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emacs 27.2 From: Ankur Arora To: Arnd Bergmann Cc: Catalin Marinas , Ankur Arora , linux-kernel@vger.kernel.org, Linux-Arch , linux-arm-kernel@lists.infradead.org, linux-pm@vger.kernel.org, bpf@vger.kernel.org, Will Deacon , Peter Zijlstra , Andrew Morton , Mark Rutland , Haris Okanovic , "Christoph Lameter (Ampere)" , Alexei Starovoitov , "Rafael J . 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X-CRM114-Status: GOOD ( 16.01 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Arnd Bergmann writes: > 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 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 ~100=C2=B5s. True. The thinking was that an adding an explicit timeout just begs the question of how closely the interface adheres to the timeout and I guess the final interface tried to sidestep all of that. > 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. That makes sense. It's a good point that the WFET+event-stream-off case would just end up using the spin lock which is quite suboptimal. >>> - 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 e= tc. > > 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. Great. I think that means that I can keep more or less the same interface with an explicit time_end. Which allows WFET to do the right thing. And, WFE can have an architecture specific slack (event-stream period). >>> 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 fo= r >>> 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. True. > 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. Will send out the next version with an interface on the following lines: /** * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no orderi= ng * guarantees until a timeout expires. * @ptr: pointer to the variable to wait on * @cond: boolean expression to wait for * @time_expr: time expression in caller's preferred clock * @time_end: end time in nanosecond (compared against time_expr; * might also be used for setting up a future event.) * * Equivalent to using READ_ONCE() on the condition variable. * * Note that the expiration of the timeout might have an architecture sp= ecific * delay. */ #ifndef smp_cond_load_relaxed_timeout #define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr, time_e= nd_ns) \ ({ \ typeof(ptr) __PTR =3D (ptr); \ __unqual_scalar_typeof(*ptr) VAL; \ u32 __n =3D 0, __spin =3D SMP_TIMEOUT_POLL_COUNT; \ u64 __time_end_ns =3D (time_end_ns); \ \ for (;;) { \ VAL =3D READ_ONCE(*__PTR); \ if (cond_expr) \ break; \ cpu_poll_relax(__PTR, VAL, __time_end_ns); \ if (++__n < __spin) \ continue; \ if ((time_expr) >=3D __time_end_ns) { \ VAL =3D READ_ONCE(*__PTR); \ break; \ } \ __n =3D 0; \ } \ (typeof(*ptr))VAL; \ }) #endif That allows for a __cmpwait_timeout() as you had outlined and similar to these two patches: https://lore.kernel.org/lkml/20241107190818.522639-15-ankur.a.arora@oracle= .com/ https://lore.kernel.org/lkml/20241107190818.522639-16-ankur.a.arora@oracle= .com/ (this one incorporating some changes that Catalin had suggested: https://lore.kernel.org/lkml/aKRTRyQAaWFtRvDv@arm.com/) -- ankur