Catalin Marinas [off-list ref] writes:

On Sun, Feb 08, 2026 at 06:31:42PM -0800, Ankur Arora wrote:

quoted

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 <redacted>

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

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

Great. Thanks for the (re-)review!

--
ankur