| Message ID | 20221004033548.2508712-1-jeffrey.lane@canonical.com |
|---|---|
| Headers | show |
| Series | limit "Dummy wait" to old Intel systems | expand |
On 10/3/22 21:35, Jeff Lane wrote: > BugLink: https://bugs.launchpad.net/bugs/1990985 > > Old, circa 2002 chipsets have a bug: they don't go idle when they are > supposed to. So, a workaround was added to slow the CPU down and > ensure that the CPU waits a bit for the chipset to actually go idle. > This workaround is ancient and has been in place in some form since > the original kernel ACPI implementation. > > But, this workaround is very painful on modern systems. The "inl()" > can take thousands of cycles (see Link: for some more detailed > numbers and some fun kernel archaeology). > > First and foremost, modern systems should not be using this code. > Typical Intel systems have not used it in over a decade because it is > horribly inferior to MWAIT-based idle. > > Despite this, people do seem to be tripping over this workaround on > AMD system today. > > Limit the "dummy wait" workaround to Intel systems. Keep Modern AMD > systems from tripping over the workaround. Remotely modern Intel > systems use intel_idle instead of this code and will, in practice, > remain unaffected by the dummy wait. > > Small patch cleanly picks to both Bionic and Focal. > > > > Dave Hansen (1): > ACPI: processor idle: Practically limit "Dummy wait" workaround to old > Intel systems > > Yin Fengwei (1): > ACPI: processor_idle: Skip dummy wait if kernel is in guest > > drivers/acpi/processor_idle.c | 38 +++++++++++++++++++++++++++++------ > 1 file changed, 32 insertions(+), 6 deletions(-) > Acked-by: Tim Gardner <tim.gardner@canonical.com>
On 04.10.22 05:35, Jeff Lane wrote: > BugLink: https://bugs.launchpad.net/bugs/1990985 > > Old, circa 2002 chipsets have a bug: they don't go idle when they are > supposed to. So, a workaround was added to slow the CPU down and > ensure that the CPU waits a bit for the chipset to actually go idle. > This workaround is ancient and has been in place in some form since > the original kernel ACPI implementation. > > But, this workaround is very painful on modern systems. The "inl()" > can take thousands of cycles (see Link: for some more detailed > numbers and some fun kernel archaeology). > > First and foremost, modern systems should not be using this code. > Typical Intel systems have not used it in over a decade because it is > horribly inferior to MWAIT-based idle. > > Despite this, people do seem to be tripping over this workaround on > AMD system today. > > Limit the "dummy wait" workaround to Intel systems. Keep Modern AMD > systems from tripping over the workaround. Remotely modern Intel > systems use intel_idle instead of this code and will, in practice, > remain unaffected by the dummy wait. > > Small patch cleanly picks to both Bionic and Focal. > > > > Dave Hansen (1): > ACPI: processor idle: Practically limit "Dummy wait" workaround to old > Intel systems > > Yin Fengwei (1): > ACPI: processor_idle: Skip dummy wait if kernel is in guest > > drivers/acpi/processor_idle.c | 38 +++++++++++++++++++++++++++++------ > 1 file changed, 32 insertions(+), 6 deletions(-) > Acked-by: Stefan Bader <stefan.bader@canonical.com>
On 04.10.22 05:35, Jeff Lane wrote: > BugLink: https://bugs.launchpad.net/bugs/1990985 > > Old, circa 2002 chipsets have a bug: they don't go idle when they are > supposed to. So, a workaround was added to slow the CPU down and > ensure that the CPU waits a bit for the chipset to actually go idle. > This workaround is ancient and has been in place in some form since > the original kernel ACPI implementation. > > But, this workaround is very painful on modern systems. The "inl()" > can take thousands of cycles (see Link: for some more detailed > numbers and some fun kernel archaeology). > > First and foremost, modern systems should not be using this code. > Typical Intel systems have not used it in over a decade because it is > horribly inferior to MWAIT-based idle. > > Despite this, people do seem to be tripping over this workaround on > AMD system today. > > Limit the "dummy wait" workaround to Intel systems. Keep Modern AMD > systems from tripping over the workaround. Remotely modern Intel > systems use intel_idle instead of this code and will, in practice, > remain unaffected by the dummy wait. > > Small patch cleanly picks to both Bionic and Focal. > > > > Dave Hansen (1): > ACPI: processor idle: Practically limit "Dummy wait" workaround to old > Intel systems > > Yin Fengwei (1): > ACPI: processor_idle: Skip dummy wait if kernel is in guest > > drivers/acpi/processor_idle.c | 38 +++++++++++++++++++++++++++++------ > 1 file changed, 32 insertions(+), 6 deletions(-) > Applied to focal,bionic:linux/master-next. Thanks. -Stefan
BugLink: https://bugs.launchpad.net/bugs/1990985 Old, circa 2002 chipsets have a bug: they don't go idle when they are supposed to. So, a workaround was added to slow the CPU down and ensure that the CPU waits a bit for the chipset to actually go idle. This workaround is ancient and has been in place in some form since the original kernel ACPI implementation. But, this workaround is very painful on modern systems. The "inl()" can take thousands of cycles (see Link: for some more detailed numbers and some fun kernel archaeology). First and foremost, modern systems should not be using this code. Typical Intel systems have not used it in over a decade because it is horribly inferior to MWAIT-based idle. Despite this, people do seem to be tripping over this workaround on AMD system today. Limit the "dummy wait" workaround to Intel systems. Keep Modern AMD systems from tripping over the workaround. Remotely modern Intel systems use intel_idle instead of this code and will, in practice, remain unaffected by the dummy wait. Small patch cleanly picks to both Bionic and Focal. Dave Hansen (1): ACPI: processor idle: Practically limit "Dummy wait" workaround to old Intel systems Yin Fengwei (1): ACPI: processor_idle: Skip dummy wait if kernel is in guest drivers/acpi/processor_idle.c | 38 +++++++++++++++++++++++++++++------ 1 file changed, 32 insertions(+), 6 deletions(-)