[dpdk-dev] [PATCH v4 02/10] eal: add power management intrinsics

[Ananyev, Konstantin]

> 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