Re: [RESEND PATCH v7 2/7] arm64: barrier: Support smp_cond_load_relaxed_timeout()

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Oct 28, 2025, at 22:17, Catalin Marinas wrote:
> On Tue, Oct 28, 2025 at 11:01:22AM -0700, Ankur Arora wrote:
>> Arnd Bergmann <arnd@arndb.de> writes:
>> > On Tue, Oct 28, 2025, at 06:31, Ankur Arora wrote:
>> >> +
>> >
>> > Since the caller knows exactly how long it wants to wait for,
>> > we should be able to fit a 'wfet' based primitive in here and
>> > pass the timeout as another argument.
>> 
>> Per se, I don't disagree with this when it comes to WFET.
>> 
>> Handling a timeout, however, is messier when we use other mechanisms.
>> 
>> Some problems that came up in my earlier discussions with Catalin:
>> 
>>   - when using WFE, we also need some notion of slack
>>     - and if a caller specifies only a small or no slack, then we need
>>       to combine WFE+cpu_relax()

I don't see the difference to what you have: with the event stream,
you implicitly define a slack to be the programmed event stream rate
of ~100µs.


I'm not asking for anything better in this case, only for machines
with WFET but no event stream to also avoid the spin loop.

>>   - for platforms that only use a polling primitive, we want to check
>>     the clock only intermittently for power reasons.

Right, I missed that bit.

>>     Now, this could be done with an architecture specific spin-count.
>>     However, if the caller specifies a small slack, then we might need
>>     to we check the clock more often as we get closer to the deadline etc.

Again, I think this is solved by defining the slack as architecture
specific as well rather than an explicit argument, which is essentially
what we already have.
 
>> A smaller problem was that different users want different clocks and so
>> folding the timeout in a 'timeout_cond_expr' lets us do away with the
>> interface having to handle any of that.
>>
>> I had earlier versions [v2] [v3] which had rather elaborate policies for
>> handling timeout, slack etc. But, given that the current users of the
>> interface don't actually care about precision, all of that seemed
>> a little overengineered.
>
> Indeed, we've been through all these options and without a concrete user
> that needs a more precise timeout, we decided it's not worth it. It can,
> however, be improved later if such users appear.

The main worry I have is that we get too many users of cpu_poll_relax()
hardcoding the use of the event stream without a timeout argument, it
becomes too hard to change later without introducing regressions
from the behavior change.

As far as I can tell, the only place that currently uses the
event stream on a functional level is the delay() loop, and that
has a working wfet based version.

     Arnd