don't have stomp_cpumask() today, but that's trivial enough to
> >> implement.
> >
> > I don't think we want to gift people a random cpumask stop_machine(),
> 
> Fair enough.
> 
> > but here's one that stops a core. It runs the @fn on every cpu since I
> > thought to have understood that was the requirement for this muck.
> 
> Yes.
> 
> > *completely* untestededed.
> 
> Looks about right neverthelessesseess.

Close enough. I made these changes:

1) Added EXPORT_SYMBOL_GPL()
2) Added protoype in <linux/stop_machine.h>
3) Moved the kerneldoc comment to the header (for some reason
   the other stop* functions document there).
4) Edited that kerneldoc a bit <<<< NEEDS REVIEW  >>>
5) Changed a ';' to a ',' to make it compile.

With that it works, and will do exactly what I need (with less code
in the IFS driver by the look of things).

A couple of thousand tests shows it works. The two threads
arrive within 20 TSC cycles of each other 60% of the time,
but I have some outliers up to 654 cycles ... which is plenty
close enough.

Patch now looks like this. Author credit to Peter ... are you willing
to add a Signed-off-by to stop checkpatch from whining at me?

There isn't a "Something-similar-suggested-by:" tag to credit Thomas
with this.  Perhaps "Inspired-by:"?

-Tony


From df5ca8024997d3d782978d154cfbff5147f451ad Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 5 May 2022 08:55:09 -0700
Subject: [PATCH] stop_machine: Add stop_core_cpuslocked() for per-core
 operations

Hardware core level testing features require near simultaneous execution
of WRMSR instructions on all threads of a core to initiate a test.

Provide a customized cut down version of stop_machine_cpuslocked() that
just operates on the threads of a single core.

Signed-off-by: Tony Luck <tony.luck@intel.com>
---
 include/linux/stop_machine.h | 16 ++++++++++++++++
 kernel/stop_machine.c        | 19 +++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
index 46fb3ebdd16e..ea7a74ea7389 100644
--- a/include/linux/stop_machine.h
+++ b/include/linux/stop_machine.h
@@ -124,6 +124,22 @@ int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus);
  */
 int stop_machine_cpuslocked(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus);
 
+/**
+ * stop_core_cpuslocked: - stop all threads on just one core
+ * @cpu: any cpu in the targeted core
+ * @fn: the function to run
+ * @data: the data ptr for @fn()
+ *
+ * Same as above, but instead of every CPU, only the logical CPUs of a
+ * single core are affected.
+ *
+ * Context: Must be called from within a cpus_read_lock() protected region.
+ *
+ * Return: 0 if all executions of @fn returned 0, any non zero return
+ * value if any returned non zero.
+ */
+int stop_core_cpuslocked(unsigned int cpu, cpu_stop_fn_t fn, void *data);
+
 int stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data,
 				   const struct cpumask *cpus);
 #else	/* CONFIG_SMP || CONFIG_HOTPLUG_CPU */
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index cbc30271ea4d..579761729836 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -633,6 +633,25 @@ int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus)
 }
 EXPORT_SYMBOL_GPL(stop_machine);
 
+int stop_core_cpuslocked(unsigned int cpu, cpu_stop_fn_t fn, void *data)
+{
+	const struct cpumask *smt_mask = cpu_smt_mask(cpu);
+
+	struct multi_stop_data msdata = {
+		.fn = fn,
+		.data = data,
+		.num_threads = cpumask_weight(smt_mask),
+		.active_cpus = smt_mask,
+	};
+
+	lockdep_assert_cpus_held();
+
+	/* Set the initial state and stop all online cpus. */
+	set_state(&msdata, MULTI_STOP_PREPARE);
+	return stop_cpus(smt_mask, multi_cpu_stop, &msdata);
+}
+EXPORT_SYMBOL_GPL(stop_core_cpuslocked);
+
 /**
  * stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
  * @fn: the function to run
-- 
2.35.1