Ankur Arora <ankur.a.arora@xxxxxxxxxx> writes:
> Add a timed variant of smp_cond_load_relaxed().
>
> This is useful because arm64 supports polling on a conditional variable
> by directly waiting on the cacheline instead of spin waiting for the
> condition to change.
>
> However, an implementation such as this has a problem that it can block
> forever -- unless there's an explicit timeout or another out-of-band
> mechanism which allows it to come out of the wait state periodically.
>
> smp_cond_load_relaxed_timeout() supports these semantics by specifying
> a time-check expression and an associated time-limit.
>
> However, note that for the generic spin-wait implementation we want to
> minimize the numbers of instructions executed in each iteration. So,
> limit how often we evaluate the time-check expression by doing it once
> every smp_cond_time_check_count.
>
> The inner loop in poll_idle() has a substantially similar structure
> and constraints as smp_cond_load_relaxed_timeout(), so define
> smp_cond_time_check_count to the same value used in poll_idle().
>
> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> ---
> include/asm-generic/barrier.h | 42 +++++++++++++++++++++++++++++++++++
> 1 file changed, 42 insertions(+)
>
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index d4f581c1e21d..77726ef807e4 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -273,6 +273,48 @@ do { \
> })
> #endif
>
> +#ifndef smp_cond_time_check_count
> +/*
> + * Limit how often smp_cond_load_relaxed_timeout() evaluates time_expr_ns.
> + * This helps reduce the number of instructions executed while spin-waiting.
> + */
> +#define smp_cond_time_check_count 200
> +#endif
> +
> +/**
> + * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no ordering
> + * guarantees until a timeout expires.
> + * @ptr: pointer to the variable to wait on
> + * @cond: boolean expression to wait for
> + * @time_expr_ns: evaluates to the current time
> + * @time_limit_ns: compared against time_expr_ns
> + *
> + * Equivalent to using READ_ONCE() on the condition variable.
> + *
> + * Due to C lacking lambda expressions we load the value of *ptr into a
> + * pre-named variable @VAL to be used in @cond.
Based on the review comments so far I'm planning to add the following
text to this comment:
Note that in the generic version the time check is done only coarsely
to minimize instructions executed while spin-waiting.
Architecture specific variations might also have their own timeout
granularity.
Meanwhile, would appreciate more reviews.
Thanks
Ankur
> + */
> +#ifndef smp_cond_load_relaxed_timeout
> +#define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr_ns, \
> + time_limit_ns) ({ \
> + typeof(ptr) __PTR = (ptr); \
> + __unqual_scalar_typeof(*ptr) VAL; \
> + unsigned int __count = 0; \
> + for (;;) { \
> + VAL = READ_ONCE(*__PTR); \
> + if (cond_expr) \
> + break; \
> + cpu_relax(); \
> + if (__count++ < smp_cond_time_check_count) \
> + continue; \
> + if ((time_expr_ns) >= time_limit_ns) \
> + break; \
> + __count = 0; \
> + } \
> + (typeof(*ptr))VAL; \
> +})
> +#endif
> +
> /*
> * pmem_wmb() ensures that all stores for which the modification
> * are written to persistent storage by preceding instructions have
