Tuesday June 14, 2005 Weblog
Tuesday June 14, 2005 Prior to Solaris 10, we used to have
a resolution of 10 milliseconds by default for
kernel's cyclic subsystem on x86 and also the high resolution user level real time
timer (see timer_create(3RT)).
Then there was a bug report (4830628) where system time as reported by mpstat
on idle CPUs was much greater than expected on multi-processor x86 systems.
At the time, the way our timer interrupts on x86 used to work was that only the
boot CPU was programmed to periodically generate timer interrupts (by default
at 10 millisecond intervals). Upon handling such interrupts on boot CPU, the cyclic
backend then would send a broadcast cross call to other CPUs. The time to handle
the interrupt from the cross call on the other CPUs were accounted for system time
in mpstat.
I could have fixed the bug in several ways. Re-implementing the timer interrupt
mechanism was the best way, originally suggested by Jonathan. This was something
that needed to be done anyway since there were a lot of benefits to it. We'd get a
higher resolution for the kernel's cyclic subsystem and also the high resolution user
level real time timer, in addition to the bug being fixed.
The new implementation is to have a per-CPU based timer source. Each CPU then
generates timer interrupts on its own instead of having the cross call from boot CPU.
I took advantage of the Local APIC support for one-shot programming, instead of
programming the timer to generate periodical interrupts, one-shot mode is used.
This way the resolution for getting the next interrupt for the cyclic subsystem was
increased to be the same as resolution of the local APIC, e.g in the order of
10 nanoseconds :-). That is just great. We also do not have the overhead of traffic
for the cross calls on the bus anymore.
The cyclic backend already had the infrastructure to support this. I updated
the interface to our pcplusmp module, a platform specific module (psm) on
x86 which deals with APIC. I added four new functions to psm_ops structure:
(see psm_types.h)
kernel's cyclic subsystem on x86 and also the high resolution user level real time
timer (see timer_create(3RT)).
Then there was a bug report (4830628) where system time as reported by mpstat
on idle CPUs was much greater than expected on multi-processor x86 systems.
At the time, the way our timer interrupts on x86 used to work was that only the
boot CPU was programmed to periodically generate timer interrupts (by default
at 10 millisecond intervals). Upon handling such interrupts on boot CPU, the cyclic
backend then would send a broadcast cross call to other CPUs. The time to handle
the interrupt from the cross call on the other CPUs were accounted for system time
in mpstat.
I could have fixed the bug in several ways. Re-implementing the timer interrupt
mechanism was the best way, originally suggested by Jonathan. This was something
that needed to be done anyway since there were a lot of benefits to it. We'd get a
higher resolution for the kernel's cyclic subsystem and also the high resolution user
level real time timer, in addition to the bug being fixed.
The new implementation is to have a per-CPU based timer source. Each CPU then
generates timer interrupts on its own instead of having the cross call from boot CPU.
I took advantage of the Local APIC support for one-shot programming, instead of
programming the timer to generate periodical interrupts, one-shot mode is used.
This way the resolution for getting the next interrupt for the cyclic subsystem was
increased to be the same as resolution of the local APIC, e.g in the order of
10 nanoseconds :-). That is just great. We also do not have the overhead of traffic
for the cross calls on the bus anymore.
The cyclic backend already had the infrastructure to support this. I updated
the interface to our pcplusmp module, a platform specific module (psm) on
x86 which deals with APIC. I added four new functions to psm_ops structure:
(see psm_types.h)
void (*psm_timer_reprogram)(hrtime_t time);
void (*psm_timer_enable)(void);
void (*psm_timer_disable)(void);
void (*psm_post_cyclic_setup)(void *arg)
and redefined the existing psm_clkinit function.
In short, psm_timer_reprogram reprograms the timer on the CPU that is being
called to generate an interrupt at the time specified by the passed in argument.
psm_timer_enable and psm_timer_disable, enable and disable the timer interrupts
on the CPU being called on. Since the cyclic setup on x86 is intertwined with the psm
setup of for timer, psm_post_cyclic_setup was introduced so that the psm module itself
can be a consumer of cyclic subsystem as well as a backend support for it.
The code is in apic.c
Technorati Tag: OpenSolaris
Technorati Tag: Solaris
( Jun 14 2005, 11:49:13 AM PDT ) Permalink Comments [14]
