From: "Nysal Jan K.A" <[email protected]>
After the addition of HOTPLUG_SMT support for PowerPC [1] there was a
regression reported [2] when enabling SMT. On a system with at least
one offline core, when enabling SMT, the expectation is that no CPUs
of offline cores are made online.
On a POWER9 system with 4 cores in SMT4 mode:
$ ppc64_cpu --info
Core 0: 0* 1* 2* 3*
Core 1: 4* 5* 6* 7*
Core 2: 8* 9* 10* 11*
Core 3: 12* 13* 14* 15*
Turn only one core on:
$ ppc64_cpu --cores-on=1
$ ppc64_cpu --info
Core 0: 0* 1* 2* 3*
Core 1: 4 5 6 7
Core 2: 8 9 10 11
Core 3: 12 13 14 15
Change the SMT level to 2:
$ ppc64_cpu --smt=2
$ ppc64_cpu --info
Core 0: 0* 1* 2 3
Core 1: 4 5 6 7
Core 2: 8 9 10 11
Core 3: 12 13 14 15
As expected we see only two CPUs of core 0 are online
Change the SMT level to 4:
$ ppc64_cpu --smt=4
$ ppc64_cpu --info
Core 0: 0* 1* 2* 3*
Core 1: 4* 5* 6* 7*
Core 2: 8* 9* 10* 11*
Core 3: 12* 13* 14* 15*
The CPUs of offline cores are made online. If a core is offline then
enabling SMT should not online CPUs of this core. An arch specific
function topology_is_core_online() is proposed to address this.
Another approach is to check the topology_sibling_cpumask() for any
online siblings. This avoids the need for an arch specific function
but is less efficient and more importantly this introduces a change
in existing behaviour on other architectures.
What is the expected behaviour on x86 when enabling SMT and certain cores
are offline?
[1] https://lore.kernel.org/lkml/[email protected]/
[2] https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ
Nysal Jan K.A (2):
cpu/SMT: Enable SMT only if a core is online
powerpc/topology: Check if a core is online
arch/powerpc/include/asm/topology.h | 13 +++++++++++++
kernel/cpu.c | 12 +++++++++++-
2 files changed, 24 insertions(+), 1 deletion(-)
base-commit: c760b3725e52403dc1b28644fb09c47a83cacea6
--
2.35.3
From: "Nysal Jan K.A" <[email protected]>
topology_is_core_online() checks if the core a CPU belongs to
is online. The core is online if at least one of the sibling
CPUs is online. The first CPU of an online core is also online
in the common case, so this should be fairly quick.
Signed-off-by: Nysal Jan K.A <[email protected]>
---
arch/powerpc/include/asm/topology.h | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index f4e6f2dd04b7..16bacfe8c7a2 100644
--- a/arch/powerpc/include/asm/topology.h
+++ b/arch/powerpc/include/asm/topology.h
@@ -145,6 +145,7 @@ static inline int cpu_to_coregroup_id(int cpu)
#ifdef CONFIG_HOTPLUG_SMT
#include <linux/cpu_smt.h>
+#include <linux/cpumask.h>
#include <asm/cputhreads.h>
static inline bool topology_is_primary_thread(unsigned int cpu)
@@ -156,6 +157,18 @@ static inline bool topology_smt_thread_allowed(unsigned int cpu)
{
return cpu_thread_in_core(cpu) < cpu_smt_num_threads;
}
+
+#define topology_is_core_online topology_is_core_online
+static inline bool topology_is_core_online(unsigned int cpu)
+{
+ int i, first_cpu = cpu_first_thread_sibling(cpu);
+
+ for (i = first_cpu; i < first_cpu + threads_per_core; ++i) {
+ if (cpu_online(i))
+ return true;
+ }
+ return false;
+}
#endif
#endif /* __KERNEL__ */
--
2.35.3
"Nysal Jan K.A." <[email protected]> writes:
> From: "Nysal Jan K.A" <[email protected]>
>
> After the addition of HOTPLUG_SMT support for PowerPC [1] there was a
> regression reported [2] when enabling SMT.
This implies it was a kernel regression. But it can't be a kernel
regression because previously there was no support at all for the sysfs
interface on powerpc.
IIUIC the regression was in the ppc64_cpu userspace tool, which switched
to using the new kernel interface without taking into account the way it
behaves.
Or are you saying the kernel behaviour changed on x86 after the powerpc
HOTPLUG_SMT was added?
> On a system with at least
> one offline core, when enabling SMT, the expectation is that no CPUs
> of offline cores are made online.
>
> On a POWER9 system with 4 cores in SMT4 mode:
> $ ppc64_cpu --info
> Core 0: 0* 1* 2* 3*
> Core 1: 4* 5* 6* 7*
> Core 2: 8* 9* 10* 11*
> Core 3: 12* 13* 14* 15*
>
> Turn only one core on:
> $ ppc64_cpu --cores-on=1
> $ ppc64_cpu --info
> Core 0: 0* 1* 2* 3*
> Core 1: 4 5 6 7
> Core 2: 8 9 10 11
> Core 3: 12 13 14 15
>
> Change the SMT level to 2:
> $ ppc64_cpu --smt=2
> $ ppc64_cpu --info
> Core 0: 0* 1* 2 3
> Core 1: 4 5 6 7
> Core 2: 8 9 10 11
> Core 3: 12 13 14 15
>
> As expected we see only two CPUs of core 0 are online
>
> Change the SMT level to 4:
> $ ppc64_cpu --smt=4
> $ ppc64_cpu --info
> Core 0: 0* 1* 2* 3*
> Core 1: 4* 5* 6* 7*
> Core 2: 8* 9* 10* 11*
> Core 3: 12* 13* 14* 15*
>
> The CPUs of offline cores are made online. If a core is offline then
> enabling SMT should not online CPUs of this core.
That's the way the ppc64_cpu tool behaves, but it's not necessarily what
other arches want.
> An arch specific
> function topology_is_core_online() is proposed to address this.
> Another approach is to check the topology_sibling_cpumask() for any
> online siblings. This avoids the need for an arch specific function
> but is less efficient and more importantly this introduces a change
> in existing behaviour on other architectures.
It's only x86 and powerpc right?
Having different behaviour on the only two arches that support the
interface does not seem like a good result.
> What is the expected behaviour on x86 when enabling SMT and certain cores
> are offline?
AFAIK no one really touches SMT on x86 other than to turn it off for
security reasons.
cheers
> [1] https://lore.kernel.org/lkml/[email protected]/
> [2] https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ
>
> Nysal Jan K.A (2):
> cpu/SMT: Enable SMT only if a core is online
> powerpc/topology: Check if a core is online
>
> arch/powerpc/include/asm/topology.h | 13 +++++++++++++
> kernel/cpu.c | 12 +++++++++++-
> 2 files changed, 24 insertions(+), 1 deletion(-)
>
>
> base-commit: c760b3725e52403dc1b28644fb09c47a83cacea6
> --
> 2.35.3
On Thu, Jun 13, 2024 at 09:34:10PM +1000, Michael Ellerman wrote:
> "Nysal Jan K.A." <[email protected]> writes:
> > From: "Nysal Jan K.A" <[email protected]>
> >
> > After the addition of HOTPLUG_SMT support for PowerPC [1] there was a
> > regression reported [2] when enabling SMT.
>
> This implies it was a kernel regression. But it can't be a kernel
> regression because previously there was no support at all for the sysfs
> interface on powerpc.
>
> IIUIC the regression was in the ppc64_cpu userspace tool, which switched
> to using the new kernel interface without taking into account the way it
> behaves.
The reported regression is in the ppc64_cpu tool behavior.
> Or are you saying the kernel behaviour changed on x86 after the powerpc
> HOTPLUG_SMT was added?
>
> > On a system with at least
> > one offline core, when enabling SMT, the expectation is that no CPUs
> > of offline cores are made online.
> >
> > On a POWER9 system with 4 cores in SMT4 mode:
> > $ ppc64_cpu --info
> > Core 0: 0* 1* 2* 3*
> > Core 1: 4* 5* 6* 7*
> > Core 2: 8* 9* 10* 11*
> > Core 3: 12* 13* 14* 15*
> >
> > Turn only one core on:
> > $ ppc64_cpu --cores-on=1
> > $ ppc64_cpu --info
> > Core 0: 0* 1* 2* 3*
> > Core 1: 4 5 6 7
> > Core 2: 8 9 10 11
> > Core 3: 12 13 14 15
> >
> > Change the SMT level to 2:
> > $ ppc64_cpu --smt=2
> > $ ppc64_cpu --info
> > Core 0: 0* 1* 2 3
> > Core 1: 4 5 6 7
> > Core 2: 8 9 10 11
> > Core 3: 12 13 14 15
> >
> > As expected we see only two CPUs of core 0 are online
> >
> > Change the SMT level to 4:
> > $ ppc64_cpu --smt=4
> > $ ppc64_cpu --info
> > Core 0: 0* 1* 2* 3*
> > Core 1: 4* 5* 6* 7*
> > Core 2: 8* 9* 10* 11*
> > Core 3: 12* 13* 14* 15*
> >
> > The CPUs of offline cores are made online. If a core is offline then
> > enabling SMT should not online CPUs of this core.
>
> That's the way the ppc64_cpu tool behaves, but it's not necessarily what
> other arches want.
>
> > An arch specific
> > function topology_is_core_online() is proposed to address this.
> > Another approach is to check the topology_sibling_cpumask() for any
> > online siblings. This avoids the need for an arch specific function
> > but is less efficient and more importantly this introduces a change
> > in existing behaviour on other architectures.
>
> It's only x86 and powerpc right?
>
> Having different behaviour on the only two arches that support the
> interface does not seem like a good result.
That's unfortunate. At the same time changing the x86 behavior would
potentially lead to similar reports from people relying on the old
behavior.
> > What is the expected behaviour on x86 when enabling SMT and certain cores
> > are offline?
>
> AFAIK no one really touches SMT on x86 other than to turn it off for
> security reasons.
In particular I am not aware of x86 suporting those middle partially
enabled states. I don't have a x86 4+ way SMT cpu at hand to test, either.
The more likely excuse is that there is little use case for enabling
previously disabled CPUs (whole cores/thread groups) by changing the SMT
state, even if the SMT code happened to do it in the past.
That is, more technically, that the value of 'off' is 1 - 1 thread of
each core is enabled, and another value representing 'core disabled'
with no thread of the core running is to be treated specially, and not
changed when setting the system-wide SMT value.
These are separate concerns, and should be addressed by separate
interfaces.
Thanks
Michal
On Thu, Jun 13, 2024 at 09:34:10PM GMT, Michael Ellerman wrote:
> "Nysal Jan K.A." <[email protected]> writes:
> > From: "Nysal Jan K.A" <[email protected]>
> >
> > After the addition of HOTPLUG_SMT support for PowerPC [1] there was a
> > regression reported [2] when enabling SMT.
>
> This implies it was a kernel regression. But it can't be a kernel
> regression because previously there was no support at all for the sysfs
> interface on powerpc.
>
> IIUIC the regression was in the ppc64_cpu userspace tool, which switched
> to using the new kernel interface without taking into account the way it
> behaves.
>
> Or are you saying the kernel behaviour changed on x86 after the powerpc
> HOTPLUG_SMT was added?
>
The regression is in ppc64_cpu. If we need the older behaviour we will need this
or an equivalent change in the kernel though. Fixing it in userspace in an
efficient way might be difficult.
> > On a system with at least
> > one offline core, when enabling SMT, the expectation is that no CPUs
> > of offline cores are made online.
> >
> > On a POWER9 system with 4 cores in SMT4 mode:
> > $ ppc64_cpu --info
> > Core 0: 0* 1* 2* 3*
> > Core 1: 4* 5* 6* 7*
> > Core 2: 8* 9* 10* 11*
> > Core 3: 12* 13* 14* 15*
> >
> > Turn only one core on:
> > $ ppc64_cpu --cores-on=1
> > $ ppc64_cpu --info
> > Core 0: 0* 1* 2* 3*
> > Core 1: 4 5 6 7
> > Core 2: 8 9 10 11
> > Core 3: 12 13 14 15
> >
> > Change the SMT level to 2:
> > $ ppc64_cpu --smt=2
> > $ ppc64_cpu --info
> > Core 0: 0* 1* 2 3
> > Core 1: 4 5 6 7
> > Core 2: 8 9 10 11
> > Core 3: 12 13 14 15
> >
> > As expected we see only two CPUs of core 0 are online
> >
> > Change the SMT level to 4:
> > $ ppc64_cpu --smt=4
> > $ ppc64_cpu --info
> > Core 0: 0* 1* 2* 3*
> > Core 1: 4* 5* 6* 7*
> > Core 2: 8* 9* 10* 11*
> > Core 3: 12* 13* 14* 15*
> >
> > The CPUs of offline cores are made online. If a core is offline then
> > enabling SMT should not online CPUs of this core.
>
> That's the way the ppc64_cpu tool behaves, but it's not necessarily what
> other arches want.
>
True, but from a user perspective it seems logical though. I think one can make
a case for either behaviour.
> > An arch specific
> > function topology_is_core_online() is proposed to address this.
> > Another approach is to check the topology_sibling_cpumask() for any
> > online siblings. This avoids the need for an arch specific function
> > but is less efficient and more importantly this introduces a change
> > in existing behaviour on other architectures.
>
> It's only x86 and powerpc right?
>
> Having different behaviour on the only two arches that support the
> interface does not seem like a good result.
>
Agree, I was originally thinking of sending out a patch changing this for both
architectures, but was unsure if there might be users who now depend on this
behaviour on x86.
> > What is the expected behaviour on x86 when enabling SMT and certain cores
> > are offline?
>
> AFAIK no one really touches SMT on x86 other than to turn it off for
> security reasons.
>
> cheers
>
Thanks for your comments. It will be good to hear if changing this behaviour
for both x86 and PowerPC might be an acceptable path forward.
Regards
--Nysal
> > [1] https://lore.kernel.org/lkml/[email protected]/
> > [2] https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ
> >
> > Nysal Jan K.A (2):
> > cpu/SMT: Enable SMT only if a core is online
> > powerpc/topology: Check if a core is online
> >
> > arch/powerpc/include/asm/topology.h | 13 +++++++++++++
> > kernel/cpu.c | 12 +++++++++++-
> > 2 files changed, 24 insertions(+), 1 deletion(-)
> >
> >
> > base-commit: c760b3725e52403dc1b28644fb09c47a83cacea6
> > --
> > 2.35.3