Re: [PATCH v9 01/12] asm-generic: barrier: Add smp_cond_load_relaxed_timeout()

[Catalin Marinas <catalin.marinas@arm.com>]

On Sun, Feb 08, 2026 at 06:31:42PM -0800, Ankur Arora wrote:
> Add smp_cond_load_relaxed_timeout(), which extends
> smp_cond_load_relaxed() to allow waiting for a duration.
> 
> We loop around waiting for the condition variable to change while
> peridically doing a time-check. The loop uses cpu_poll_relax() to slow
> down the busy-waiting, which, unless overridden by the architecture
> code, amounts to a cpu_relax().
> 
> Note that there are two ways for the time-check to fail: the usual
> timeout case or, @time_expr_ns returning an invalid value (negative
> or zero). The second failure mode allows for clocks attached to the
> clock-domain of @cond_expr, which might cease to operate meaningfully
> once some state internal to @cond_expr has changed.
> 
> Evaluation of @time_expr_ns: in the fastpath we want to keep the
> performance close to smp_cond_load_relaxed(). To do that we defer
> evaluation of the potentially costly @time_expr_ns to when we hit
> the slowpath.
> 
> This also means that there will always be some hardware dependent
> duration that has passed in cpu_poll_relax() iterations at the time of
> first evaluation. Additionally cpu_poll_relax() is not guaranteed to
> return at timeout boundary. In sum, expect timeout overshoot when we
> exit due to expiration of the timeout.
> 
> The number of spin iterations before time-check, SMP_TIMEOUT_POLL_COUNT
> is chosen to be 200 by default. With a cpu_poll_relax() iteration
> taking ~20-30 cycles (measured on a variety of x86 platforms), we expect
> a tim-check every ~4000-6000 cycles.
> 
> The outer limit of the overshoot is double that when working with the
> parameters above. This might be higher or lower depending on the
> implementation of cpu_poll_relax() across architectures.
> 
> Lastly, config option ARCH_HAS_CPU_RELAX indicates availability of a
> cpu_poll_relax() that is cheaper than polling. This might be relevant
> for cases with a prolonged timeout.
> 
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: linux-arch@vger.kernel.org
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>

This series evolved a bit since last time I looked, so going through it
again:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>