Re: [PATCH v4 5/5] rqspinlock: use smp_cond_load_acquire_timewait()

[Ankur Arora <ankur.a.arora@oracle.com>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Mon, Sep 1, 2025 at 4:28 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>>
>> On Fri, Aug 29, 2025 at 01:07:35AM -0700, Ankur Arora wrote:
>> > diff --git a/arch/arm64/include/asm/rqspinlock.h b/arch/arm64/include/asm/rqspinlock.h
>> > index a385603436e9..ce8feadeb9a9 100644
>> > --- a/arch/arm64/include/asm/rqspinlock.h
>> > +++ b/arch/arm64/include/asm/rqspinlock.h
>> > @@ -3,6 +3,9 @@
>> >  #define _ASM_RQSPINLOCK_H
>> >
>> >  #include <asm/barrier.h>
>> > +
>> > +#define res_smp_cond_load_acquire_waiting() arch_timer_evtstrm_available()
>>
>> More on this below, I don't think we should define it.
>>
>> > diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
>> > index 5ab354d55d82..8de1395422e8 100644
>> > --- a/kernel/bpf/rqspinlock.c
>> > +++ b/kernel/bpf/rqspinlock.c
>> > @@ -82,6 +82,7 @@ struct rqspinlock_timeout {
>> >       u64 duration;
>> >       u64 cur;
>> >       u16 spin;
>> > +     u8  wait;
>> >  };
>> >
>> >  #define RES_TIMEOUT_VAL      2
>> > @@ -241,26 +242,20 @@ static noinline int check_timeout(rqspinlock_t *lock, u32 mask,
>> >  }
>> >
>> >  /*
>> > - * Do not amortize with spins when res_smp_cond_load_acquire is defined,
>> > - * as the macro does internal amortization for us.
>> > + * Only amortize with spins when we don't have a waiting implementation.
>> >   */
>> > -#ifndef res_smp_cond_load_acquire
>> >  #define RES_CHECK_TIMEOUT(ts, ret, mask)                              \
>> >       ({                                                            \
>> > -             if (!(ts).spin++)                                     \
>> > +             if ((ts).wait || !(ts).spin++)                \
>> >                       (ret) = check_timeout((lock), (mask), &(ts)); \
>> >               (ret);                                                \
>> >       })
>> > -#else
>> > -#define RES_CHECK_TIMEOUT(ts, ret, mask)                           \
>> > -     ({ (ret) = check_timeout((lock), (mask), &(ts)); })
>> > -#endif
>>
>> IIUC, RES_CHECK_TIMEOUT in the current res_smp_cond_load_acquire() usage
>> doesn't amortise the spins, as the comment suggests, but rather the
>> calls to check_timeout(). This is fine, it matches the behaviour of
>> smp_cond_load_relaxed_timewait() you introduced in the first patch. The
>> only difference is the number of spins - 200 (matching poll_idle) vs 64K
>> above. Does 200 work for the above?
>>
>> >  /*
>> >   * Initialize the 'spin' member.
>> >   * Set spin member to 0 to trigger AA/ABBA checks immediately.
>> >   */
>> > -#define RES_INIT_TIMEOUT(ts) ({ (ts).spin = 0; })
>> > +#define RES_INIT_TIMEOUT(ts) ({ (ts).spin = 0; (ts).wait = res_smp_cond_load_acquire_waiting(); })
>>
>> First of all, I don't really like the smp_cond_load_acquire_waiting(),
>> that's an implementation detail of smp_cond_load_*_timewait() that
>> shouldn't leak outside. But more importantly, RES_CHECK_TIMEOUT() is
>> also used outside the smp_cond_load_acquire_timewait() condition. The
>> (ts).wait check only makes sense when used together with the WFE
>> waiting.
>
> +1 to the above.

Ack.

> Penalizing all other architectures with pointless runtime check:
>
>> -             if (!(ts).spin++)                                     \
>> +             if ((ts).wait || !(ts).spin++)                \
>
> is not acceptable.

Is it still a penalty if the context is a busy wait loop.

Oddly enough the compiler does not eliminate this check on x86 (given
that it is statically defined to be 0 and ts does not escape the
function.)

--
ankur