On 09-Oct-20 4:39 PM, Ananyev, Konstantin wrote:
On 08-Oct-20 6:15 PM, Ananyev, Konstantin wrote:
Add two new power management intrinsics, and provide an implementation
in eal/x86 based on UMONITOR/UMWAIT instructions. The instructions
are implemented as raw byte opcodes because there is not yet widespread
compiler support for these instructions.
The power management instructions provide an architecture-specific
function to either wait until a specified TSC timestamp is reached, or
optionally wait until either a TSC timestamp is reached or a memory
location is written to. The monitor function also provides an optional
comparison, to avoid sleeping when the expected write has already
happened, and no more writes are expected.
I think what this API is missing - a function to wakeup sleeping core.
If user can/should use some system call to achieve that, then at least
it has to be clearly documented, even better some wrapper provided.
I don't think it's possible to do that without severely overcomplicating
the intrinsic and its usage, because AFAIK the only way to wake up a
sleeping core would be to send some kind of interrupt to the core, or
trigger a write to the cache-line in question.
Yes, I think we either need a syscall that would do an IPI for us
(on top of my head - membarrier() does that, might be there are some other
syscalls too),
or something hand-made. For hand-made, I wonder would something like that
be safe and sufficient:
uint64_t val = atomic_load(addr);
CAS(addr, val, &val);
?
Anyway, one way or another - I think ability to wakeup core we put to sleep
have to be an essential part of this feature.
As I understand linux kernel will limit max amount of sleep time for these
instructions:
https://lwn.net/Articles/790920/
But relying just on that, seems too vague for me:
- user can adjust that value
- wouldn't apply to older kernels and non-linux cases
Konstantin
This implies knowing the value the core is sleeping on. That's not
always the case - with this particular PMD power management scheme, we
get the address from the PMD and it stays inside the callback.
--
Thanks,
Anatoly
