Paulo, this is what I'm going to send upstream tomorrow. FYI.
P.
----8<---
I have a system on which I have disabled threading in the BIOS, and I am booting
the kernel with the option "idle=poll".
The kernel displays
process: WARNING: polling idle and HT enabled, performance may degrade
which is incorrect -- I've already disabled HT.
This warning is issued here:
void select_idle_routine(const struct cpuinfo_x86 *c)
{
if (boot_option_idle_override == IDLE_POLL && smp_num_siblings > 1)
pr_warn_once("WARNING: polling idle and HT enabled, performance
may degrade\n");
>From my understanding of the other areas of kernel that use
smp_num_siblings, the value is supposed to be the the number of threads
per core.
The value of smp_num_siblings is incorrect. In theory, it should be 1 but it
is reported as 2. When I looked into how smp_num_siblings is calculated I
found the following call sequence in the kernel:
start_kernel ->
check_bugs ->
identify_boot_cpu ->
identify_cpu ->
c_init = init_intel
init_intel ->
detect_extended_topology
(sets value)
OR
c_init = init_amd
init_amd -> amd_detect_cmp
-> amd_get_topology
(sets value)
-> detect_ht()
... (sets value)
detect_ht()
(also sets value)
ie) it is set three times in some cases and is overwritten by the call
to detect_ht() from identify_cpu() in all cases.
It should be noted that nothing in the identify_cpu() path or the cpu_up()
path requires smp_num_siblings to be set, prior to the final call to
detect_ht().
For x86 boxes, smp_num_siblings is set to a value read in a CPUID call in
detect_ht(). This value is the *factory defined* value in all cases; even
if HT is disabled in BIOS the value still returns 2 if the CPU supports
HT. AMD also reports the factory defined value in all cases.
That is, even with threading disabled,
crash> p smp_num_siblings
smp_num_siblings = $1 = 0x2
on processors that support multi-threading.
smp_num_siblings should be calculated a single time on cpu 0 to determine
whether or not the system is multi-threaded or not.
On a system with HT enabled,
crash> p smp_num_siblings
smp_num_siblings = $1 = 0x2
On a system with HT disabled,
crash> p smp_num_siblings
smp_num_siblings = $1 = 0x1
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: [email protected]
Cc: Borislav Petkov <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Dave Jones <[email protected]>
Cc: Torsten Kaiser <[email protected]>
Cc: Jan Beulich <[email protected]>
Cc: Jan Kiszka <[email protected]>
Cc: Toshi Kani <[email protected]>
Cc: Andrew Jones <[email protected]>
Signed-off-by: Prarit Bhargava <[email protected]>
Prarit Bhargava (2):
x86, Clean up smp_num_siblings calculation
x86, Calculate smp_num_siblings once
arch/x86/kernel/cpu/amd.c | 1 -
arch/x86/kernel/cpu/common.c | 23 +++++++++++------------
arch/x86/kernel/cpu/topology.c | 2 +-
arch/x86/kernel/smpboot.c | 10 +++++++---
4 files changed, 19 insertions(+), 17 deletions(-)
--
1.7.9.3
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