RE: [RFC PATCH v5 1/4] topology: Represent clusters of CPUs within a die

quoted

-----Original Message-----
From: Greg KH [mailto:gregkh@linuxfoundation.org]
Sent: Friday, March 19, 2021 11:02 PM
To: Jonathan Cameron <redacted>
Cc: Song Bao Hua (Barry Song) <redacted>;
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Subject: Re: [RFC PATCH v5 1/4] topology: Represent clusters of CPUs within
a die

On Fri, Mar 19, 2021 at 09:36:16AM +0000, Jonathan Cameron wrote:
On Fri, 19 Mar 2021 06:57:08 +0000
"Song Bao Hua (Barry Song)" [off-list ref] wrote:

-----Original Message-----
From: Greg KH [mailto:gregkh@linuxfoundation.org]
Sent: Friday, March 19, 2021 7:35 PM
To: Song Bao Hua (Barry Song) <redacted>
Cc: tim.c.chen@linux.intel.com; catalin.marinas@arm.com;
will@kernel.org;
rjw@rjwysocki.net; vincent.guittot@linaro.org; bp@alien8.de;
tglx@linutronix.de; mingo@redhat.com; lenb@kernel.org;
peterz@infradead.org;
dietmar.eggemann@arm.com; rostedt@goodmis.org; bsegall@google.com;
mgorman@suse.de; msys.mizuma@gmail.com; valentin.schneider@arm.com;
Jonathan
Cameron [off-list ref]; juri.lelli@redhat.com;
mark.rutland@arm.com; sudeep.holla@arm.com; aubrey.li@linux.intel.com;
linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org;
linux-acpi@vger.kernel.org; x86@kernel.org; xuwei (O)
[off-list ref];
Zengtao (B) [off-list ref]; guodong.xu@linaro.org;
yangyicong
[off-list ref]; Liguozhu (Kenneth) [off-list ref];
linuxarm@openeuler.org; hpa@zytor.com
Subject: Re: [RFC PATCH v5 1/4] topology: Represent clusters of CPUs within
a die

On Fri, Mar 19, 2021 at 05:16:15PM +1300, Barry Song wrote:

diff --git a/Documentation/admin-guide/cputopology.rst
b/Documentation/admin-guide/cputopology.rst
index b90dafc..f9d3745 100644
--- a/Documentation/admin-guide/cputopology.rst
+++ b/Documentation/admin-guide/cputopology.rst
@@ -24,6 +24,12 @@ core_id:
 	identifier (rather than the kernel's).  The actual value is
 	architecture and platform dependent.

+cluster_id:
+
+	the Cluster ID of cpuX.  Typically it is the hardware platform's
+	identifier (rather than the kernel's).  The actual value is
+	architecture and platform dependent.
+
 book_id:

 	the book ID of cpuX. Typically it is the hardware platform's
@@ -56,6 +62,14 @@ package_cpus_list:
 	human-readable list of CPUs sharing the same physical_package_id.
 	(deprecated name: "core_siblings_list")

+cluster_cpus:
+
+	internal kernel map of CPUs within the same cluster.
+
+cluster_cpus_list:
+
+	human-readable list of CPUs within the same cluster.
+
 die_cpus:

 	internal kernel map of CPUs within the same die.
Why are these sysfs files in this file, and not in a Documentation/ABI/
file which can be correctly parsed and shown to userspace?
Well. Those ABIs have been there for much a long time. It is like:

[root@ceph1 topology]# ls
core_id  core_siblings  core_siblings_list  physical_package_id
thread_siblings  thread_siblings_list
[root@ceph1 topology]# pwd
/sys/devices/system/cpu/cpu100/topology
[root@ceph1 topology]# cat core_siblings_list
64-127
[root@ceph1 topology]#

Any chance you can fix that up here as well?
Yes. we will send a separate patch to address this, which won't
be in this patchset. This patchset will base on that one.

Also note that "list" is not something that goes in sysfs, sysfs is "one
value per file", and a list is not "one value".  How do you prevent
overflowing the buffer of the sysfs file if you have a "list"?
At a glance, the list is using "-" rather than a real list
[root@ceph1 topology]# cat core_siblings_list
64-127

Anyway, I will take a look if it has any chance to overflow.
It could in theory be alternate CPUs as comma separated list.
So it's would get interesting around 500-1000 cpus (guessing).

Hopefully no one has that crazy a cpu numbering scheme but it's possible
(note that cluster is fine for this, but I guess it might eventually
happen for core-siblings list (cpus within a package).

Shouldn't crash or anything like that but might terminate early.
We have a broken sysfs api already for listing LED numbers that has had
to be worked around in the past, please do not create a new one with
that same problem, we should learn from them :)
Another place I am seeing a cpu list is in numa topology:
/sys/devices/system/node/nodex/cpulist.

But the code has a BUILD_BUG_ON to guard the pagebuf:

static ssize_t node_read_cpumap(struct device *dev, bool list, char *buf)
{
	ssize_t n;
	cpumask_var_t mask;
	struct node *node_dev = to_node(dev);

	/* 2008/04/07: buf currently PAGE_SIZE, need 9 chars per 32 bits. */
	BUILD_BUG_ON((NR_CPUS/32 * 9) > (PAGE_SIZE-1));

	if (!alloc_cpumask_var(&mask, GFP_KERNEL))
		return 0;

	cpumask_and(mask, cpumask_of_node(node_dev->dev.id), cpu_online_mask);
	n = cpumap_print_to_pagebuf(list, buf, mask);
	free_cpumask_var(mask);

	return n;
}

For lists in cpu topology, I haven't seen this while I believe we need it.
Or am I missing something?

thanks,

greg k-h
Thanks
Barry


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