linux-kernel - Re: [PATCH 1/5] asm-generic: add smp_vcond_load

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Message-ID: <b62d938111c6ce52b91d0f2e3922857c5d4ef253.camel@amazon.com>
Date: Wed, 6 Nov 2024 18:13:35 +0000
From: "Okanovic, Haris" <harisokn@...zon.com>
To: "catalin.marinas@....com" <catalin.marinas@....com>
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Subject: Re: [PATCH 1/5] asm-generic: add smp_vcond_load_relaxed()

On Wed, 2024-11-06 at 11:08 +0000, Catalin Marinas wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> On Tue, Nov 05, 2024 at 12:30:37PM -0600, Haris Okanovic wrote:
> > diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> > index d4f581c1e21d..112027eabbfc 100644
> > --- a/include/asm-generic/barrier.h
> > +++ b/include/asm-generic/barrier.h
> > @@ -256,6 +256,31 @@ do {                                                                     \
> >  })
> >  #endif
> > 
> > +/**
> > + * smp_vcond_load_relaxed() - (Spin) wait until an expected value at address
> > + * with no ordering guarantees. Spins until `(*addr & mask) == val` or
> > + * `nsecs` elapse, and returns the last observed `*addr` value.
> > + *
> > + * @nsecs: timeout in nanoseconds
> 
> FWIW, I don't mind the relative timeout, it makes the API easier to use.
> Yes, it may take longer in absolute time if the thread is scheduled out
> before local_clock_noinstr() is read but the same can happen in the
> caller anyway. It's similar to udelay(), it can take longer if the
> thread is scheduled out.
> 
> > + * @addr: pointer to an integer
> > + * @mask: a bit mask applied to read values
> > + * @val: Expected value with mask
> > + */
> > +#ifndef smp_vcond_load_relaxed
> > +#define smp_vcond_load_relaxed(nsecs, addr, mask, val) ({    \
> > +     const u64 __start = local_clock_noinstr();              \
> > +     u64 __nsecs = (nsecs);                                  \
> > +     typeof(addr) __addr = (addr);                           \
> > +     typeof(*__addr) __mask = (mask);                        \
> > +     typeof(*__addr) __val = (val);                          \
> > +     typeof(*__addr) __cur;                                  \
> > +     smp_cond_load_relaxed(__addr, (                         \
> > +             (VAL & __mask) == __val ||                      \
> > +             local_clock_noinstr() - __start > __nsecs       \
> > +     ));                                                     \
> > +})
> 
> The generic implementation has the same problem as Ankur's current
> series. smp_cond_load_relaxed() can't wait on anything other than the
> variable at __addr. If it goes into a WFE, there's nothing executed to
> read the timer and check for progress. Any generic implementation of
> such function would have to use cpu_relax() and polling.

How would the caller enter wfe()? Can you give a specific scenario that
you're concerned about?

This code already reduces to a relaxed poll, something like this:

```
start = clock();
while((READ_ONCE(*addr) & mask) != val && (clock() - start) < nsecs) {
  cpu_relax();
}
```

> 
> --
> Catalin