Re: [dpdk-dev] [PATCH v1 1/7] power_intrinsics: allow monitor checks inversion

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Burakov, Anatoly <anatoly.burakov@intel.com>
> Sent: Wednesday, June 23, 2021 10:43 AM
> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>; dev@dpdk.org; Richardson, Bruce <bruce.richardson@intel.com>
> Cc: Loftus, Ciara <ciara.loftus@intel.com>; Hunt, David <david.hunt@intel.com>
> Subject: Re: [PATCH v1 1/7] power_intrinsics: allow monitor checks inversion
> 
> On 21-Jun-21 1:56 PM, Ananyev, Konstantin wrote:
> >
> > Hi Anatoly,
> >
> >> Previously, the semantics of power monitor were such that we were
> >> checking current value against the expected value, and if they matched,
> >> then the sleep was aborted. This is somewhat inflexible, because it only
> >> allowed us to check for a specific value.
> >>
> >> This commit adds an option to reverse the check, so that we can have
> >> monitor sleep aborted if the expected value *doesn't* match what's in
> >> memory. This allows us to both implement all currently implemented
> >> driver code, as well as support more use cases which don't easily map to
> >> previous semantics (such as waiting on writes to AF_XDP counter value).
> >>
> >> Since the old behavior is the default, no need to adjust existing
> >> implementations.
> >>
> >> Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
> >> ---
> >>   lib/eal/include/generic/rte_power_intrinsics.h | 4 ++++
> >>   lib/eal/x86/rte_power_intrinsics.c             | 5 ++++-
> >>   2 files changed, 8 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/lib/eal/include/generic/rte_power_intrinsics.h b/lib/eal/include/generic/rte_power_intrinsics.h
> >> index dddca3d41c..1006c2edfc 100644
> >> --- a/lib/eal/include/generic/rte_power_intrinsics.h
> >> +++ b/lib/eal/include/generic/rte_power_intrinsics.h
> >> @@ -31,6 +31,10 @@ struct rte_power_monitor_cond {
> >>                          *   4, or 8. Supplying any other value will result in
> >>                          *   an error.
> >>                          */
> >> +     uint8_t invert;  /**< Invert check for expected value (e.g. instead of
> >> +                       *   checking if `val` matches something, check if
> >> +                       *   `val` *doesn't* match a particular value)
> >> +                       */
> >>   };
> >>
> >>   /**
> >> diff --git a/lib/eal/x86/rte_power_intrinsics.c b/lib/eal/x86/rte_power_intrinsics.c
> >> index 39ea9fdecd..5d944e9aa4 100644
> >> --- a/lib/eal/x86/rte_power_intrinsics.c
> >> +++ b/lib/eal/x86/rte_power_intrinsics.c
> >> @@ -117,7 +117,10 @@ rte_power_monitor(const struct rte_power_monitor_cond *pmc,
> >>                const uint64_t masked = cur_value & pmc->mask;
> >>
> >>                /* if the masked value is already matching, abort */
> >> -             if (masked == pmc->val)
> >> +             if (!pmc->invert && masked == pmc->val)
> >> +                     goto end;
> >> +             /* same, but for inverse check */
> >> +             if (pmc->invert && masked != pmc->val)
> >>                        goto end;
> >>        }
> >>
> >
> > Hmm..., such approach looks too 'patchy'...
> > Can we at least replace 'inver' with something like:
> > enum rte_power_monitor_cond_op {
> >          _EQ, NEQ,...
> > };
> > Then at least new comparions ops can be added in future.
> > Even better I think would be to just leave to PMD to provide a comparison callback.
> > Will make things really simple and generic:
> > struct rte_power_monitor_cond {
> >       volatile void *addr;
> >       int (*cmp)(uint64_t val);
> >       uint8_t size;
> > };
> > And then in rte_power_monitor(...):
> > ....
> > const uint64_t cur_value = __get_umwait_val(pmc->addr, pmc->size);
> > if (pmc->cmp(cur_value) != 0)
> >          goto end;
> > ....
> >
> 
> I like the idea of a callback, but these are supposed to be
> intrinsic-like functions, so putting too much into them is contrary to
> their goal, and it's going to make the API hard to use in simpler cases
> (e.g. when we're explicitly calling rte_power_monitor as opposed to
> letting the RX callback do it for us). For example, event/dlb code calls
> rte_power_monitor explicitly.

Good point, I didn't know that.
Would be interesting to see how do they use it.

> 
> It's going to be especially "fun" to do these indirect function calls
> from inside transactional region on call to multi-monitor.

But the callback is not supposed to do any memory reads/writes.
Just mask/compare of the provided value with some constant.

> I'm not
> opposed to having a callback here, but maybe others have more thoughts
> on this?
> 
> --
> Thanks,
> Anatoly