Hi All,

At 03/13/2018 05:35 PM, Rafael J. Wysocki wrote:
On Tue, Mar 13, 2018 at 9:39 AM, Artem Bityutskiy <dedeki...@gmail.com> wrote:
On Tue, 2018-03-13 at 16:35 +0800, Ming Lei wrote:
Then looks this issue need to fix by making possible CPU count
accurate
because there are other resources allocated according to
num_possible_cpus(),
such as percpu variables.


Short term the regression should be fixed. It is already v4.16-rc6, we
have little time left.

Right.

Longer term, yeah, I agree. Kernel's notion of possible CPU count
should be realistic.


I did a patch for that, Artem, could you help me to test it.

----------------------->8-------------------------

diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
index 449d86d39965..878abfa0ce30 100644
--- a/drivers/acpi/acpi_processor.c
+++ b/drivers/acpi/acpi_processor.c
@@ -671,6 +671,18 @@ static acpi_status __init acpi_processor_ids_walk(acpi_handle handle,

 }

+static void __init acpi_refill_possible_map(void)
+{
+       int i;
+
+       reset_cpu_possible_mask();
+
+       for (i = 0; i < nr_unique_ids; i++)
+               set_cpu_possible(i, true);
+
+       pr_info("Allowing %d possible CPUs\n", nr_unique_ids);
+}
+
 static void __init acpi_processor_check_duplicates(void)
 {
        /* check the correctness for all processors in ACPI namespace */
@@ -680,6 +692,9 @@ static void __init acpi_processor_check_duplicates(void)
                                                NULL, NULL, NULL);
acpi_get_devices(ACPI_PROCESSOR_DEVICE_HID, acpi_processor_ids_walk,
                                                NULL, NULL);
+
+       /* make possible CPU count more realistic */
+       acpi_refill_possible_map();
 }

 bool acpi_duplicate_processor_id(int proc_id)

------------------------------------------------------------------
I agree.

Moreover, there are not too many systems where physical CPU hotplug
actually works in practice AFAICS, so IMO we should default to "no
physical CPU hotplug" and only change that default in special cases
(which may be hard to figure out, but that's a different matter).


Yes, I think so.



What platform firmware tells us may be completely off.

Rafeal,

Sorry, I am not sure what you mean :-) . Did you mean no platform
firmware can tell us whether physcial CPU hotplug is supported or not?

My colleagues also told to me that there is no way in OS to know whether
it is supported or not.

Thanks
        dou




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